Audio Engineering

Compact Disc 501

This diffi culty is illustrated by the graph of Figure 16.4 , which shows the relative
group delay and phase shift introduced by a conventional low-pass analogue
fi lter circuit of the kind shown in Figure 16.5. The circuit shown gives only a modest
−90-dB/octave attenuation rate, while the actual slope necessary for the required
antialiasing characteristics (say, 0 dB at 20 kHz and 60 dB at 22.05 kHz) would be
426 dB/octave. If a group of fi lters of the kind shown in Figure 16.5 were connected in
series to increase the attenuation rate from 90 to 426 dB/octave, this would cause a
group delay, at 20 kHz, of about 1 ms with respect to 1 kHz and a relative phase shift of

0dB

50 dB

100 dB

150 dB

200 dB

10 100 1 K 10 K 100 K

Frequency (Hz)

0

 200

 400

 600

 800

200

100

0

Delay

Phase

Gain (dB)

Phase (degrees)

Delay (microseconds)

Figure 16.4 : Responses of a low-pass LC fi lter.

L 1

17.35 mH 24.84 mH 25.26 mH 24.84 mH 17.35 mH

L 2 L 3 L 4 L 5

C 1

8.905 nF

C 2

9.467 nF

C 3

9.467 nF

C 4

8.905 nF

R Load

Source impedance 1 K0 1K0

Input

0V

Output

0V

Figure 16.5 : Steep-cut LP fi lter circuit.