this has seldom been reflected in the design of domestic receivers. white noise at a level of 4dB below the inherent quantising noise. In Increasing consumer awareness of the fidelity provided by other practice for point to point links, a single parity check digit is added media, including FM stereo radio, stereo cassette tapes and digital to each 13 bit sample word. CD records, has led to the development of systems for dual channel A 13 channel PCM sound distribution system introduced by BBC and multi channel audio for television capable of providing suffi- Radio in 1972 had a transmission bit rate of 6.336Mbit/s. This has cient separation to permit simultaneous dual language broadcasting. been progressively superseded by a 6 channel system with near The desirable requirements include: general improvement of instantaneous companding (NICAM-3) (see Figure 57.18) with a sound quality; possibility of receiving stereo sound programmes in total transmission bit rate of 2.048Mbit/s (676Kbit/s for each stereo mono (direct compatibility); possibility of receiving mono pro- pair). grammes on stereo receivers (reverse compatibility); no discernible High quality audio links are more vulnerable to bit errors than interference between picture and audio channels; possibility of digital video. Without error protection, 'just perceptible' impair- transmitting either a stereo or two or more separate high quality ment can be detected with a BER of the order of 1:107. With mono channels; maximum possible technological compatibility protection of the five most significant bits (MSBs), an equivalent with existing systems and equipment, and receiver decoder costs impairment would need a BER of 1:105 compatible with perceived benefits to the viewer. For (L + R)/(L- With instantaneous companding, a limited number of digits is R) stereo a separation of 25dB to 30dB is considered sufficient. For transmitted per sample word together with a scale factor indicating bilingual transmission, separation of the order of at least 55dB is the sign