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.
