tereo or bilingual sound systems have been around. They arose because of the difficulty of transmission has (for System G) a separate carrier located achieving good quality reception; this often required the aerial to be 5.742MHz above the vision carrier at -20dB in addition to the placed in an advantageous position, probably on high ground. It normal sound carrier at 5.5MHz above the vision carrier at -13dB. became natural then to share this facility by cabling to local house- The 242kHz difference between the sound carrier is an odd har- holds. monic of the vision line frequency. The usual sound FM carrier is From the early local enterprises, perhaps initiated by electrical modulated with (L + R)/2, and the second carrier with only the R goods shops wishing to increase the sales of receivers, there arose signal (or with a second language or separate audio signal) with a specific cable TV firms. The first heyday of cable distribution deviation of +2.5kHz. A mode identification signal is provided on a systems in the 1950s and 1960s was as an off air TV, and radio, relay tone modulated 54.6875kHz subcarrier (unmodulated for mono, system. Though far from universal, it was a significant business in 117.5Hz AM for stereo, 274.1Hz AM for separate audio). many countries covering areas of poor reception, hotels, appartment The Nicam 728 system uses digital quadrature phase shift keying blocks, and sometimes towns where external aerials were not to digitally modulate the second carrier located (for System 1) allowed for aesthetic reasons. 6.552MHz above the vision carrier with a carrier level of -20dB As over the air broadcast transmissions improved their coverage, with respect to peak vision. The two audio channels (stereo or the relay systems fell into disuse and cable TV had to look to bilingual) are combined in time division multiplex as a digital bit providing additional services to prove its worth. Systems using Table 57.6 Main characteristics of NICAM-728 Characteristic Number of sound channels with 15kHz bandwidt