H.261

H.261 is an ITU-T video compression standard, first ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group (VCEG), and was the first video codec that was useful in practical terms.

H.261 was originally designed for transmission over ISDN lines on which data rates are multiples of 64 kbit/s. The coding algorithm was designed to be able to operate at video bit rates between 40 kbit/s and 2 Mbit/s. The standard supports two video frame sizes: CIF (352x288 luma with 176x144 chroma) and QCIF (176x144 with 88x72 chroma) using a 4:2:0 sampling scheme. It also has a backward-compatible trick for sending still images with 704x576 luma resolution and 352x288 chroma resolution (which was added in a later revision in 1993).

History

Whilst H.261 was preceded in 1984 by H.120 (which also underwent a revision in 1988 of some historic importance) as a digital video coding standard, H.261 was the first truly practical digital video coding standard (in terms of product support in significant quantities). In fact, all subsequent international video coding standards (MPEG-1 Part 2, H.262/MPEG-2 Part 2, H.263, MPEG-4 Part 2, H.264/MPEG-4 Part 10, and HEVC) have been based closely on the H.261 design. Additionally, the methods used by the H.261 development committee to collaboratively develop the standard have remained the basic operating process for subsequent standardization work in the field. It was developed by the CCITT Study Group XV Specialists Group on Coding for Visual Telephony (which later became part of ITU-T SG16), chaired by Sakae Okubo of NTT.