Section 6.2. A major difference is in the sampling rate. Broadcast quality TV is generally oversampled. Here we mean that the sampling rate is greater than the Nyquist rate. The Nyquist rate, as we will remember, requires that the sampling rate be two times the highest frequency of interest. In this case, for the video, the highest frequency of interest is 4.2 MHz, the video bandwidth. Using the Nyquist rate, the sampling rate would be 4.2 X 106 X 2 8.4 million samples per second. One example is the ADC Video Systems scheme, which uses a sampling rate of 13.524 X 106 samples per second. One alternative is an 8-bit system; another is a 10-bit system. The resulting equivalent bit rates are 108.195 Mbps and 135.24 Mbps, respec- tively. The 20-kHz audio channel is sampled at 41,880 samples per second using 16-bit PCM. The resulting bit rate is 2.68 Mbps for four audio channels (quadraphonic). ADC Video Systems, Meriden, Connecticut, multiplexes and frames a 16-channel TV configuration for transmission over a fiber-optic trunk in an HFC system. The bit rate on each system is 2.38 Gbps. The frame structure is illustrated in Figure 17.14, and Figure 17.15 is an equipment block diagram for a 16-channel link. A major advantage of digital transmission is the regeneration capability just as it is in PSTN 8-bit PCM. As a result, there is no noise accumulation on the digital portion of the network. These digital trunks can be extended hundreds of miles or more. The complexity Audio out Sync & low: 176 bits speed mux Ch. 1 Ch. 2 Ch. 3 Ch. 4 Ch. 14 Ch. 15 Ch. 16 2.38 Gbps address 6 bit 10 bit 10 bit 10 bit 10 bit 10 bit 10 bit 10 bit Byte Interleave Figure 17.14 Typical frame structure on a single fiber in a CATV trunk. (Courtesy of ADC Video Systems.) COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Baseband Channel Channel Baseband Video Video video decoder video encoder Baseband Audio Audio Baseband audio encoder High- decoder audio speed receiver Baseband Fiber Baseband Video Video video encoder decoder video (-90 km) Baseba