 out any accumulated errors before they up to 2Mbit/s) become too large. This approach and the same IDCT specification Telephony quality speech Wideband speech quality was adopted for the above CCIR and ISO algorithms and supported elsewhere (IEEE, 1990b). 57.4.15 However, for some applications at least, the boundaries between HDTV coding the two services are becoming less distinct; for example, desktop videoconferencing terminals operating over switched 56kbit/s and All the methods outlined above are directly applicable to High 64kbit/s networks are becoming available. The CCITT, which is Definition Television (HDTV) coding. The higher sampling rates responsible for the video coding, audio coding and protocol stand- needed, in the region of 70MHz for luminance, currently present ards for audiovisual services, has appreciated the future need for some technological challenges in the implementation of the more both types of service to interwork. As a result, the relevant standards complex algorithms. The economic balance between the saving in ensure that any audiovisual terminal adhering to the standards will transmission or storage cost gained by more efficient coding and the automatically interwork with any other standard terminal with the increased cost of the coding and decoding equipment must be same or lesser capabilities. The standards are discussed further in considered. There is some interest in compatible coding schemes for later sections. HDTV which simultaneously provide decoded pictures for viewing on conventional displays. 57.5.1 General design considerations 57.4.16 New approaches This section deals with the general design considerations for video- phone and videoconferencing systems and terminals, but since a Though the techniques mentioned thus far have only recently been number of the considerations are a consequence of the CCITT Rec. incorporated in standards, they have been known for many years, H.261 video coding algorithm used (CCITT, 1990a), the reader well investigated and further improvemen