US20090097551A1

Movatterモバイル変換

Info

Publication number: US20090097551A1
Application number: US12/336,479
Authority: US
Inventors: Ji Zhang; Yi Tong Tse
Original assignee: Cisco Technology Inc
Current assignee: Cisco Technology Inc
Priority date: 2000-01-26
Filing date: 2008-12-16
Publication date: 2009-04-16
Also published as: US7477688B1

Abstract

The present invention relates to systems and methods for efficient bit rate alteration of a bitstream to match an available channel capacity. The efficient bit rate alteration includes selective re-quantization of the compressed bitstream. Selective re-quantization according to the present invention applies multiple re-quantization schemes to different portions of a bitstream. In one embodiment, the multiple re-quantization schemes each have a different computational load. By selectively choosing which type of re-quantization is performed on each portion, efficient bandwidth scaling and data transmission may be achieved both when computational capacity is limited and when video data integrity is important.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 60,178,123, entitled “Methods for Efficient Bandwidth Scaling of Compressed Video Signals” and filed Jan. 26, 2000, and to U.S. patent application Ser. No. 09/766,020, entitled “Methods for Efficient Bandwidth Scaling of Compressed Video Data” and filed on Jan. 18, 2001, both of which are incorporated by reference herein. This application cross references U.S. Pat. No. 6,181,711, entitled “A System and Method for Transporting Compressed Video and Data Bitstream Over a Communication Channel” by Ji Zhang et al., filed on Oct. 10, 1997, which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates generally to systems and methods for transmitting data. More specifically, the present invention relates to systems and methods for transmitting compressed digital video data over communication channels at bit rates that comply with the available bandwidth of the channels. In particular, the present invention relates to systems and methods for adapting the bit rate of compressed digital video data by flexible re-quantization.

Transmission of video and voice over digital networks is increasing in demand. Exemplary digital networks include broadband IP networks, ATM networks, DWDM (Dense Wave Division Multiplexing) networks, wireless networks and local last-mile access networks such as DSL networks, cable networks and terrestrial broadcast networks. These networks have limited transmission capacities. Often, there is a need to scale a transmission bitstream including the video data in order to fit within the available bandwidth of a network connection, or channel.

For the purposes of the present application a channel is defined broadly as a connection facility to convey digital information from one point to another. A channel includes some or all of the following elements: 1) physical devices that generate and receive the data (e.g., a modulator/demodulator); 2) a medium that carries the data; 3) mathematical schemes used to encode and decode the data; 4) proper communication protocols used to establish, maintain and manage the connection created by the channel; and 5) storage systems used to store the data such as magnetic tapes and optical disks. The concept of a channel includes but is not limited to a physical channel and also includes logical connections established on top of different network protocols, such as xDSL, ATM, IP, wireless, HFC, coaxial cable, Ethernet, Token Ring, etc.

The channel is used to transport a bitstream, or a continuous sequence of binary bits used to digitally represent compressed video, audio or data. The bit rate is the number of bits per second that the channel is able to transport. The bit error rate is the statistical ratio between the number of bits in error due to transmission and the total number of bits transmitted. The channel capacity is the maximum bit rate at which a given channel can convey digital information with a bit error rate no more than a given value.

Since transmission of video image sequence data with existing communication channels is often excessive in terms of bandwidth requirement, compression is an approach that has been used to make digital video data more transportable. Digital video compression schemes allow digitized video data to be represented digitally in a much more efficient manner. Compression of video data makes it practical to transmit the compressed signal using channels at a fraction of the bandwidth required to transmit the original signal without compression.

International standards have been created on video compression schemes. These include MPEG-1, MPEG-2, MPEG-4, H.261, H.262, H.263, H.263+, etc. These standardized compression schemes rely on several algorithm schemes such as motion compensated transform coding (for example, DCT transforms or wavelet/sub-band transforms), quantization of the transform coefficients, and variable length coding (VLC). The motion compensated encoding removes the temporally redundant information in video sequences. The transform coding enables orthogonal spatial frequency representation of spatial domain video data. Quantization of the transformed coefficients reduces the number of levels required to represent a given digitized video. The other factor contributing to the compression is the use of variable length coding (VLC) so that most frequently used symbols are represented by the shortest code word. In general, the number of bits used to represent a given image determines the quality of the decoded picture. The more bits used to represent a given image, the better the image quality. The system that is used to compress digitized video sequence using the above-described schemes is called an encoder or encoding apparatus.

Commonly, transmission of video data is intended for real-time playback. This implies that all of the information required to represent a digital picture must be delivered to the destination in time for decoding and display in a timely manner. The channel must be capable of making such a delivery. However, a channel imposes a bit rate constraint for data being sent through the channel. This bit rate constraint often falls below the bit rate required to transport the compressed video bitstream.

Thus, there is often a need to scale the transmission bandwidth required for the video data in order to fit within the available bandwidth of a network connection, or channel. However, compression introduces significant complexity to manipulating the bitstream. The compressed video bitstreams, at any given bit rate, cannot be altered to a different bit rate without at least partial decoding and re-encoding. In addition, the number of bits required to represent digital video pictures varies from picture to picture. The coded pictures are sensitive to this variation and bit rate alteration of compressed video data must be as well.

There have been several proposed methods for performing bandwidth scaling of compressed video bitstreams. These bandwidth-scaling methods differ in computational complexity and video quality. Typically, there is a trade-off between computational complexity and video quality. Common to the methods is that only one scaling, or recoding, technique is used on a bitstream.

Quantization has a direct effect on the compressed bit usage and decoded video quality. For many compressed video bitstream schemes, it is possible to change the bit rate of a bitstream by changing the quantization step value. This approach is called re-quantization. Currently, real-time re-quantization of compressed video data is performed in one of two ways: basic re-quantization and motion compensated re-quantization.

Basic re-quantization involves partial de-coding and re-encoding of the compressed bitstream. Partial de-coding includes VLC decoding and de-quantization of a compressed bitstream. Re-encoding of the partially decoded bitstream then includes re-quantization and VLC encoding. The data is typically re-quantized to a larger step size, thus reducing the amount of information in the compressed bitstream and the resultant bit rate.

Re-quantization in this manner without motion compensation to a larger step size often results in a larger loss of video information compared to re-quantization scheme with motion estimation. This video information cannot be subsequently recovered. The resulting loss of video quality is reflected as blocky images and inconsistent image texture. To help reduce the video quality loss as a result of basic re-quantization, more complicated re-quantization schemes such as re-quantization with motion compensation are often used.

Motion compensated re-quantization includes a much more complex set of operations. The additional processing includes DCT transforms, motion compensation, and inverse DCT transforms. Motion compensation recoding may include motion vector re-use or motion vector estimation.

When the bit rate reduction range is small, the quality difference between the two re-quantization schemes is largely negligible. However, the video quality difference between the two schemes is much more pronounced when the bit rate reduction range is large. For example, when a 5 Mbps is reduced to 2 Mbps, motion compensation based recoding yields noticeably better video quality than the re-quantization-only recoding. However, if a 5 Mbps is reduced to 4 Mbps, the video quality impact by the two schemes are largely similar. Another major difference between basic re-quantization and motion compensated re-quantization is the additional processing required for motion compensation of image pixel blocks, which is very computationally intensive.

