Infinite Impulse Response Filter Design - Digital Signal Processing: Fundamentals and Applications - page 341

8.5 APPLICATION: DIGITAL AUDIO EQUALIZER
In this section, the design of a digital audio equalizer is introduced. For an audio application such as the
CD player, the digital audio equalizer is used to adjust the sound as one desires by changing filter gains
for different audio frequency bands. Other applications include adjusting the sound source to take
room acoustics into account, removing undesired noise, and boosting the desired signal in the specified
passband. The simulation is based on the consumer digital audio processor d such as a CD player d
handling the 16-bit digital samples with a sampling rate of 44.1 kHz and an audio signal bandwidth at
22.05 kHz. A block diagram of the digital audio equalizer is depicted in Figure 8.24 .
A seven-band audio equalizer is adopted for discussion. The center frequencies are listed in
Table 8.10 . The 3-dB bandwidth for each bandpass filter is chosen to be 50% of the center frequency.
As shown in Figure 8.24 , g 0 through g 6 are the digital gains for each banspass filter output and can be
adjusted to make sound effects, while y 0 ðnÞ through y 6 ðnÞ are the digital amplified bandpass filter
outputs. Finally, the equalized signal is the sum of the amplified bandpass filter outputs and itself. By
changing the digital gains of the equalizer, many sound effects can be produced.

x ()

bandpass filter

yn

0 ()

g 0

bandpass filter yn

1 ()

g 1

x () y ()

yn

6 ()

bandpass filter

g 6

FIGURE 8.24

Simplified block diagram of the audio equalizer.

Table 8.10 Specifications for an Audio Equalizer

Center frequency (Hz) 100 200 400 1000 2500 6000 15000

Bandwidth (Hz) 50 100 200 500 1250 3000 7500