THERE IS effectively an infinite range of values available for the encoded signal in analogue modulation methods like AM and FM. The same can be said for the analogue pulse-modulation techniques, PAM, PWM and PPM. Digital modulation employs a small number of discrete pulse sizes, often just two, and consequently the signal amplitude must be encoded in some way. The significant words here are ‘in some way’, for that determines the capacity of the communications channel — how much information it can convey in a given time. The chief advantage of digital communications is that the encoded message can be made as free from errors as desired and moreover, given the appropriate parameters of the channel, the error rate is predictable. It is inherently more difficult to maintain the relative levels of a continuously varying waveform than to recognise the presence or absence of a pulse. Thus signal distortion is cumulative in an analogue communications system, whereas in a digital system, no matter how many repeaters are used, the signal can be recovered with a guaranteed level of distortion.
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