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H.261 is a procedure and data format standardized by the ITU-T in November 1988 for the compression or decompression of digital video signals in real-time use. The aim was to be able to operate video telephony or video conferencing over ISDN lines with several bundled B channels . H.261 was the first digital video coding standard in use. Today it no longer has any significant use.

Data format of a single image


In H.261, a maximum delay time for compression and decompression of a total of 150 ms is specified. H.261 can only process images that have been read in progressively. The images are encoded as luminance (Y) and two chrominance components (Cr and Cb), the chrominance matrices being a quarter of the size of the luminance matrix ( color subsampling 4: 2: 0). The process supports two image formats. These are CIF and QCIF video frames that have a luminance resolution of 352 × 288 and 176 × 144, respectively. H.261 is also backwards compatible for picture graphics (still images) with a luminance resolution of 704 × 576.


The draft of H.261 was a pioneering effort and all subsequent international video coding standards ( MPEG-1 , MPEG-2 , H.262 , H.263 and also H.264 ) are based on this draft. In addition, the methods used by the H.261 Product Development Committee (headed by Sakae Okubo) to collaboratively develop the standard are the basic functioning process for standardization work in this area. The H.261 standard describes the compression of image sequences for video conferences and video telephony and defines the basic principles of digital video transmission and data compression. The compression rates range from 100: 1 to 2,000: 1. H.261 is part of H.320 .


In order to reduce image data to the low target bandwidth , DPCM is used as a central element . H.261 works according to the 4: 2: 0 YCbCr color model with intra-frames and predicted frames. It has a JPEG- like intra-coding with block-wise discrete cosine transformation (DCT) and entropy coding of quantized DCT coefficients according to Huffman as well as pixel-precise, motion-compensated temporal prediction for the respective following image (forward). A filter within the codec loop can be used against high-frequency image interference, the effects of which are also taken into account in the forecast (in-loop filtering). A common use of this standard was in the MBone , a system for broadcasting video conferences on the Internet over a spanning tree of communication lines to a large number of logged-in users.

See also

  • H.120 , the first digital video coding standard


Web links

Individual evidence

  1. Sakae Okubo, "Reference model methodology - A tool for the collaborative creation of video coding standards," Proceedings of the IEEE , Volume 83, Number 2, February 1995, pages 139-150