Transmission of real-time video is highly sensitive to delays. A compressed video bitstream, when transmitted under excessive and jittery delays, will cause a real-time decoder buffer to underflow or overflow, causing the decoded video sequence to be jerky, or causing associated audio video data to be out of synchronization. Another consequence of the real-time nature of compressed video decoding is that lost compressed data will not be re-transmitted. For these reasons, the additional computational load and time required for motion compensated re-quantization often limits its usage. Additionally, a higher computational load on the recoding processor implies that the processor must handle a bitstream of a lower bit rate when performing motion compensation based recoding as compared with re-quantization-only based recoding.

Traditionally, only one of the above re-quantization methods is used. Simpler re-quantization schemes may result in additional video quality loss. More complex schemes are often unsuitable or too costly for many transmission applications such as transmission of real time video. Therefore, there is a need for methods of transmitting digital video information over data channels that can match the bit rate to the channel bandwidth, without incurring an undesirable bit error rate or encountering excessive computational load, and also providing the best trade-off between video quality and processor load requirement.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies and limitations of the prior art with systems and methods for efficient bit rate alteration of a bitstream to match an available channel capacity. The efficient bit rate alteration includes selective re-quantization of the compressed bitstream. Selective re-quantization according to the present invention applies multiple re-quantization schemes to different portions of a bitstream. In one embodiment, the multiple re-quantization schemes each have a different computational load. By selectively choosing which type of re-quantization is performed on each portion, efficient bandwidth scaling and data transmission may be achieved both when computational capacity is limited and when video data integrity is important.

In one aspect, the present invention relates to a method for converting the bit rate of a compressed bitstream to use an available bandwidth of a channel. The method comprises re-quantizing a first portion of the bitstream containing video data using a first re-quantization scheme. The method also comprises re-quantizing a second portion of the bitstream containing video data using a second re-quantization scheme.

In another aspect, the present invention relates to a method for converting the bit rate of a compressed bitstream to use an available bandwidth of a channel. The method comprises selectively re-quantizing a portion of the bitstream according to one of two re-quantization schemes. The first re-quantization scheme comprises variable length decoding the portion, inverse quantizing the portion, re-quantizing the portion with a different quantization step size and variable length encoding the portion. The second re-quantization scheme comprising motion compensated re-quantization of the portion.

In yet another aspect, the present invention relates to a network device for providing compressed video data onto a network. The network device comprises a re-quantization apparatus that receives a compressed video bitstream having a first bit rate and outputs the compressed video bitstream having a second bit rate. The re-quantization apparatus includes a first portion configured to receive a first portion of the compressed video bitstream and output the first portion after re-quantization by a first re-quantization scheme. The re-quantization apparatus also includes a second portion configured to receive a second portion of the compressed video bitstream and output the second portion after re-quantization by a second re-quantization scheme. The network device also comprises a transmitter configured to transmit the compressed video bitstream having the second bit rate.

In still another aspect, the present invention relates to a system for converting the bit rate of a compressed bitstream to use an available bandwidth of a channel. The system comprising means for re-quantizing a first portion of the bitstream containing video data using a first re-quantization scheme. The system also comprising means for re-quantizing a second portion of the bitstream containing video data using a second re-quantization scheme.

In yet another aspect, the present invention relates to a computer readable medium including instructions for converting the bit rate of a compressed bitstream to use an available bandwidth of a channel. The instructions comprising instructions for re-quantizing a first portion of the bitstream containing video data using a first re-quantization scheme. The instructions also comprising instructions for re-quantizing a second portion of the bitstream containing video data using a second re-quantization scheme.

These and other features and advantages of the present invention will be described in the following description of the invention and associated Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for transmitting a bitstream having compressed video data at multiple bit rates over multiple channels in accordance with one embodiment of the present invention.

FIG. 2A is a high level block diagram of the network device ofFIG. 1 for converting the bit rate of a compressed bitstream using multiple quantization schemes in accordance with one embodiment of the present invention.

FIG. 2B illustrates the network device ofFIG. 1 in accordance with another embodiment of the present invention.

FIG. 3 shows a high level illustration of a transmission bitstream having an MPEG-2 format.

FIG. 4A illustrates an exemplary block diagram of the processing required for a basic re-quantization scheme.

FIG. 4B illustrates an exemplary block diagram of the processing required for motion compensated re-quantization with motion vector re-use.

FIG. 4C illustrates an exemplary block diagram of the processing required for motion compensated re-quantization with motion estimation.

FIG. 4D illustrates a process flow for selective re-quantization in accordance with one embodiment of the present invention.

FIG. 5A illustrates an exemplary portion of a compressed bitstream to be re-quantized by the present invention.

FIG. 5B illustrates a process flow for selective re-quantization of the portion ofFIG. 5A in accordance with a specific embodiment of the present invention.

FIG. 5C illustrates a process flow for selective re-quantization of a portion of a compressed video data including color and brightness video data in accordance with a specific embodiment of the present invention.

FIG. 5D illustrates a table that illustrates combinations of the process flows ofFIGS. 5B and 5C in accordance with a specific embodiment of the present invention.

FIG. 6 illustrates a high-level block diagram of an exemplary general-purpose router suitable for implementing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In some instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention.

In some multimedia delivery systems, compressed video programs are delivered to numerous digital receiver/decoders via one or more digital transmission channels. In such multi-cast situations, the same compressed video bitstream, which has a predetermined bit rate R, must be delivered to different end receiver/decoders via different channels. Some of the channels may not have a capacity sufficient to transmit the digital data at the requested bit rate R. In these cases, the present invention applies a selective re-quantization process to the originally compressed bitstream so that the resulting bit rate is no more than the channel capacity. Selective re-quantization results in a reduction of the bit usage required to represent compressed video images. This is due to the fact that when the re-quantization is performed, the quantization step value may be increased for various portions of the bitstream. As a result, the re-quantized data contains less information, which can be compressed with fewer bits.

The invention also relates to a set of network devices. Each of the network devices is adapted to flexibly adapt an incoming compressed bitstream to a compressed bitstream having a bit rate that is appropriate for a channel that the network device transmits onto. For example, if a second channel has a channel capacity R1, which is less than R, the capacity of a first channel, the network device converts an original compressed bitstream to have a bit rate that matches the channel capacity, namely R1. Similarly, if a third channel has a capacity R2, where R2 is less than R but greater than or equal to R1, the network device converts the original compressed bitstream to have a rate that matches the third channel capacity, namely R2. Finally, if a fourth channel has a capacity R3, and R3 is equal to R, then the network device simply passes the original compressed bitstream through the channel.

