ance expressed as error rate. Errors are cumulative, and as we go down a PCM-repeatered line, the error performance degrades. A decision in error, whether a 1 or a 0, made anywhere in the digital system, is not recoverable. Thus such an incorrect decision made by one regenerative repeater adds to the existing error rate on the line, and errors taking place in subsequent repeaters further down the line add in a cumulative manner, thus deteriorating the received signal. In a purely binary transmission system, if a 22-dB signal-to-noise ratio is maintained, the system operates nearly error free. 8 In this respect, consider Table 6.1. Table 6.1 Error Rate of a Binary Transmission System Versus Signal-to-rms-Noise Ratio Error Rate S/N (dB) Error Rate S/N (dB) 10-10 10-11 As discussed in Section 6.4, PCM, in practice, is transmitted on-line with alternate mark inversion (in the bipolar mode). The marks (1s) have a 50% duty cycle, permitting signal energy concentration at a frequency equivalent to half the transmitted bit rate. Thus it is advisable to add 1 dB or 2 dB to the values shown in Table 6.1 to achieve the desired error performance in a practical system. 7 Gaussian noise is the same as thermal noise. It is against the laws of physics to have a completely tror-free systems. 6.6 REGENERATIVE REPEATERS 6.6 REGENERATIVE REPEATERS As we are probably aware, pulses passing down a digital transmission line suffer atten- uation and are badly distorted by the frequency characteristic of the line. A regenerative repeater amplifies and reconstructs such a badly distorted digital signal and develops a nearly perfect replica of the original at its output. Regenerative repeaters are an essential key to digital transmission in that we could say that the "noise stops at the repeater." Figure 6.11 is a simplified block diagram of a regenerative repeater and shows typical waveforms corresponding to each functional stage of signal processing. As illustrated in the figure, at the first stage of signal processing is amplification an