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This paper describes many of the pseudoternary (PT) codes (twinned, bipolar, partial response, etc.) used in data transmission. In these, binary information is transmitted through three-level rather than binary pulse codes for controlling the power distribution in the frequency spectrum, improving clock recovery, allowing error detection or for just increasing the binary data rate. Linear and nonlinear PT codes are considered, the latter being divided into alphabetic and nonalphabetic codes. Among the nonalphabetic codes, emphasis is given to the modified bipolar codes used in pulse code modulation systems. Two recently developed codes of this type are described: High Density Bipolar (HDB) and Compatible High Density Bipolar (CHDB). They are particularly suitable for PCM transmission on repeatered lines. Another nonalphabetic code, the Transparent Interleaved Bipolar (TIB) is presented for the first time. This code features all the advantages of partial-response (or Interleaved Bipolar) signalling and, in addition, guarantees a minimum density of pulses, regardless of the data.
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