Spectral nulls and coding with large alphabets

Generalizations of the simple alternate mark inversion (AMI) line code that provide enhanced immunity to additive noise as well as spectral shaping are considered. The first technique is the extension of balanced disparity methods to multilevel signaling alphabets. This is a small step beyond simple codes such as AMI that are used to transmit binary PCM over twisted-pair cables. An important feature of this method (and later methods) is the use of suboptimal decoders. The most sophisticated technique used was Tomlinson (1971) filtering, and here it was possible to require a spectral null in the line code spectrum with a certain minimum width, and to minimize line code power subject to this requirement. This technique is compared to methods introduced by Marcus and Siegel (1987) in magnetic recording that provide spectral nulls at rational multiples of the symbol frequency. Theoretical possibilities, rather than techniques proven superior for a particular application, are addressed.<>