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## Coding for videoconferencing (H.261)

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The H.261 standard defines the video coding and decoding methods for digital trans mission over ISDN at rates of p x 64kbit/s, where p is in the range of 1-30. The video bit rates will lie between approximately 64kbit/s and 1920kbit/s. The recommendation is aimed at meeting projected customer demand for videophone, videoconferencing and other audio-visual services. The coding structure of H.261 is very similar to that of the generic codec of Chapter 3. That is, it is an interframe DCT-based coding technique. Interframe prediction is first carried out in the pixel domain. The prediction error is then transformed into the frequency domain, where the quantisation for bandwidth reduction takes place. Motion compensation can be included in the prediction stage, although it is optional. Thus the coding technique removes temporal redundancy by interframe prediction and spatial redundancy by transform coding. Techniques have been devised to make the codec more efficient, and at the same time suitable for telecommunications.

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