FIG. 1 illustrates asystem10 for transmitting a bitstream having compressed video data at multiple bit rates over

multiple channels

17,18 and19 in accordance with one embodiment of the present invention. Thechannel17 comprises anencoder14, atransmitter16, areceiver20, a medium15 coupling thetransmitter16 to thereceiver20, anetwork device22 and adecoder24. Theencoder14 receives an input bitstream and compresses the input bitstream to provide a compressed bitstream. The compressed bitstream is then received by thetransmitter16 and output over the medium15 at a first bit rate. In a specific embodiment, the medium is a satellite based communication system where thetransmitter16 transmits to a satellite that bounces the signal back to satellite receivers that act as thereceiver20. In another embodiment, thetransmitter16 is any one of a variety of those known in the art for DSL networks or ATM networks.

The compressed bitstream sent over the medium15 is received by thereceiver20 and input to thenetwork device22. Thenetwork device22 is a bit rate converter that changes the bit rate of the compressed bitstream according to the available bandwidth on alocal network23 between thenetwork device22 and thedecoder24. Thenetwork device22 changes the bit rate by selective re-quantization of video data in the bitstream. Thenetwork device22 then sends the re-quantized compressed bitstream to thedecoder24 over thelocal network23. In one embodiment, thenetwork device22 is a headend and thelocal network23 corresponds to a geographic service area of a cable network serviced by the headed. Thedecoder24 decompresses the compressed bitstream.

Thelocal network23 is included in alarger network26 that also includes

local networks

27 and29.Channel18 comprises theencoder14, thetransmitter16, the medium15, thereceiver20, thenetwork device22, thelocal network27 and adecoder30 for transmitting data from theencoder14 to thedecoder30.Channel19 comprises theencoder14, thetransmitter16, the medium15, thereceiver20, thenetwork device22, thelocal network29 and adecoder36 for transmitting data from theencoder14 to thedecoder36. Each of the

local networks

23,27, and29 has a different available bandwidth. The available bandwidth determines the maximum bit rate that may be used between thenetwork device22 and each

respective decoders

24,30 and36. By way of example, thelocal network23 has an available bandwidth that allows a bit rate of 2.5 million bits per second (Mbps), thelocal network27 has an available bandwidth that allows a bit rate of 1.5 Mbps, and thelocal network29 has an available bandwidth that allows a bit rate of 2.0 Mbps. The present invention allows the compressed video stream produced by theencoder14 to be sent across any of the

Thenetwork device22 may alter the modified compressed bitstream provided by theencoder14 to fit the available bandwidth provided by the

local networks

23,27 and29. Thenetwork device22 may alter the modified compressed bitstream by selectively re-quantizing video data in different portions of the incoming compressed bitstream. It does so using multiple re-quantization schemes and in a timely manner that does not sacrifice real-time transmission of the video data. By way of example, theencoder14 produces a compressed bitstream that requires a bit rate of 3.0 Mbps. Thelocal network23 has an available bandwidth that allows a bit rate of 2.5 million bits per second (Mbps) and thenetwork device22 performs selective re-quantization using multiple re-quantization schemes to change the bit rate of the compressed bitstream to 2 Mbps. Thelocal network27 has an available bandwidth that allows a bit rate of 1.5 Mbps and thenetwork device22 performs applies a simplified re-quantization scheme on some portions of the bitstream and a computationally more expensive re-quantization scheme on other portions of the bitstream to change the bit rate to fit the available bandwidth of thelocal network27. Thelocal network29 has an available bandwidth that allows a bit rate of 2.0 Mbps and thenetwork device22 performs applies selective re-quantization using multiple re-quantization schemes to change the bit rate of the compressed bitstream to 2.0 Mbps. In some cases, the available bandwidth on the

channels

17,18 and19 may vary over time and thenetwork device22 adapts accordingly. Thenetwork device22 also provides bandwidth sharing between the output compressed bitstream and other video and data bitstreams.

Thesystem10 is suitable for real-time bit rate alteration and real-time decoded playback at a different location. The decoded real-time playback may be done according to the NTSC standard and PAL standard, for example. While the present invention will now be described in the context of cable networks, IP networks, ATM networks, Digital Subscriber Line networks (DSL) such as xDSL, ADSL, HDSL, VDSL, and SDSL, those skilled in the art will realize that the present invention is applicable to other types of communication channels.

Asymmetric Digital Subscriber Loop, or ADSL, is a physical line code modulation scheme for transmitting digital bitstream over a single pair of plain old telephone system (POTS) grade twisted copper wires, that are usually noisy and are subject to outside interference. Several schemes have been developed to achieve high channel capacity of the twisted copper wires. xDSL systems provide simplex or full duplex raw bit pipes between the Central Office (CO) and the remote site receivers. The material content and the format of the bitstream or the protocols used by the connections being established by the bit pipe is immaterial to the xDSL system. In ADSL, the downstream link, from central office (CO) to remote site, typically has a higher bandwidth than the upstream direction. Downstream channel capacity is typically up to 8 Mbps while the upstream channel capacity is up to about 1 Mbps. The actual channel capacity may also depend on the noise level and the distance between the transmitter and the receiver.

Because broadcast video quality can be achieved with compressed video, such as MPEG-2, at 3-6 Mbps, ADSL provides a useful delivery scheme for compressed digital video and other high speed data connecting COs with remote sites such as consumer subscribers or business establishments. However, because of the variation in physical line conditions mentioned above, ADSL transmission schemes do not specify channel capacity in either direction. Alternatively, the channel capacity is determined at connection set up time via channel initialization and training by the transceivers at the CO location and at the remote locations. The initialization and training process determines the proper coding configuration best matched to the current channel condition in order to achieve the maximum channel capacity. During the connection, due to a change in line condition or due to loss of data, the transceivers may also re-initialize and retrain to settle on a new coding configuration.

FIG. 2A is a high level block diagram of thenetwork device22 for converting the bit rate of a compressed bitstream using selective re-quantization in accordance with one embodiment of the present invention. Thenetwork device22 adjusts the bit rate of an input compressedbitstream88 including video data to match the bandwidth capacity of a channel that it transmits onto. The output compressed bitstream of thenetwork device22 is either at the original bit rate or at a lower bit rate. Thenetwork device22 comprises areceiver82, are-quantization apparatus84 and atransmitter86. Thereceiver82 receives thebitstream88 from a network channel. An output of thereceiver82 is coupled to there-quantization apparatus84.

There-quantization apparatus84 converts thecompressed video stream88 to anoutput bitstream87 using multiple re-quantization schemes. There-quantization apparatus84 may be a single structure or multiple structures. In one embodiment, there-quantization apparatus84 is implemented in software and the software is capable of flexibly applying multiple re-quantization schemes. Afirst portion84aof there-quantization apparatus84 re-quantizes a first portion of the compressedbitstream88 using a first re-quantization scheme. Asecond portion84bof there-quantization apparatus84 re-quantizes a second portion of the compressedbitstream88 using a second re-quantization scheme. In one embodiment, the second re-quantization scheme is computationally more demanding than the first re-quantization scheme. In this case, thefirst portion84ais included within thesecond portion84b. In another embodiment, there-quantization apparatus84 consists of separate and discrete hardware portions each dedicated to a single re-quantization scheme.

