Evaluation of Message Circuit Noise

From the time of the first commercial telephone system, transmission engineers have been concerned with the problem of message circuit noise evaluation. Prior to the turn of the century, progress was slow and limited to crude estimates because of the lack of means for performing measurements. One's hearing mechanism had to serve as the indicating meter, and the scale of measurement was simply one's expression of a t t i t u d e concerning the noise. Initially, the most accurate determination of noise magnitudes was through frequency analysis and computation. A given noise voltage was first analyzed into its spectrum, which was then weighted according to a predetermined curve t h a t characterized the relative interfering effect of frequencies in the telephone channel. The total noise magnitude was then obtained by summing the weighted components on a power basis. A quicker b u t less accurate way was to make an over-all noise measurement. "Circuit noise" was simply compared by ear to a variable-ampli879