Sampling and quantization - Continuous and Discrete Time Signals and Systems - page 414

Fig. 9.14. Storing digital music
on a compact disk.
(a) Preprocessing stage to
convert CT music channels into
PCM data. (b) Multiplexing stage
to interleave data from multiple
channels.

pulse code modulation (PCM)

channel C m of
CT music signal

digital data
for channel C m

anti-aliasing
filter

sample and
hold

analog to digital
conversion

amplifier

(a)

channel C 1

PCM

channel C 2

PCM

error
protection

multiplexer

CD

channel C N

PCM

(b)

the sample-and-hold circuit is 44.1 ksamples/s, which exceeds the Nyquist rate
by a margin of 4.1 ksamples/s. The additional margin reduces the complexity
of the anti-aliasing filter by allowing a fair transition bandwidth between the
pass and stop bands of the filter. The audio samples obtained from the sample-
and-hold circuit are quantized using 2 16 -level uniform quantization. Finally,
each quantized sample is encoded with a 16-bit codeword, which results in a
raw data rate of (44 100 samples/s 16 bits/sample) = 705.6 kbits per second
(kbps) or 705.6/8 = 88.2 kBytes per second (kBps).
For high-fidelity performance, several channels of the music signal are
recorded on a CD. For commonly used stereo systems, only two channels
corresponding to the left and right speakers are recorded. Many home theatre
systems now record a much higher number of channels to simulate surround
sound and other audio effects. Each channel of the music signal is prepro-
cessed by the system illustrated in Fig. 9.14(a) and converted into raw PCM
data. Figure 9.14(b) illustrates the multiplexing stage, where data streams from
different channels are interleaved together into a single continuous bit stream.
The final step in the multiplexing stage is an error control scheme, which adds
an additional layer of protection to the music data. Any scanning errors that
were introduced whilst data were being read out from the CD are concealed
by the error control scheme. The output of the error control circuit is stored on
the CD. To record more music on a single CD, PCM data may be compressed
using an audio compression standard such as MP3.
A CD player reverses each step illustrated in Fig. 9.14. Data read from the
CD is checked for possible scanning errors. After correcting or concealing
the detected errors, the data streams for the individual channels are derived from
the interleaved bit stream. By following the reconstruction procedure outlined
in Section 9.1.1, each data stream is used to reconstruct the corresponding music