An output of there-quantization apparatus84 is coupled to the input of thetransmitter86. Thetransmitter86 transmits the compressed bitstream produced by there-quantization apparatus84 onto achannel89. As illustrated inFIG. 2A, there-quantization apparatus84 and thetransmitter86 are separate devices or performed in software separate modules. In another embodiment, there-quantization apparatus84 andtransmitter86 are combined in a single device or software module.

Thechannel89 may correspond to any one of a number of conventional transmission systems, including but not limited to xDSL, ATM/ADSL, ATM, ISDN links, Ethernets, public data networks, T1, T3, DS-3, OC-3, wireless/terrestrial networks, digital satellites, digital cable networks, etc.

The present invention describes bit rate alteration methods applied onto compressed video bitstreams by performing selective re-quantization. Although the remaining discussion will focus primarily on re-quantization of an MPEG-2 transmission bitstream, the present invention is not limited to re-quantizing a bitstream of MPEG, or any other specific format. In other words, re-quantization in accordance with the present invention may be implemented in any public or proprietary compression format including quantization as a compression step as one skilled in the art will appreciate. Examples of such compression formats include MPEG-1, MPEG-2, MPEG-4, H.261, H.262, H.263, H.263+, Microsoft streaming format, QuickTime, and RealNetworks.

FIG. 3 illustrates atransmission bitstream60 having an MPEG-2 format in accordance with one embodiment of the present invention. The MPEG-2 compression standard consists of two layers: asystem layer61 anelementary stream layer62. Thesystem layer61 comprises two sub layers: a packetized elementary stream (PES)layer64 and apacket layer65 above thePES layer64. Thepacket layer65 can be either atransport stream66 or aprogram stream68. For compressed video and audio data, the data follows a hierarchical structure, namely, the elementary stream is contained in the PES payload, and the PES packets are contained in the packet layer payload.

Theelementary stream layer62 typically contains the coded video and audio data. It also defines how compressed video (or audio) data are sampled, motion compensated (for video), transform coded, quantized and represented by different variable length coding (VLC) tables.

The basic structure for a coded video picture data is a block that is an 8 pixel by 8 pixel array. Multiple blocks form a macroblock, which in turn forms part of a slice. A coded picture consists of multiple slices. Multiple coded pictures form a group of pictures. Such hierarchical layering of data structures localizes the most basic processing on the lowest layer, namely blocks and macroblocks. In one embodiment, processing in accordance with the present invention occurs on this lowest level. For example, motion compensation re-quantizing of a precompressed MPEG-2 bitstream is a process that occurs on theelementary stream layer62.

Each block contains variable length codes (VLC) for DCT coefficients. In the MPEG-2 syntax, the picture data section contains the bulk of the compressed video images. This is where the DCT coefficients are encoded as VLCs. For a typical bitstream, this portion of the data takes somewhere between 70%-90% of the total bit usage of a coded picture, depending on the coded bit rate.

The access unit level information relates to coded pictures and may specify whether a picture is an intra frame (I frame), a predicted frame (P frame), or a bi-directional frame (B frame). An I frame contains full picture information. A P frame is constructed using a past I frame or P frame. A bi-directional frame (B frame) is bi-directionally constructed using both a past and a future I or P frame, which are also called anchor frames.

Referring back toFIG. 3, the next layer is thesystem layer61. Thesystem layer61 is defined to allow an MPEG-2 decoder to correctly decode audio and video data, and present the decoded result to the video screen in time continuous manner. Thesystem layer61 consists of two sublayers. The first sublayer in thesystem layer61 is thePES layer64. ThePES layer64 defines how the elementary stream layer is encapsulated into variable length packets called PES packets. In addition, thePES layer64 may include presentation and decoding timestamps for the PES packets, which are used by a decoder to determine the timing to decode and display the video images from the decoding buffers.

Thetransport layer65 defines how the PES packets are further packetized into fixed sized transport packets, e.g. packets of 188 bytes to produce a transport stream. Additional timing information and multiplexing information may be added by thetransport layer65. Thetransport stream66 is optimized for use in environments where errors are likely such as transmission in a lossy or noisy media. Applications using thetransport stream66 include Direct Broadcast Service (DBS), digital or wireless cable services, broadband transmission systems, etc. Theprogram stream68 defines how the PES packets are encapsulated into variable sized packets and may also include additional timing in multiplexing information. Theprogram stream68 is designated for use in relatively error free environments and is suitable for applications that may involve software processing of system information such as interactive multimedia applications. Applications ofprogram stream68 include Digital Versatile Disks (DVD) and video servers.

Video data can be contained in the elementary stream (ES), which means that no PES, transport or program system layer information is added to the bitstream. The video data can also be contained in thePES stream64,transport stream66 orprogram stream68. For a given video bitstream, the difference between these different layers lies in the timing information, multiplexing information and other information not directly related to the re-encoding process. In one embodiment, the information required to perform re-encoding is contained in the elementary stream layer. However, the present invention is not limited to bitstreams in the elementary stream layer. In other words, the present invention can be extended to the PES stream, transport stream or program stream as one of skill in the art will appreciate.

The structure of the MPEG standard is well known to one of skill in the art and described by the MPEG standards. The present invention is suitable for use with the MPEG-1 specification, described in ISO/IEC International Standard 11172; “Coding of moving pictures and associated audio for digital storage media up to about 1.5 Mbits/s”, November 1993, which is herein incorporated by reference. The present invention is also suitable for use with the MPEG-2 specification, described in ISO/IEC International Standard 13818; “Generic coding of moving pictures and associated audio information”, November 1994, which is herein incorporated by reference. The present invention is also suitable for use with the MPEG-4 specification, described in ISO/IEC International Standard; “Coding of moving pictures and associated audio information”, March 2000, which is herein incorporated by reference.

Having briefly discussed an exemplary system for transmitting compressed video data as well the MPEG syntax, thenetwork device22 ofFIG. 1 will be described in accordance with another embodiment of the present invention.

Referring now toFIG. 2B, a second embodiment of anetwork device22 ofFIG. 1 is illustrated in accordance with another embodiment of the present invention. Thenetwork device22 uses selective re-quantization to alter video data in a compressed bitstream to provide a new compressed bitstream having a new bit rate. ForFIG. 2B, the data and bit rate conversion process is intended to remove bits from the modified compressed bitstream so that the new bitstream is still compliant to the given compression syntax, and can be decoded by the receiver, albeit at a possibly lower quality than the originally compressed bitstream. In some cases, the new compressed bitstream may be further multiplexed with other bitstreams to share the same channel capacity.

The second embodiment of the bitrate conversion device22 is adapted for use on a MPEG-2 transport stream and comprises aseparation unit500, a de-multiplexer502, anelementary stream parser504, aDCT VLC decoder506, a motionvector VLC decoder508, arate controller512, are-quantization apparatus514, aDCT VLC encoder520, a motionvector VLC encoder522, abitstream multiplexer526, and atransport multiplexer528. While the present invention will now be described as an apparatus composed of units, those skilled in the area will recognize that the present invention encompasses a method, process or software having as steps the actions performed by each unit and described below.

