Introduction to Digital Signal Processing - Digital Signal Processing: Fundamentals and Applications - page 7

Digital notch filter for
eliminating 60-Hz
interference

ECG signal
with 60-Hz
interference

ECG recorder with
the removed 60 Hz
interference

60-Hz
interference

ECG
preamplifier

FIGURE 1.8
Elimination of 60-Hz interference in electrocardiography (ECG).

This technique can also be applied to remove 60-Hz interference in audio systems. This topic is
explored in depth in Chapter 8.

1.3.3 Speech Coding and Compression
One of the speech coding methods, called waveform coding , is depicted in Figure 1.9 A, describing the
encoding process, while Figure 1.9 B shows the decoding processing. As shown in Figure 1.9 A, the
analog signal is first sent through an analog lowpass filter to remove high frequency noise components
and is then passed through the ADC unit, where the digital values at sampling instants are captured by
the DS processor. Next, the captured data are compressed using data compression rules to reduce the
storage requirements. Finally, the compressed digital information is sent to storage media.
The compressed digital information can also be transmitted efficiently, since compression reduces the
original data rate. Digital voice recorders, digital audio recorders, and MP3 players are products that
use compression techniques (Deller et al., 1993; Li and Drew, 2004; Pan 1985).
To retrieve the information, the reverse process is applied. As shown in Figure 1.9 B, the DS
processor decompresses the data from the storage media and sends the recovered digital data to DAC.
The analog output is acquired by filtering the DAC output via the reconstruction filter.

1.3.4 Compact-Disc Recording System
A compact-disc (CD) recording system is described in Figure 1.10 A. The analog audio signal is sensed
from each microphone and then fed to the anti-aliasing lowpass filter. Each filtered audio signal is
sampled at the industry standard rate of 44.1 kilo-samples per second, quantized, and coded to 16 bits for
each digital sample in each channel. The two channels are further multiplexed and encoded, and extra
bits are added to provide information such as playing time and track number for the listener. The encoded