In 2003, the MPEG completed the first version of a compression standard known as MPEG-4 Part 10, or H.264. Like other standards in the MPEG family, it uses temporal compression. Although H.264 is not the only compression method, it has become the most commonly used format for recording, compression, and streaming high definition video, and it is the method used to encode content on Blu-ray discs.
H.264 is a “family” of standards that includes a number of different sets of capabilities, or “profiles.” All of these profiles rely heavily on temporal compression and motion prediction to reduce frame count. The three most commonly applied profiles are Baseline, Main, and High. Each of these profiles defines the specific encoding techniques and algorithms used to compress files.
This is the simplest profile used mostly for low-power, low cost devices, including some videoconferencing and mobile applications. Baseline profiles can achieve a compression ratio of about 1000:1 — i.e. a stream of 1 Gbps can be compressed to about 1 Mbps. They uses 4:2:0 chrominance sampling, which means that color information is sampled at half the vertical and half the horizontal resolution of the black and white information. Other important features of the Baseline Profile are the use of Universal Variable Length Coding (UVLC) and Context Adaptive Variable Length Coding (CAVLC) entropy coding techniques.
Main Profile includes all of the functionality of Baseline, but with improvements to frame prediction algorithms. It is used for SD digital TV broadcasts that use the MPEG-4 format, but not for HD broadcasts.
H.264 High Profile is the most efficient and powerful profile in the H.264 family, and is the primary profile for broadcast and disc storage, particularly for HDTV and Bluray disc storage formats. It can achieve a compression ratio of about 2000:1. The High Profile also uses an adaptive transform that can select between 4x4 or 8x8-pixel blocks. For example, 4x4 blocks are used for portions of the picture that are dense with detail, while portions that have little detail are compressed using 8x8 blocks. The result is the preservation of video image quality while reducing network bandwidth requirements by up to 50 percent. By applying H.264 High Profile compression, a 1 Gbps stream can be compressed to about 512 Kbps.
In the H.264 standard, there are a number of different “levels” which specify constraints indicating a degree of required decoder performance for a profile. In practice, levels specify the maximum data rate and video resolution that a device can play back.
For example, a level of support within a profile will specify the maximum picture resolution, frame rate, and bit rate that a decoder may be capable of using. Lower levels mean lower resolutions, lower allowed maximum bitrates, and smaller memory requirements for storing reference frames. A decoder that conforms to a given level is required to be capable of decoding all bit streams that are encoded for that level and for all lower levels.