Theseparation unit500 has an input and a plurality of outputs. The input of theseparation unit500 is coupled toline501 to receive an input compressed digital video bitstream. Theseparation unit500 de-multiplexes the system layer stream, removing filler packets/frames as appropriate, to obtain a video stream, an audio stream, and a data stream. The video bitstream is provided on a first output coupled toline511. The audio stream and data stream are output directly to thetransport multiplexer528 which recombines these bitstreams with a rate convertedvideo stream527.

The de-multiplexer502 is coupled to receive the video stream from theseparation unit500. The de-multiplexer502 extracts a video elementary stream payload from the video stream and in turn sends the video elementary stream payload to the elementarystream bit parser504. The elementarystream bit parser504 receives the output of the de-multiplexer502 and divides it into a transform coefficient component and a motion vector component. Each of these components is output to a

respective decoder

506 and508. More specifically, the discrete cosine transform (DCT) variable length coding (VLC)decoder506 is provided to decompress the transform coefficient component while the motion vector (MV) variable length coding (VLC)decoder508 is provided to decompress the motion vector component.

The present invention also provides therate controller512 to control the bit rate at which thetransport multiplexer528 outputs data. In one embodiment, therate controller512 determines the bit usage for each video frame so that the resulting output bitstream maintains a desired bit rate profile. Therate controller512 is coupled to receive commands that specify the bit rate for the output stream, and therefore, the rate by which there-quantization apparatus514 andencoder components520 and524 alter the data bitstreams. For example, when the rate controller is used in the context of RADSL, the input to therate controller512 is coupled toline532 which in turn is coupled to receive feedback from an RADSL transmitter or ADSL modem to control the target output bit rate. In such an arrangement, the signal provided online532 is a value or rate for there-quantization apparatus514 to output data. Therate controller512 is shown diagrammatically inFIG. 2B as being coupled to there-quantization apparatus514 bycontrol signal line530. While therate controller512 has been described as being a discrete device, those skilled in the art will realize that therate controller512 could be software that provides a control signal to there-quantization apparatus514.

Therate controller512, and

decoders

506 and508, are coupled to there-quantization apparatus514. There-quantization apparatus514 is used to modify portions of the bitstream received atline501. The modification of the these portions comprises flexibly applying multiple re-quantization schemes to change the quantization step size of the video images. In one embodiment, the modification of the encoded bitstream comprises changing the quantization step size on a frame by frame basis. The particular quantization step size used may be determined by the control signal from therate controller512. The present invention performs rate control by adjusting the quantization step size of video data in the video stream using multiple re-quantization schemes. In many cases, one re-quantization scheme is used for some frames an another re-quantization scheme for other frames. In a specific embodiment, the present invention also performs rate control by adjusting the resolution of the video data. Resolution alteration of the video data typically requires complete decoding to a raw video image before encoding back into a compressed bitstream and may include additional hardware and software not illustrated inFIG. 2B. For example, frame buffers may be included to store images while motion vectors are generated. The performance of there-quantization apparatus514 will be described in further detail with respect toFIGS. 4A-4C.

The outputs of thedownconverter514 are in turn coupled to respectively to theDCT VLC encoder520 and the motionvector VLC encoder522. Each of the

encoders

520 and522 code the re-quantized video data back into the compressed format. In one embodiment, theencoder520 performs DCT, motion residual calculation via motion compensation, variable length coding of all the bits. The outputs of the

encoders

520 and522 are then combined bystream bit multiplexer526, and then again combined with the audio and data by thetransport multiplexer528. The output of themultiplexer528 provides asignal529 that is converted to match the bit rate of a channel that thenetwork device22 transmits onto. The output compressedbitstream529 is compliant to the MPEG standard. Although the discussion so far has focused on video data in the transport stream, one of skill in the art will appreciate that thebit rate converter22 is capable of handling the program stream as well.

Having briefly discussed the MPEG syntax, two exemplary embodiments of thenetwork device22, selective re-quantization of compressed data in accordance with one aspect of the present invention will now be expanded upon.

Upon receiving a compressed bitstream at a headend or any other suitable network device, the network device applies selective re-quantization and bit rate alteration. In general, the selective re-quantization comprises re-quantizing a first portion of the bitstream containing video data using a first re-quantization scheme and re-quantizing a second portion of the bitstream containing video data using a second re-quantization scheme. In one embodiment, the present invention uses a combination of a computationally varying re-quantization schemes. More specifically, the present invention may selectively apply a computationally less demanding re-quantization scheme and a computationally more demanding re-quantization scheme. The selective re-quantization may then bypass some of the computationally expensive steps in the more demanding re-quantization scheme when advantageous.

In one embodiment, selective re-quantization combines basic re-quantization and a more computationally demanding re-quantization scheme including motion compensation re-encoding. As mentioned before, motion compensated re-quantization is generally more computationally demanding than basic re-quantization.FIGS. 4A-D illustrate exemplary re-quantization schemes suitable for use with the present invention. The re-quantization schemes may take place in thenetwork device22 ofFIG. 1. Processes in accordance with the present invention may include up to several additional steps not described or illustrated here in order not to obscure the present invention. While re-quantization will now be described as a method, those skilled in the area will recognize that the present invention encompasses a system or software having units capable of performing the actions as described below.

FIG. 4A illustrates a block diagram of the processing required for abasic re-quantization scheme100.Basic re-quantization100 begins by receiving a portion of a compressed bitstream having video data (101). The video data is then re-encoded. As the term is used herein, re-encoding refers to the process of performing at least partial decoding and subsequent encoding on a compressed bitstream. Partial decoding of the video data includesVLC decoding102 andde-quantization104. Re-encoding then comprisesre-quantization106 andVLC encoding108. For bit rate reduction of the video data,re-quantization106 is performed with a larger quantization step value. As one of skill in the art will appreciate, the exact quantization step value will depend on a number of factors such as the channel capacity, bit rate reduction ratio, picture type, original quantization step size, video resolution, general scene activity measures (e.g., a scene cut, fade, dissolves, wipes, rapid motion, panning), etc. The motion vectors and mode info are then encoded back into the compressed domain for transmission by the network device (110).

FIG. 4B illustrates a block diagram of the processing required for re-quantization via one type of motioncompensation re-quantization scheme120 and motion vector re-use. Recoding in this case begins by receiving compressed video data (121). The video data is then decoded comprisingvariable length decoding122,de-quantization124,inverse transform coding126 andmotion compensation128.Motion compensation126 includes an iterative process where I, P and B frames are reconstructed using aframestore memory130. Encoding includes processing the video data withtransform coding132,re-quantization134, andVLC encoding136. For bit rate reduction of the video data,re-quantization134 is performed with a larger quantization step value.

