DV is a video format used for digital video cameras. Unlike highCompression formats likeMPEG-2 andMPEG-4, DV uses only IntraframeCompression (compressing eachFrame individually), resulting in a format that's well suited to editing as eachFrame can be decoded with no information from surrounding frames. There are a number of variations of DV, includingDVCPro andDVCAM.

Firewire
A technical specification calledIEEE 1394 defines the standard commonly referred to asFireWire. You may also see it referred to as i.Link, particularly in Sony products.FireWire was developed in conjunction with DV, and in fact DV is optimized for maximum speed when transferred across aFireWire connection. Firewire is the standard interface between DV devices and computers, as well as between multiple DV devices such as camcorders.

miniDV
The standard videotape used for most DV devices is called minDV. It uses a 6mm videotape cassette. In addition to DV camcorders,miniDV tapes are also used inHDV cameras.HDV is an early high definitionCamcorder format unrelated to DV except in the use ofminiDV tapes. A singleminiDV tape can record 60 minutes of DV video with 16 bit audio. Although longer recordings are possible by reducing audio sampling to 12 bits, this isn't recommended as it will require resampling for conversion to nearly any other consumer format.

Digital8
Digital8 is a videotape format used almost exclusively by Sony. It uses the same 8mm videotape cassette as (analog)Hi8 camcorders. In fact mostDigital8 camcorders are capable of playing analog recordings inHi8, although they can't create them. Despite differences in tape size,Digital8 uses standard DV video and audio encoding, resulting in the same quality as any other DVCamcorder. As with miniDV, aDigital8 camcorder can record 60 minutes of DV video with 16 bit audio.

DVCAM and DVCPro
DVCAM andDVCPro camcorders are intended for use by video professionals and generally cost significantly more than a typical miniDV camcorder.DVCAM uses the same miniDV tapes as standard DV camcorders, while DVCPro models use a slightly larger tape, but can still read standard miniDV with the use of a cassette adapter. While both use the same video and audio encoding as consumer camcorders, the quality of the camera components is generally much higher, and both change the way data is stored to make it more editor friendly.

VideoEncoding
DV video is encoded as a series of I frames. This means eachFrame contains all the information toDecode it, so any frame can be removed without affecting playback of the others or requiring any re-encoding. Because of this it's also a fairly highBitrate format, requiring a fair amount of disk space on your computer to work with it. Although this presented serious challenges in the early days of the format, today it's relatively cheap to buy a hard drive that would hold several hours of DV video, with itsBitrate of 25Mbps. An hour of DV video, with accompanyingLPCM (uncompressed) 16 bit audio results in a file that's approximately 13GB.

Resolution andFramerate
Although there are higher definition variants used in some professional applications, consumer DV cameras have standard definition resolutions of 720x480 forNTSC and 720x576 forPAL. This matches the standards in ITU-Rec.601 for sampling analog video for both TV systems. Likewise, the framerates for NTSC and PAL are 29.97fps and 25fps respectively.

Chroma Subsampling
Like all consumer digital video formats, DV uses aYUVColorspace and subsamples theChroma (color) information to reduce file size. The subsampling used varies fromNTSC toPAL, although DVCPro uses theNTSC standard for both.PAL DV (except DVCPro) uses 4:2:0 subsampling, which is identical in name, but different in implementation than theYV12Colorspace used forMPEG video. WhileMPEG usesChroma samples that represent an entire 4x4Block ofPixels, PAL DV hasChroma samples that represent only 1 pixel in the same sizeBlock. This means forMPEG video every pixel's chroma must be calculated usingInterpolation, while PALDVD chroma is exact for 25% of thePixels in each frame, and must be interpolated for the other 75%.


The diagram above shows the position of each chroma sample in PAL DV's 4:2:0Colorspace. Notice that each one is located in the top-left pixel of each 4x4Block. Below is a diagram showing the three planes that make upYV12, as used in formats likeDVD. Notice that the chroma samples are located at the nexus of each block, representing the color for all of them simultaneously.


