nd High- Audio High- Audio Baseband speed High- audio encoder speed transmitter speed decoder audio multiplexer (-2.4 Gpbs) demulti- (LSM) plexer (LSD) Ch 12 Ch 12 Baseband Video video Video Baseband encoder Channel decoder Channel video Baseband Audio Audio Baseband audio encoder decoder audio Figure 17.15 Functional block diagram of a 16-channel digital TV system. (Courtesy of ADC Video Systems.) is only marginally greater than an FM system. The 10-bit system can easily provide an S/N ratio at the conversion hub of 67 dB in a video channel and an S/N value of 63 dB with an 8-bit system. With uncompressed video, BER requirements are not very stringent because video contains highly redundant information. 17.5.3 Compressed Video MPEG compression is widely used today. A common line bit rate for MPEG is 1.544 Mbps. Allowing 1 bit per hertz of bandwidth, BPSK modulation, and a cosine roll-off of 1.4, the 1.544-Mbps TV signal can be effectively transported in a 2-MHz bandwidth. Certainly 1000-MHz coaxial cable systems are within the state of the art. With simple division we can see that 500-channel CATV systems are technically viable. If the modulation scheme utilizes 16-QAM (4 bits/Hz theoretical), three 1.544-Mbps compressed channels can be accommodated in a 6-MHz slot. We select 6 MHz because it is the current RF bandwidth assigned for one NTSC TV channel. 17.6 TWO-WAY CATV SYSTEMS 17.6.1 Introduction Figures 17.16a and 17.16b are two views of a CATV system as they might appear on coaxial cable. Of course, with conventional CATV systems, each NTSC television channel is assigned 6 MHz of bandwidth as shown in Chapter 16, Figure 16.6. In Figure 17.17a, only 25 MHz is assigned to upstream6 6 services. Not all the bandwidth may be used for voice and data. A small portion should be set aside for upstream telemetry from active CATV equipment in the system (e.g., broadband amplifiers). On the other 6 Remember, upstream is the direction from the CATV subscriber to the headend, and downstream is in the direction from the he