Performance Characteristics of Various Carrier Telegraph Methods

OST of the carrier telegraph methods mentioned below are well known,1-2-3-4-5 but the selection of the best method for a particular application is difficult without comparative tests on specific designs. It is the purpose of this paper to record data taken during such tests and to explain the results so that they may be helpful to those concerned with the selection of the optimum method for a given set of requirements. The conclusions here reached regarding methods of telegraph transmission do not necessarily apply to transmission of sound, pictures, or television, because their requirements differ. In telegraph transmission it is important t h a t signal transitions be received at a p p r o x i m a t e l y the correct times, a n d M wave rounding is permissible. Computations for a square cift-off band-pass filter with zero phase distortion6 show that the shape and duration of the transient in the received wave are about the same for a sudden transition in both on-off and frequency-shift arrangements (explained in the next section), when the total frequency shift is not more than half the channel width. As telegraph distortion depends largely upon the transient, one might therefore infer that, if the transients are about alike, there is no particular advantage in frequency-shift over the on-off method as far as signal speed is concerned. However, the computation for the idealized filter gives no assurance that a physical filter will per1 H. Nyquist: "Certain Topics in Telegraph Transmission Theory", A.I.