Antex Electronics SX-26 Digital Audio Adapter - Manual (Page 5)

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In essence, digital audio is a technological process whereby an
analog audio signal (produced when sound waves in the air excite
a microphone) is first converted into a continuous stream of
numbers (or digits). Once in digital form, the signal is extremely
immune to degradation caused by system noise or defects in the
storage or transmission medium (unlike previous analog systems).
The digitized audio signal is easily recorded onto a variety of
optical or magnetic media, where it can be stored indefinitely
without loss. The digitized signal is then reconverted to an analog
signal by reversing the digitizing process. In digital audio
record/playback systems, each of these two functions is
performed separately. In digital audio signal processing systems
(where no record/playback function occurs) both analog-to-digital
and digital-to-analog conversion processes occur simultaneously.
A variety of techniques are possible, but the most common
method by which audio signals are processed digitally is known as
linear pulse code modulation, or PCM. Let's take a brief look at
how PCM works.
Converting an analog signal to digital is a two-stage process,
sampling and quantization. This is illustrated in Figure 1. At regular
intervals, a sample-and-hold circuit instantaneously freezes the
audio waveform voltage and holds it steady while the quantizing
circuit selects the binary code which most closely represents the
sampled voltage. Most digital audio is based on a 16-bit PCM
system. This means that the quantizer has 65,536 (2
16 ) possible
signal values to choose from, each represented by a unique
sequence of the ones and zeroes which make up the individual
code "bits" of the digital signal.
The number of these bits generated each second is a function of
sampling rate. At a relatively low sampling rate of 8 kHz (suitable
for voice) far fewer code bits are produced each second than, for
example, at the 44.1 kHz sampling rate used for commercial
compact disks. For a two-channel stereo signal at a 44.1 kHz
sampling rate, some 1.4 million bits are generated each second.
That's about five billion bits per hour,which is why you'll need at
least an 800 Megabyte hard disk to record an hour of compact disk
quality music.