coded qualities. This is more likely with the above CCITT and ISO codecs where the relatively low bit Videophony and videoconferencing rates mean that coding distortion is larger and more often visible than in the CCIR case. Videophony and videoconferencing are companion audiovisual ser- vices which are, at the present time, generally distinguishable by the 57.4.14 Inverse DCT Specification factors given in Table 57.8. In the above schemes it is important that the loops at coder and Table 57.8 Comparison between videophony and decoder track each other. Any divergences manifest themselves as videoconferencing noise and because of the recirculating nature of the loop this noise will steadily increase if there is a continual though small source of error. All the mathematical operations except the IDCT can be Videophony Videoconferencing precisely specified and implemented. The IDCT, however, contains Primarily person-to-person Primarily group-to-group cosine terms and some of these are transcendental numbers which Office environment can never be represented exactly with the finite precision inherent Studio or specially equipped room environment in practical implementations. Rather than define a unique fixed precision approximation to the Compact desk top terminals Large floor standing terminals IDCT which would have translated to a single architecture with On demand service via Bookable service via digital specified accuracy and rounding at every intermediate step, the customer switched digital leased lines CCITT chose in Recommendation H.261 to allow some flexibility networks to manufacturers. The IDCT is specified with an error limit and the accumulation of mismatch between different implementations at Picture quality consistent Picture quality consistent coder and decoder is controlled by stipulating the minimum rate at with low bit rate transmission with high bit rate which intra coding must be selected. This action of temporarily (e.g. 64kbit/s or 128kbit/s) transmission (e.g. 384kbit/s breaking the loop clears