Sampling and quantization - Continuous and Discrete Time Signals and Systems - page 413

Table 9.2. Raw data rates for digital audio used in commercial applications

Bandwidth Sampling rate Quantization Raw data rate

Applications (Hz) (samples/s) levels ( L ) (bytes/s)

2 8 Telephone [phone]

2 8

AM radio 11 025 11 025

FM radio (stereo) 22 050 2 16 88 200

CD (stereo) 20-20 000 44 100 2 16 176 400

Digital audio tape (stereo) 20-20 000 48 000 2 16 192 000

and the AM radio, are sampled at a relatively low sampling rate followed by
a coarse quantizer to generate the PCM sequence. The quality of the recon-
structed audio is moderate in such applications. In high-fidelity applications,
for example the FM radio, compact disc (CD), and digital audio tape (DAT),
the sampling rate is much higher to ensure accurate reconstruction of the high-
frequency components. The number of levels in the quantizer is also increased
to 2 16 to reduce the effect of the quantization error. Two channels, one for
the right speaker and the other for the left speaker, are transmitted for high-
fidelity applications. Compared to a single channel, the data rate is effectively
doubled with the transmission of two channels. The CD and DAT provide
excellent audio quality and are generally recognized as world standards for
achieving fidelity of audio reproduction that surpasses any other existing tech-
nique. In the following section, we discuss the CD digital audio system in more
detail.

9.4 Compact discs

The compact disc (CD) digital audio system was defined jointly in 1979 by the
Sony Corporation of Japan and the Philips Corporation of the Netherlands. The
most important component of the CD digital audio system is an optical disc
about 120 mm in diameter, which is used as the storage medium for recording
data. The optical disc is referred to as the compact disc (CD) and stores about
10 10 bits of data in the form of minute pits. To read data, the CD is optically
scanned with a laser.
Before music can be recorded on a CD, it is preprocessed and converted
into PCM data. The schematic diagram of the preprocessing stage for a single
music channel is illustrated in Fig. 9.14(a). Each channel of the music signal is
amplified and applied at the input of a lowpass filter (LPF), referred to as the anti-
aliasing filter. Since the human ear is only sensitive to frequency components
within the range 20 Hz-20 kHz, the anti-aliasing filter limits the bandwidth of
the input channel to 20 kHz. Following the anti-aliasing filter is the PCM system,
which converts the CT music channel into binary data. The sampling rate used in