Multirate Digital Signal Processing, Oversampling of Analog-to-Digital Conversion, and Undersampling of Bandpass Signals - Digital Signal Processing: Fundamentals and Applications - page 601

12.4 APPLICATION EXAMPLE: CD PLAYER
Figure 12.37 illustrates a CD playback system, also described earlier in this chapter. A laser
optically scans the tracks on a CD to produce a digital signal. The digital signal is then
demodulated, and parity bits are used to detect bit errors due to manufacturing defects, dust, and so
on, and to correct them. The demodulated signal is again oversampled by a factor of 4 and hence
the sampling rate is increased to 176.4 kHz for each channel. Each digital sample then passes
through a 14-bit DAC, which produces the sample-and-hold voltage signals that pass the anti-image
lowpass filter. The output from each analog filter is fed to its corresponding loudspeaker. Over-
sampling relaxes the design requirements of the analog anti-image lowpass filter, which is used to
smooth out the voltage steps.
The earliest system used a third-order Bessel filter with a 3-dB gain attenuation at 30 kHz. Notice
that first-order sigma delta modulation (first-order SDM) is added to the 14-bit DAC unit to further
improve the 14-bit DAC to 16-bit DAC.
Let us examine the single-channel DSP portion as shown in Fi g ure 12.38 .
The spectral plots for the oversampled and interpolated signal xðnÞ , the 14-bit SDM output yðnÞ ,
and the final analog output audio signal are given in Figure 12.39 . As we can see in plot (a) in the
figure, the quantization noise is uniformly distributed, and only in-band quantization noise (0 to 22.05
kHz) is expected. Again, 14 bits for each sample are kept after oversampling. Without using the first-
order SDM, we expect the effective ADC resolution due to oversampling to be

176 : 4
44 : 1

n ¼ 14 þ 0 : 5 log 2 ¼ 15 bits

which is fewer than 16 bits. To improve quality further, the first-order SDM is used. The in-band
quantization noise is then shaped. The first-order SDM pushes quantization noise to the high-
frequency range, as illustrated in plot (b) in Figure 12.39 . The effective ADC resolution now becomes

176 : 4
44 : 1

n ¼ 14 þ 1 : 5 log 2 0 : 86 z 16 bits

Amplified
left speaker

14-bit
DAC
1st-order
SDM
14-bit
DAC
1st-order
SDM

A nti-image
LP filter

Optical pickup
Demodulation
Error correction

4x
Over-
sampling

Anti-image
LP filter

CD

Amplified
right speaker

FIGURE 12.37
Simplified decoder of a CD recording system.