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4
the quantizer generates a continuous "bit stream" which repre-
sents the quantized signal. The continuous stream of digital audio
information is converted into a digitally modulated signal using a
technique known as linear pulse code modulation.
Digital-to-analog conversion (used in playback) is the exact oppo-
site of the analog-to digital conversion process and is illustrated in
Figure 2.
In digital-to-analog conversion, the PCM bitstream is converted at
the sampling frequency to a continuously changing series of
quantization levels which are individual "steps" of discrete voltage
equal to the quantization levels in the analog-to-digital process.
The shape of this continuously changing stream of quantization
levels approximates the shape of the original wave. This is shown
in the top half of Figure 2. This signal is then passed through a
low-pass filter, which removes the digital "switching noise." The
end result, shown in the bottom half of Figure 2 is an analog out-
put signal whose waveshape is a very close approximation of the
original analog input signal.
The foregoing is a very brief and, of necessity, oversimplified ex-
planation of how digital audio works. For the interested reader,
the book Principles of Digital Audio by Ken C. Pohlmann, copy-
right 1985 by Howard W. Sams, is highly recommended.