Aftertransform coding132 andre-quantization134, each image is decoded comprisingde-quantization138 andinverse transform coding140 beforemotion compensation142 with the motion vectors and mode info provided byVLD122.Motion compensation142 includes an iterative process where I, P and B frames are reconstructed using aframestore memory144.Motion compensation142 produces a predicted picture that is summed146 with the nextdecoded picture148 and encoded bytransform coding132,re-quantization134, andVLC encoding136. This iterative process ofmotion compensation142 with motion vector re-use and re-encoding (132,134 and136) produces compressedvideo data150 having a lower bit rate than received (121). The re-quantizedcompressed video data150 may then be combined with other re-quantized compressed video data and transmitted onto a channel. Drift in the reference I and P pictures is represented by the difference between the two

framestore memories

130 and144 due to re-quantization. The motioncompensation re-quantization scheme120 takes into account the drift in generating thevideo data150, and hence provides better video quality thanscheme100, particularly when the bit rate reduction ratio is large.

FIG. 4C illustrates an exemplary block diagram of the processing required for re-quantization viacomplete recoding160. Recoding in this case begins by receiving compressed video data (161). The video data is then decoded comprisingvariable length decoding162,de-quantization164,inverse transform coding166 andmotion compensation168.Motion compensation166 includes an iterative process where I, P and B frames are reconstructed using a framestore memory170. Recoding includes processing the video data withtransform coding172,re-quantization174, andVLC encoding176. For bit rate reduction of the video data,re-quantization174 is performed with a larger quantization step value.

Aftertransform coding172 andre-quantization174, each image is decoded comprisingde-quantization178 andinverse transform coding180 beforemotion compensation182 with motion vectors provided bymotion estimation186.Motion estimation186 is applied to generate motion vectors on a frame by frame basis.Motion compensation182 includes an iterative process where I, P and B frames are reconstructed using aframestore memory184.Motion compensation182 produces a predicted picture that is summed186 with the nextdecoded picture188 and encoded bytransform coding172,re-quantization174, andVLC encoding176. This iterative process ofmotion compensation182 including generation of motion vectors bymotion estimation186 produces compressedvideo data190 having a lower bit rate than received (161). The re-quantizedcompressed video data190 may then be combined with other re-quantized compressed video data and transmitted onto a channel. There-quantization scheme160 is advantageous if the resolution of the video data is also to be changed, e.g., to further reduce the bit rate.

FIG. 4C represents a complete re-encoding, i.e., decoding followed encoding. In this case, motion estimation is required because motion vectors are not re-used from the input. This approach is generally significantly more computationally complex since motion estimation is much more computationally intensive than the re-quantization only and motion compensation operations. Generally, the benefit of such complete re-encoding is not significant when the bit rate reduction ratio is minor, i.e., not greater than 80%. One benefit of complete re-encoding is that it allows a resolution change of the video data. For example, a full D-1 resolution video, when compressed to, say, 6 Mbps, can be re-encoded into a SIF resolution at, say, 800 Kbps, and still maintain acceptable video quality.

In one embodiment, the portions of a compressed bitstream that are selectively re-quantized correspond to individual frames. As mentioned before, an MPEG bitstream comprises I, P and B frames.

FIG. 5A illustrates anexemplary frame sequence300 included in a compressed bitstream to be re-quantized by the present invention. Thesequence300 corresponds to a group of pictures in an MPEG-2 bitstream. Thesequence300 includes aninitial I frame302, P frames304a-dand ten B frames306a-j. TheI frame302 contains full picture information. The P and B frames are constructed from other frames as illustrated byarrows308. Each P frame304a-cis constructed using theI frame302 or a previous P frame304 ac, whichever immediately precedes the P frame (e.g., theP frame304buses theP frame304a). The B frames306a-jare bi-directionally constructed using the nearest past and future reference picture. A reference picture is either an I or a P picture. For example, the B frames306aand306bare constructed using the past I frame302 andfuture P frame304a.

Because of the construction relationships of the frames in thesequence300, errors in one frame may propagate to another. More specifically, re-quantization errors in an I frame may propagate to P and B frames constructed using the I frame. Re-quantization errors in a P frame may propagate to B frames and subsequent P frames produced using the P frame. The re-quantization error may also cumulatively propagate, e.g. from I frame302 to theP frame304ato theP frame304bto theB frame306e. As a result of this error propagation, re-quantization errors typically have more effect on video quality for P frames than B frames. In addition, re-quantization errors typically have more effect on video quality for I frames than P frames.

In one embodiment, the selective re-quantization methods of the present invention consider the amount of bit rate reduction required and the available computation capacity to process the bitstream. If the amount of bit rate reduction is small, then the basic re-quantization scheme100 (FIG. 4A) may be used for all coded picture types. If the amount of bit rate reduction is large and there is an abundant amount of computational capacity, then re-quantization via motion compensation recoding120 ofFIG. 4B may be used for all coded picture types. As each frame is rebuilt individually, errors will not propagate to the P and B frames and error propagation is minimized—at the cost of the increased computational load. If the amount of bit rate reduction for the compressed bitstream is significant and computational capacity is limited, a more selective method of re-quantization may be used.

FIG. 5B illustrates aprocess flow320 for selective re-quantization of theframe sequence300 in accordance with a specific embodiment of the present invention. For each frame in the compressed bitstream, theprocess flow320 determines which type of re-quantization is to be used (324). The process flow320 uses two different re-quantization schemes, each having a different computational load.

The process flow320 selectively re-quantizes B frames206a-jusing a simple re-quantization scheme such as thebasic re-quantization scheme100 ofFIG. 4A (326). Thebasic re-quantization100 does not impose a large processing load for re-quantizing the B frames206a-j. The P frames204a-dare selectively re-quantized based on position of each P frame within its group of picture (GOP). Due to the sensitivity of error propagation, the P frames204a-care re-quantized using the re-quantization via motion compensation recoding120 with motion vector re-use ofFIG. 4B (328). The re-quantization via motion compensation recoding120 ofFIG. 4B is more computationally demanding and imposes a large processing load for re-quantizing the video data than the basic re-quantization scheme (328), but introduces less error in the P frames204a-cthat can be subsequently propagated to other frames. Since any re-quantization from the P frame204dwill only propagate into the B frame206i, the P frame204dis re-quantized using the basic re-quantization scheme100 (FIG. 4A). In a more aggressive re-quantization to save computational load, additional P frames are re-quantized using thebasic re-quantization scheme100, e.g. the last two or three P frames in a sequence. TheI frame202 contains no motion vectors and is re-quantized using thebasic re-quantization scheme100 ofFIG. 4A (326). A smaller quantization step size may be used in this case to maintain video data quality in theI frame202. The selection process (324) proceeds for each frame of the compressed bitstream that was received (330). The re-quantized I frame may also be stored in the frame store for the future P and B frames to use as a reference frame.

When desirable, theprocess flow320 selectively avoids computationally demanding re-quantization schemes at the remote network devices and headends and may use a less computationally demanding re-quantization scheme. More specifically, theprocess flow320 avoids motion compensation recoding for less impacted portions of the bitstream while minimizing the resulting loss in quality for the entire bitstream. Motion compensation of video images and data is typically the most computationally expensive process for encoding. As the B frames may comprise up to 70 percent of many MPEG-2 compressed bitstreams, the selective methods ofprocess flow320 may result in considerable computational savings. For a transmission system having numerous remote network devices (e.g. hundreds of headends), this results in considerable computational savings for the system. Alternatively, it also reduces the computational load for a single network device having to perform re-quantization for a large number of compressed video bitstreams. This may result in improved computational efficiency at the remote network devices and may increase processing speed of some compressed bitstreams in the range of 30-40 percent (depending on video content).

