H.261

H.261 is standardized for the first time in November 1988 by the ITU- T method of digital compression and decompression of analog video signals. The goal was to be able to operate over ISDN lines with several bundled B channels video telephony or video conferencing. To perform the low bandwidth required for this reduction of the image data is used as the central element DPCM.

H.261 can only process progressively scanned images. The pictures are luminance Y and two chrominance components Cr and Cb coded, the Chrominanzmatrizen have a quarter of the size of the Luminanzmatrix. The method supports two image formats. These are CIF and QCIF video frames that have a luminance resolution of 352 × 288 or 176 × 144. H.261 is also backwards compatible for illustration graphics ( still images) with a resolution of the luminance of 704 × 576 H.261 was the first digital video coding standard.

The design of H.261 was a pioneering effort, and all subsequent international video coding standards (MPEG- 1, MPEG -2, H.262, H.263 and H.264) based on this design. In addition, the methods used by the H.261 Development Committee ( headed by Sakae Okubo ) to jointly develop the standard, the basic working process of standardization work in this area. The H.261 standard describes the compression of image sequences for video conferencing and video telephony and determines the basic principles of digital moving picture transmission and data compression. The compression rates are 100:1 to 2000: 1 H.261 is part of H.320.

In H.261, a maximum delay time for compression and decompression of a total of 150 ms is set. H.261 uses the YCbCr color model with DCT and Huffman coding and has, as well as MPEG, a prediction mechanism for the following series of images with intra-frames and predicted frames. A common use of this standard will take place in the MBone, a system for broadcasting of video conferencing on the Internet via a spanning tree of the communication cables to a lot of registered users.