As was previously reviewed in Sec. 3.3.6, the CATV MODEM is a key building block in hybrid fiber coax (HFC) systems. There are two types of MODEMs, 8 VSB (vestigial side band) shown in Fig. 3.21, and QAM. The basic link block diagram of a QAM HFC MODEM is given in Fig. 15.1 with more details on its block functionality and its operation.
The first block in a QAM or VSB MODEM after Moving Picture Experts Group (MPEG) frame synchronization is the data randomizer, which is also called a scrambler. Its role is to achieve dc balance and eliminate long sequences of consecutive zeros to ensure accurate timing recovery on the receiver side. Next is the Reed-Solomon (RS) encoder. The RS encoder provides block encoding for block identification. After the RS encoder is the interleaver block that disperses the symbols in time. This way, it prevents a burst-error situation from occurring at the RS coder. The interleaver spreads the error burst so that the RS encoder refers to them as if they were random errors. This way, errors are corrected and the RS encoder is not overloaded. After the interleaver, there is an additional convolution encoding done by Trellis coding. Then sync signals are inserted.
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