In another embodiment, the selective re-quantization methods of the present invention consider the type of video data. Human vision is more sensitive to brightness than color. In other words, human vision is less sensitive to the picture quality degradation of chroma (color) information than luma (brightness) information. As a result, the present invention may selectively apply different re-quantization schemes to chroma and luma video data in a compressed video bitstream.

If the amount of bit rate reduction for the compressed bitstream is small, then the basic re-quantization scheme100 (FIG. 4A) may be used for both chroma and luma compressed data. If the amount of bit rate reduction for the compressed bitstream is significant, a more selective method of re-quantization may be used.

FIG. 5C illustrates aprocess flow340 for selective re-quantization of a frame sequence including compressed chroma and luma video data in accordance with a specific embodiment of the present invention. For each frame in the compressed bitstream, theprocess flow320 determines which type of re-quantization is to be used (344). The process flow340 uses two different re-quantization schemes of different computational load.

The process flow340 selectively re-quantizes chroma information using a simple re-quantization scheme such as thebasic re-quantization scheme100 ofFIG. 4A (346). Thebasic re-quantization100 does not impose a large processing load for re-quantizing the chroma information. The luma information is selectively re-quantized using the re-quantization via motion compensation recoding120 with motion vector re-use ofFIG. 4B (348). The re-quantization via motion compensation recoding120 ofFIG. 4B is more computationally demanding and imposes a large processing load for re-quantizing the luma video data than the first re-quantization scheme, but introduces less error in the luma information that may be subsequently detected by human vision. The selection process (344) proceeds for each frame of the compressed bitstream that was received (350).

The selective re-quantization methods of the present invention may combine re-quantization criteria. In a specific embodiment, the process flows320 and340 are combined. As one of skill in the art will appreciate, there are numerous ways to combine the process flows320 and340.FIG. 5D illustrates a table360 that illustratescombinations362 of the process flows320 and340 in accordance with one embodiment of the present invention. Thecombinations362 are ranked according to computational load of re-quantization. A higher ranking364 (lower number) refers to computationally more demanding re-quantization and, typically, a higher quality video image. Of course, the ranking may differ based on the content of the compressed bitstream. For example, a high motion black and white film may have much more data in the P and B frames relative to the chroma. In this case, re-quantization of the P and B frames will be more computationally expensive. It is noted that the selection of re-quantization schemes may vary for a single bitstream. For example,combination1 may be used initially on a compressed bitstream untilcombination5 is applied, e.g., when a network device must suddenly process a large number of bitstreams.

In another embodiment, a third re-quantization scheme is used such as complete re-encoding. When a re-quantization scheme including motion compensation is used, the re-quantized I frame is stored in a frame buffer. However, an I frame does not include a motion compensation process by itself since the I frame does not contain motion vectors. The use of a re-quantized I frame is for reconstruction of one or more subsequent P and B frames. Specifically, P and B frames use the stored I frame, along with the coded residual DCT coefficients and motion vectors to form the re-coded P or B frame. The re-quantized I frame is then added to the motion compensated motion residuals. Thus, in this case, a motion compensation operation is applied to I frame, albeit in a different manner than the P and B frames.

Other criteria may be used in determining which re-quantization scheme is used. In a specific embodiment, the present invention monitors the processing load of a CPU in a network device. Based on the CPU cycles, if the processor cannot keep up with a more demanding re-quantization scheme, than the network device applies a more aggressive application of the less computationally demanding re-quantization scheme (e.g., a lower ranking in the table360). As processor overload may lead to buffer underflow in compressed data transmission, which may then result in data transmission failure, monitoring the CPU in this manner may avoid data transmission failure.

Although the present invention describes bit rate alteration based on selective re-quantization, the present invention may also include bit rate alteration according to one or more additional bit rate alteration techniques. In one embodiment, the present invention relates to bit rate alteration methods that use a combination of bit rate alteration techniques. Several suitable techniques are described in commonly co-pending U.S. patent application entitled “A System and Method for Transporting Compressed Video and Data Bitstream Over a Communication Channel” by Ji Zhang et al., filed on Oct. 10, 1997 (U.S. application Ser. No. 08/947,480), which was previously incorporated by reference. Resolution conversion is a specific bit rate alteration scheme that is suitable for use with the present invention and further described in commonly co-pending U.S. patent application entitled “Methods and Apparatus for Bandwidth Scalable Transmission of Compressed Video Data Through Resolution Conversion” by Ji Zhang et al., filed on Jun. 30, 2000 (U.S. application Ser. No. 09/608,128), which is incorporated by reference herein for all purposes.

The methods of this present invention may be implemented on various systems. For example, the invention may be implemented on routers and/or switches. In a specific embodiment, the systems of this invention may be specially configured routers such as, for example, specially configured router models 1600, 2500, 2600, 3600, 4500, 4700, 7200, and 7500 available from Cisco Systems, Inc. of San Jose, Calif. A general architecture for some of these machines will be given below. In an alternative embodiment, the methods of this invention may be implemented on a video streaming server, or a general-purpose network host machine such as a personal computer or workstation. Further, the invention may be at least partially implemented on a card (e.g., an interface card) for a network device or a general-purpose computing device.

Referring now toFIG. 6, ageneral purpose router810 suitable for implementing the present invention includes a master central processing unit (CPU)862,interfaces868, and a bus815 (e.g., a PCI bus). When acting under the control of appropriate software or firmware, theCPU862 is responsible for such router tasks as routing table computations and network management. It preferably accomplishes all these functions under the control of software including an operating system (e.g., the Internetwork Operating System (IOS®) of Cisco Systems, Inc.) and any appropriate applications software.CPU862 may include one ormore processors863 such as a processor from the Motorola family of microprocessors or the MIPS family of microprocessors. In an alternative embodiment,processor863 is specially designed hardware for controlling the operations ofrouter810. In a preferred embodiment, a memory861 (such as non-volatile RAM and/or ROM) also forms part ofCPU862. However, there are many different ways in which memory could be coupled to the system.

Theinterfaces868 are typically provided as interface cards (sometimes referred to as “line cards”). Generally, they control the sending and receiving of data packets over the network and sometimes support other peripherals used with therouter810. Among the interfaces that may be provided are Ethernet interfaces, frame relay interfaces, cable interfaces, DSL interfaces, token ring interfaces, and the like. In addition, various very high-speed interfaces may be provided such as fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces and the like. Generally, these interfaces may include ports appropriate for communication with the appropriate media. In some cases, they may also include an independent processor and, in some instances, volatile RAM. The independent processors may control such communications intensive tasks as packet switching, media control and management. By providing separate processors for the communications intensive tasks, these interfaces allow themaster microprocessor862 to efficiently perform routing computations, network diagnostics, security functions, etc.

