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What is H.261?

Video Format Description

H.261 is a codec designed by ITU (International Telecom Union) for video conferencing over PSTN (Public Switched Telephone Network). Published in 1990, it is considered a major milestone in the development of video coding.

H.261 is the most important application in the video conferencing and video communications systems. Its uses include studio based video conferencing, desktop video conferencing, surveillance, monitoring, computer training, and tele-medicine. But it does not work well over Frame Relay or TCP/IP Internet as it is optimized only for low data rates and relatively low motion video.

An H.261 codec is also included in an LGPL-licensed libav codec, which is used in many programs like VLC Media Player and Multimedia MPlayer, as well as in FFdshow and FFmpeg decoders.

Often called p x 64K bit/s as it runs in multiples of 64K (in the range of 1-30) which suit ISDN lines (for which it was originally designed), H.261 was the first standard to use the macro-block concept. According to this concept, each macro-block consists of one 16x16 array of luma samples, and two of 8x8 array one of chroma samples, and one of YCbCr color space. This allows for sending of still picture graphics with both 704x576 luma resolution and 352x288 chroma resolution. The standard also supports two video frame sizes CIF (Common Interchange Format): 352x288 luma with 176x144 chroma; and QCIF (Quarter Common Interchange format): 176x144 with 88x72 chroma.

As an option, H.261 can also support motion compensation in the encoder, in which there is a search area in the previous frame which selects the best reference macro-block. This prediction reduces chronological redundancy between pictures.

An H.261 may be encoded on a Windows or Mac OS application and decoded with a Pentium or PowerMac.