MPEG-4 Part 10 is a standard for videoCompression.MPEG-4 Part 2 (ASP and SP), which includes implementations likeDivX andXviD, was originally designed with video conferencing and other relatively primitive types of video.MPEG-4 Part 10 was designed specifically withHigh Definition (HD) video for home theater applications in mind as one potential application. It also includes specifications for lower quality video for portable devices, allowing a single standard to be compatible across a wide variety of devices and applications.

AVC / H.264
MPEG-4 Part 10, also known as MPEG-4AVC (Advanced Video Coding), is actually defined in an identical pair of standards maintained by different organizations, together known as theJoint Video Team (JVT). While MPEG-4 Part 10 is a ISO/IEC standard, it was developed in cooperation with theITU, an organization heavily involved in broadcast television standards. Since the ITU designation for the standard isH.264, you may see MPEG-4 Part 10 video referred to as either AVC or H.264. Both are valid, and refer to the same standard.

Advanced Video Coding
MPEG-4 Part 10 defines video encoding that's equally useful for low resolution/bitrate players like mobile phones and media players, medium resolutions and bitrates comparable toStandard Definition (SD)DVD, to high definition movie encoding forHDTV,Blu-ray, andHD DVD. LikeMPEG-2 and MPEG-4 Part 2 video, AVC is divided into a number of profiles, primarily to define what features a particular device or class of devices (like 3G phones for example) supports. Some of the more advanced features require a fair amount of CPU power, and therefore aren't suitable for devices like 3G phones or many portable media players. Other features, like higher resolutions and bitrates, aren't even helpful for many devices.

Codecs
Just because all MPEG-4 AVC encoders are designed to the same standard doesn't mean they're all the same. MPEG-4 Part 10 is a standard that codecs are written to. The codecs are the actual implementation, which my include all or just some of the complete feature set. Lossy encoding is basically a series of estimations and approximations. Even within a single video encoder there will usually be options to trade encoding speed for accuracy. This is possible because many values have an acceptable margin of error, allowing varying levels of precision in the calculations.

Compatibility
Regardless of what encoder was used to create an AVC video stream, as long as it's MPEG-4 Part 10 compliant any decoder equipped to handle the standard features used will be able to play it. This is usually just a matter of supporting the properProfile and level (see below).

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Partitions allow for more accuracy in motion estimation because areas with high motion can be isolated from those with less movement.

Profiles
For consumer video there are two main profiles used, Main and High, with the lower Baseline and Extended profiles getting used for some mobile devices. There are also higher profiles available, but they aren't used for standard consumer formats.

Baseline Profile
The Baseline profile is the simplest, but due to a lack of B Frame support should be avoided if a higher profile is supported. Baseline profile is suitable for video conferencing, 3G mobile phones and portable media players with AVC support. This profile also includes features forStreaming delivery not available in the Main or High profile, which are primarily intended to be used with more reliable delivery methods like broadcast TV or optical disc. There's no support for interlaced encoding, but due to both the displays you'll be encoding for and theLimitedBitrate this shouldn't be a concern. CABAC encoding isn't supported either, but once again the limitations of the hardware used for playback wouldn't generally support it.

Extended Profile
The Extended profile has more standard AVC features, like interlaced encoding and B Frames, along with the streaming enhancements it shares with the Baseline profile. Aside from the additional streaming support it's nearly identical to the Main profile, except that once again CABAC encoding isn't supported.

Main Profile
Although the Main profile was originally intended to be the standard home theater /DTV profile, by the time AVC began to see widespread adoption the High profile had replaced it for the majority ofHD content. For the majority ofSD content Main profile is probably still adequate. For HD resolutions High profile is almost always a better choice. Unlike the Baseline and Extended profiles, Main profile allows the use of CABAC encoding. While CABAC offers higher compression at comparable quality to the default CAVLC encoding, it's also more CPU intensive, both for the encoder and decoder. If either the computer you'll be encoding video on or the computer you'll be playing it back on doesn't have enough power you may not want to use it.

High Profile
The High profile is commonly used for HD video. The major differences are High profile's support for both 8x8 and 4x4 blocks to be encoded (compared to just 4x4 for Main profile). High profile also allows for the use of custom quantization matrices, which may be desirable for the content you're encoding or bitrate you're using.

Levels
In addition to Profiles, which define which features are available, and to some extent are designed around particular applications, Levels simply define limits to the features available in a profile. Parameters like bitrate are limited by the Level, although those limits may be different for different profiles. Rather than specify maximum resolutions and framerates for different levels, the maximum number of macroblocks per second is defined instead. These macroblocks can be used for lower resolutions at higher framerates or higher resolutions at lower framerates.

The following is a list of some common Profile and Level combinations, and where applicable standard applications that use them. It also gives standard resolutions and bitrates used with them.

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