A Note Regarding the Performance of Various Communications Systems in a Noisy Background

Abstract

It is possible to compare the performance of different communications systems in a noisy background by plotting the performance described in terms of the percentage of character errors or percentage of errors received correctly as a function of rms carrier to rms noise in a given bandwidth. Figure 1 shows the results of measurements taken with typical frequency-shift keying teletype receivers employing a start stop teletype printer operating at 60 words per minute and also a good and a fair operator receiving random five letter groups at 16 wpm. The noise background for the solid curves was typical VLF noise which contains rather large impulses. The amplitude probability distribution for this noise has a very wide dynamic range. The dashed line was taken in a thermal or gaussian type of noise background and shows that the performance varies much faster in terms of a changing signal to noise than is true for the atmospheric noise background. It is important to note that when human operators are involved, there is an upper limit of performance which appears to vary from individual to individual as well as with the rate of transmission and the general state of well being of the operator. This upper level of performance is in the order of 1% character errors for a good operator and something in the order of 10% for fair to poor operators.