In one embodiment, the network device276 shown inFIG. 2B is implemented as a video bit rate conversion line card. The line card behaves as video network device without any physical interface. In some cases, it may include an appropriate interface for digital video such as ASI and DHEI. The line card may also include multiple on-board video processors for bit rate conversion. While the video processors perform the typical IP routing functions and decide the network loading, these video processors may also change the bit rate for each video channel dynamically, based on the network loading statistics. In another embodiment, a video processing line card includes a physical interface to twisted pair for xDSL output or a physical interface to coaxial cable for QAM or QPSK output. QAM/QPSK format may be converted into satellite or wireless signal, one-way or two-way.

Although the system shown inFIG. 6 is one specific router of the present invention, it is by no means the only router architecture on which the present invention can be implemented. For example, an architecture having a single processor that handles communications as well as routing computations, etc. would also be acceptable. Further, other types of interfaces and media could also be used with the router.

Regardless of network device's configuration (for cable plants or otherwise), it may employ one or more memories or memory modules (e.g., memory861) configured to store program instructions for the network operations and other functions of the present invention described herein. The program instructions may specify an operating system and one or more applications, for example. Such memory or memories may also be configured to store data bitstreams, data structures or other specific non-program information described herein.

Because such information and program instructions may be employed to implement the systems/methods described herein, the present invention relates to machine readable media that include program instructions, state information, etc. for performing various operations described herein. Examples of machine-readable media include, but are not limited to, magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM). The invention may also be embodied in a carrier wave travelling over an appropriate medium such as airwaves, optical lines, electric lines, etc. Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter.

The present invention is suitable for use on a variety of networks. In one embodiment, the present invention suitable toward the integration of bit rate conversion using resolution down conversion with an ADSL rate adaptation feature to achieve relatively lossless transmission. In this case, one element of the invention is to create a synergy between the bit rate conversion of compressed video bitstream and the use of RADSL to transport the compressed video bitstreams.

One particular communication channel that is becoming increasingly common is the cable modem channel, such as that defined by the DOCSIS cable modem standard by CableLabs and the DAVIC cable modem standard by DVB. In DOCSIS based cable data networks, operators and digital service providers use the radio frequency spectrum in the coaxial cable network to deliver digital data signal in a 6 MHz frequency slot. The aggregate data bit rate capacity of the slot may be shared by multiple, possibly different, types of information services. Cable modem service is one such information service that allows service operators to provide a number of two-way interactive digital services to subscribers. One particular digital service delivers a compressed digital video signal for video streaming or video on demand. In this case, depending on the overall utilization of the channel capacity, the available bandwidth available to the compressed digital video signal may not be adequate. For example, the compressed digital video signal may require a constant bit rate of 8 Mbps, but the available bandwidth is only 6 Mbps, or even 500 Kbps. In some cases, service operators may deliberately limit the available bandwidth for certain video data based on a subscriber's level of premium service. For example, a popular movie title may require a per title premium charge to the subscriber, but a lower bit rate (thus lower quality) video of the same video content may be available for free to all subscribers. Thus, it is important for service providers in this case to handle compressed video data at multiple bit rates.

Yet another particular communication channel that is becoming increasingly common is IP networks and Asynchronous Transfer Mode (ATM) networks. Both networks are based on different transport protocols that can be used to transport data or multimedia bitstreams with a pre-specified quality of service. For example, ATM protocol specifies how data is first packetized into fixed sized data units, called cells. It also specifies how such a cell stream can be multiplexed, de-multiplexed, switched and routed between different locations to support end-to-end connections at given bit rate and/or quality of service (QOS). The present invention also includes the concept of traffic shaping, used on ATM networks to change the bit rate profile of ATM cell bitstreams through the bit rate conversion of compressed video data. In ATM networks, connections may be established with pre-determined grade of QOS. Conditions of network utilization, along with call admission control, sometimes may prevent a new connection being established at the given bit rate or given quality of service. In such cases, the requested connection may either have to be rejected or a new set of admissible connection parameters have to be negotiated between the network service provider and the user requesting the connection.

In another communication channel example, integrated services (IntServ), differential services (DiffServ), and Multiple Protocol Label Switch (MPLS) are used to establish a QOS for different network traffic. These services may use the Common Open Policy Service (COPS) for admission control. Internet Group Management Protocol (IGMP) and a variety of multicast routing protocols are also used to establish and tear down service on a multicast tree, which is commonly used for video transmission. In all these cases, the requested connection must provide the available bandwidth for the data being sent or the requested connection may either have to be rejected or a new set of admissible connection parameters may have to be negotiated.

While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention which have been omitted for brevity's sake. For example, the present invention may be used to perform bandwidth sharing, despite the fact that the available bandwidth from the transmission facility, which includes but is not limited to, xDSL, ATM, wireless channel, is sufficient to send the incoming compressed bitstream. In general, for data traffic, such as TCP/IP based traffic, the data bit rate cannot be determined. In addition, the video bitstream may not have constant bit rate, therefore, the resulting total bandwidth cannot be determined before the connection is established. The application of bit rate conversion through re-quantization, however, will ensure that the resulting total bandwidth will always be less than the total available channel bandwidth. It is therefore intended that the scope of the invention should be determined with reference to the appended claims.

Claims

1. A method for converting the bit rate of a compressed bitstream to use an available bandwidth of a channel, the method comprising: selectively re-quantizing a portion of the bitstream according to one of two re-quantization schemes, the first re-quantization scheme comprising variable length decoding the portion, inverse quantizing the portion, re-quantizing the portion with a different quantization step size and variable length encoding the portion, the second re-quantization scheme comprising motion compensated re-quantization of the portion.

2. The method ofclaim 1, wherein the portion is a frame of the compressed bitstream.

3. The method ofclaim 2, wherein the frame is re-quantized using the first re-quantization scheme when the frame is a B frame and the frame is re-quantized using the second re-quantization scheme when the frame is a P frame.

4. The method ofclaim 2, wherein the frame is re-quantized using the first re-quantization scheme when the frame includes chroma information and the frame is re-quantized using the second re-quantization scheme when includes luma information.

5. The method ofclaim 1, further including performing motion estimation to produce motion vectors for the motion compensated re-quantization.

6. A network device for providing compressed video data onto a network, the network device comprising: a re-quantization apparatus that receives a compressed video bitstream having a first bit rate and outputs the compressed video bitstream having a second bit rate, the re-quantization apparatus including a first portion configured to receive a first portion of the compressed video bitstream and output the first portion after re-quantization by a first re-quantization scheme, the re-quantization apparatus including a second portion configured to receive a second portion of the compressed video bitstream and output the second portion after re-quantization by a second re-quantization scheme; and a transmitter configured to transmit the compressed video bitstream having the second bit rate onto the network.

7. The network device ofclaim 6, further including a network interface configured to receive the compressed video bitstream having the first bit rate from the network.

8. The network device ofclaim 6, further including a rate controller coupled to the re-quantization apparatus.

9. The network device ofclaim 6, wherein the first portion of the re-quantization apparatus is included in the second portion of the re-quantization apparatus.

10. The network device ofclaim 6, further including a processor whose processing load at least partially determines which of the first portion and the second portion of the re-quantization apparatus is used.