When receiving a digital signal such as pulse code modulation (PCM), a digital-to-analog (D/A) converter is used. This reverses the process of A/D conversion at the transmitter, so that the original analog data is recovered.

You might ask, “Why convert a signal to digital form in the first place, if it’s going to be changed back to analog form at the receiver anyway?” The reason is that a digital signal is inherently simpler than an analog signal, in the sense that is less random.

Thus, a digital signal resembles noise less than an analog signal. It’s good to make a signal as different from noise as possible, in as many ways (or senses) as possible.

This is because the more different a signal is from unwanted noise, the easier it is to separate the data from the noise, and the better is the realizable S/N ratio.

You might think of signal/noise separation in terms of apples, oranges, and a watermelon. It takes awhile to find an orange in a tub of apples. (You’ll probably have to dump the tub).

Think of the orange as an analog signal and the apples as noise. But suppose there’s a watermelon in a bushel basket with apples.

You’ll have no trouble at all finding the watermelon. Think of the melon as a digital signal and the apples as noise.

Another, more interesting feature of digital communications arises when you think of a watermelon in a tub of oranges. It’s as easy to separate a digital signal from a jumble of analog signals as it is to extract a digital signal from noise.

In a band occupied by thousands of analog signals, a lone digital signal can be picked out easily—far more easily than any of the analog signals.

In recent years, digitization has become commonplace not only in data communications, but in music recording and even in video recording. The main advantage of digital recording is that a selection can be recorded, re-recorded, re-re-recorded, etc., and the quality does not diminish.

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