Parametric and Hybrid Coding

The desire to conserve spectrum is a very widespread one, prevalent not only with respect to the radiated spectrum, but also significant in terms of the conducted spectrum that exists on any cable transmission facility. Numerous multilevel modulation techniques (see Chapter 6) have been devised to increase the number of bits per second that can be transmitted per hertz of bandwidth. It should be no surprise, therefore, that significant efforts are being made to encode voice at lower information rates (b/s) than required for 64-kb/s PCM, as described in Chapter 3. The fact that 64-kb/s PCM, as such, generally requires much more bandwidth than does a 3-kHz analog signal is of itself a continuing incentive to seek lower-bit-rate forms of digitized speech.