 the provision of television line. The duobinary data burst conveys 206 bits analogue wide screen (16:9 aspect ratio) video information in and is followed by time compressed vision signals various MAC enhanced modes or for the use of digitally assisted (DATV) vector motion enhancement. 57/10 Video transmission All MAC systems facilitate the use of 'conditional access' encryp- Wide screen displays with an aspect ratio of at least 16:9. tion systems offering a high level of security for subscription tele- 5. Economic conversion of material from tape to film and from vision channels. One possibility is the use of 'double cut' film to tape, without noticeable degradation of picture quality. component rotation, with the decoding key transmitted in the digital 6. Transmission systems compatible with existing receivers. packets, addressed to individual receivers or groups of receivers. Sufficient flexibility for an HDTV standard to permit advant- The digital channel capacities of MAC/packet systems while age to be taken of continuing improvements in display systems, analogue vision signals are present are: C-MAC and D-MAC digital signal processing, etc. 3.04Mbit/s; D2-MAC 1.52Mbit/s. In the absence of vision signals, the full channel capacity of a D-MAC channel can be used. While it is feasible that future fibre optic cables may permit, at Within an analogue component area, MAC techniques can be used least theoretically, the distribution direct to the home of HDTV for routeing, contribution, ENG links, etc. T-MAC for studio inter- RGB component signals with a bit rate of the order of 1Gbit/s, in connection, etc. retains the multiplexed analogue video component practical terms an HDTV signal should be amenable to digital portion (4-2-0) of the MAC family of transmission formats, but bandwidth compression (bit rate reduction) techniques to the ISDN dispenses with the packet data portion, substituting in its place a line level of 140Mbit/s. repetitive sync pulse and sampling frequency reference burst. T- It has been shown 