Compact Disc
John Linsley Hood16.1 Problems with Digital Encoding ...........................................................................
16.1.1 Quantization Noise
Although a number of ways exist by which an analogue signal can be converted into its
digital equivalent, the most popular, and the technique used in the CD, is the one known
as “ pulse code modulation, ” usually referred to as “ PCM. ” In this, the incoming signal is
sampled at a suffi ciently high repetition rate to permit the desired audio bandwidth to be
achieved. In practice, this demands a sampling frequency somewhat greater than twice the
required maximum audio frequency. The measured signal voltage level, at the instant of
sampling, is then represented numerically as its nearest equivalent value in binary coded
form (a process which is known as “ quantization ” ).
This has the effect of converting the original analogue signal, after encoding and
subsequent decoding, into a voltage “ staircase ” of the kind shown in Figure 16.1.
Obviously, the larger the number of voltage steps in which the analogue signal can be
stored in digital form (that shown in the fi gure is encoded at “ 4-bit ” –2^4 or 16 possible
voltage levels), the smaller each of these steps will be and the more closely the digitally
encoded waveform will approach the smooth curve of the incoming signal.
The difference between the staircase shape of the digital version and the original analogue
waveform causes a defect of the kind shown in Figure 16.2 , known as “ quantization
error, ” and because this error voltage is not directly related in frequency or amplitude to
the input signal, it has many of the characteristics of noise and is therefore also known
as “ quantization noise. ” This error increases in size as the number of encoding levels is
reduced. It will be audible if large enough, and is the fi rst problem with digitally encoded
CHAPTER 16