there is and the digital transmission of colour television signals, encoded a need for A-law/u-law conversion when international digital tele- according to Rec. H.120 (CCITT, 1988e) at the primary digital phone traffic is exchanged between incompatible regions. hierarchical rates (2048kbit/s in Europe, 1544kbit/s in North Better quality speech, with a bandwidth of 7kHz, can be encoded America and Japan). Again, digital leased circuits were almost into 64, 56 or 48kbit/s using the ADPCM algorithm of Rec. G.722 universally used although some network operators introduced a (CCITT, 1988c). The systems aspects of wideband audio are limited switched capability (under their control, not the users) to covered in Rec. G.725 (CCITT, 1988d). A draft Recommendation, provide some flexibility in connecting pre-booked calls. AV 254, for 16kbit/s speech using a codebook excited linear predic- In the 1990s, the trend will be towards user switched (dial up) tion (CELP) algorithm is close to completion and is expected to be videophone and videoconferencing services at bit rates in the range formally ratified by the CCITT in 1992 as new Recommendation 64kbit/s to 384kbit/s using the coding algorithm specified in Rec. G.728. H.261, although premium services at rates up to 1920kbit/s will still Although other audio coding standards exist, the above are the be a user option. only ones applicable to audiovisual services at the present time. 57.5.3.2 57.5.2.2 Video coding The new Integrated Services Digital Network (ISDN), the standards Rec. H.261 specifies the video coding algorithm for audiovisual for which have been generated and refined by the CCITT over the services at rates from 64kbit/s to 1920kbit/s (video bit rate in the past decade and more, is the customer switched public digital range 46.4kbit/s to 1856kbit/s). network most likely to be used for videophone and videoconferenc- ing services. Basic rate access to the ISDN (CCITT, 1984a) provides two full duplex 64bit/s digital connections (the bearer or B-chan- 57.5.2.3 Audiovis