The H.261 standard describes the compression of image sequences for videoconferencing and videophony and defines the basic principles of digital moving image transmission and data compression.
The H.261 process is based on JPEG and is designed for ISDN with n*64 kbit/s channels, typically for 128 kbit/s to 768 kbit/s. Compression ratios range from 100:1 to 2,000:1. H.261 is part of H.320.
H.261specifies a maximum combined compression and decompressiondelay time of 150 ms. The file formats used for the component video are the CIF display format with 352 x 288 pixels and QCIF with 176 x 144. H.261 works according to the YUV color model, with DCT transformation and Huffman coding and, like MPEG, has a prediction mechanism for the subsequent picture sequence with intra frames and predicted frames. ITU-T documents and more information on H.261 at: