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 Study Group 16 Video Coding Experts Group (VCEG, then Specialists Group on Coding for Visual Telephony). It was the first video coding standard 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 (352×288 luma with 176×144 chroma) and QCIF (176×144 with 88×72 chroma) using a 4:2:0 sampling scheme. It also has a backward-compatible trick for sending still images with 704×576 luma resolution and 352×288 chroma resolution (which was added in a later revision in 1993). History The first digital video coding standard was H.120, created by the CCITT (now ITU-T) in 1984. H.120 was not usable in practice, as its performance was too poor. H.120 was based on differential pulse-code modulation (DPCM), which had inefficient compression. During the late 1980s, a number of companies began experimenting with the much more efficient DCT compression for video coding, with the CCITT receiving 14 proposals for DCT-based video compression formats, in contrast to a single proposal based on vector quantization (VQ) compression. The H.261 standard was subsequently developed based on DCT compression. H.261 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. Since H.261, DCT compression has been adopted by all the major video coding standards that followed. 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. Although H.261 was first approved as a standard in 1988, the