2-52Figure 2-52.—Block diagram of quantizer and pcm coder.The pcm demodulator will reproduce the correct standard amplitude represented by the pulse-codegroup. However, it will reproduce the correct standard only if it is able to recognize correctly the presenceor absence of pulses in each position. For this reason, noise introduces no error at all if the signal-to-noiseration is such that the largest peaks of noise are not mistaken for pulses. When the noise is random (circuitand tube noise), the probability of the appearance of a noise peak comparable in amplitude to the pulsescan be determined. This probability can be determined mathematically for any ration of signal-to-average-noise power. When this is done for 105pulses per second, the approximate error rate for three values ofsignal power to average noise power is:17 dB — 10 errors per second20 dB — 1 error every 20 minutes22 dB — 1 error every 2,000 hoursAbove a threshold of signal-to-noise ration of approximately 20 dB, virtually no errors occur. In allother systems of modulation, even with signal-to-noise ratios as high as 60 dB, the noise will have someeffect. Moreover, the pcm signal can be retransmitted, as in a multiple relay link system, as many times asdesired, without the introduction of additional noise effects; that is, noise is not cumulative at relaystations as it is with other modulation systems.The system does, of course, have some distortion introduced by quantizing the signal. Both thestandard values selected and the sampling interval tend to make the reconstructed wave depart from theoriginal. This distortion, called QUANTIZING NOISE, is initially introduced at the quantizing andcoding modulator and remains fixed throughout the transmission and retransmission processes. Itsmagnitude can be reduced by making the standard quantizing levels closer together. The relationship ofthe quantizing noise to the number of digits in the binary code is given by the following standardrelationship:Where:n is the number of digits in the binary code