were non linearly coded 5 bit differences. This HDPCM 8-5-5-5 mode results in an to carry video, audio, data facsimile and telephony signals with equal facility, and to carry such services without requiring a long average word length of 5.75bits, reducing the 175.5Mbit/s data to 'lead time' to set up. Digital transmission is well suited to the use of just over 126Mbit/s with virtually no perceptible loss of quality. optical fibre links with widely spaced regenerators. It is foreseen that eventually intercity links for both television and 57.2.7 Teletext transmission sound radio will use digital transmission within the ISDN hierarchy. The economics of digital distribution and contribution circuits is During the 1970s, a data transmission system riding 'piggy-back' on less certain. The introduction by telecommunications carriers of, for conventional television transmission was developed in the UK and example, digital stereo links for outside broadcasts has tended to be introduced under the service designations Ceefax (BBC) and Oracle accompanied by higher costs, leading to increasing use of satellite (IBA-ITV). An agreed technical specification was introduced in links for outside broadcast of sound radio. 1974, Teletext transmission systems have since been introduced in The serial bit rate of digital video to the CCIR Recommendation a number of countries, and basically similar systems but with rather 601 (4:2:2) standard within studio centres is 243Mbit/s (216Mbit/s different technical specifications have also been developed in some of video data). However, once the requirement for editorial process- countries. Teletext (Figure 57.16) uses the broadcast television ing has been completed, the bit rate can be significantly reduced signal to carry extra information. These extra signals do not inter- without noticeable loss of quality. For intercity links, the bit rate will fere with the transmission and reception of normal programmes. normally be reduced to meet the 140 or 70Mbit/s ISDN hierarchy teletext receiver, a 