NTSC DV and all DVCPro camcorders use 4:1:1 subsampling, where one pixel out of 4 on every horizontal line contains chroma. As with PAL DV, the values for the remainingPixels must be interpolated.


Frame Structure
Although many consumer DV camcorders aren't capable ofProgressive scanning, essentially meaning they can't take a single picture using all the lines in a frame, that doesn't actually mean that the video is encoded interlaced. In fact every frame is encoded using the same algorithms whether it consists of a single picture or two interlaced fields. While this can sometimes cause artifacts, particularly at the edges of moving objects, DV's relatively lowCompression (compared toMPEG-2 orMPEG-4 for example), usually makes up for this. If your camcorder supports progressive encoding that's still likely to give you better results, particularly on modern progressiveHDTV displays.

AudioEncoding
DV camcordersEncode audio encoded asLPCM, sampled at 48kHZ, the standardDVDSamplerate. Samples can be either 16 bit or 12 bit. For best results you should stick to the standard 16 bit samples for DVD compatibility. 12 bit audio must be upsampled to 16 bits for DVDAuthoring, resulting in a loss of quality. The only exception would be a situation where you don't care as much about audio quality as the slight increase in recording time gained by the lowerSamplerate.

Raw DV
DV stored on digital videotape is muxed like other digital video formats, but doesn't use aContainer. Because of this its referred to asRaw DV. DV can also be stored on a computer inRAW format, usually with a file extension of .DV, but few consumer tools can read it unless its stored in aContainer such asAVI.

Transferring DV To A Computer
By using a camcorder's firewire interface it's possible to transferRaw DV from a camcorder to a computer with no quality loss. Rather than outputting an analog signal, theRAW data is copied to the computer, resulting in data original to what's stored on tape. Some camcorders can even act as a sort of passthrough device, encoding video from either the camera lens or analog video and audio inputs for output directly over firewire, bypassing the tape completely. Although most people use a video editing program to transfer from their camcorder, free programs like WinDV and DVIO can handle the transfer for you, and even give you the option to create Type 1 orType 2 DV files.

DV InAVI
When DV is transferred to a PC it can be stored in a number of formats, but the most common isAVI. Although AVI is a standard Windows container format, the way it's used for storing DV isn't always so standard. In fact there are two different ways to store it. Either the raw DV (muxed video and audio) can be stored directly in the AVI container, or it can be demuxed and remuxed to standard AVI standards.

Type 1 DV
When the raw DV is put directly in an AVI it's referred to asType 1 DV. Although the video inType 1 DV can be read using the standard AVI parsing components in Windows, DV'sMuxing is different than what AVI uses, resulting in a file that doesn't appear to have audio in it. In order to read the audio its necessary to use additional software capable of splitting raw DV into separate streams. Windows 2000 and Windows XP include aDirectShow DV decoder for this purpose, but no tool forVfW (the native AVI technology). Fortunately most consumer editing programs are designed with DV in mind and use either their ownDirectShow interface, or the built in Windows decoder for reading Type 1 DV.

Type 2 DV
Type 2 DV is created by demuxing the video and audio streams from Raw DV andMuxing them into a standard AVI container. Although Type 1 DV files are more convenient if you're relying on Windows'VfW interface to read them, they do require a third party decoder because Windows only includes DirectShow DV support. Fortunately there are some high quality free decoders available, including the one in the Cedocida DVCodec and another from Canopus. If you buy a miniDV camcorder or editing software with DV support a DVCodec may come with it.

Professional DV Formats
Although all the formats described here are essentially the same in terms of video encoding, there are some higher quality variations on DV capable of very high quality. These formats aren't used in consumer grade equipment.

HD Camcorders
DV encoding has been replaced in high definition camcorders byMPEG-2 andMPEG-4, both of which offer significant advantages in terms of size, however both achieve this size at the expense of lossless editing capabilities. By using P andB Frames to drastically reduce size, if you remove a single frame it could be necessary to re-encode all the surrounding frames, which depend on it for decoding.

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