Abstract
This letter clarifies some of the properties of the Hedeman codes and examines their application to band-limited channels. Attention is drawn to the equivalence of H-2 and H-3 to simple linear modulation schemes. All three codes are dc free by virtue of a charge constraint which additionally ensures a minimum distance two (considered as rate one-half codes). The asymptotic error performance at high signal-to-noise ratio can thus be similar to NRZ or biphase signaling, although the Viterbi algorithm is required to achieve this performance in the case of H-1 code. Like Miller code, both H-1 and H-2 will pass through unequalized low-pass channels with bandwidths little greater than required for NRZ signaling.
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