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Now, you might think, “If human speech uses frequencies between 200-9,000 Hz and the
normal telephone channel only transmits frequencies from 300-3,400 Hz, how can you
understand human conversation over the phone?” That's a great question! Studies have found
that telephone equipment can accurately transmit understandable human conversation by
sending only a limited range of frequencies. The telephone channel frequency range
(300-3,400 Hz) gives you enough sound quality to identify the remote caller and sense
their mood. The telephone channel frequency range does not send the full spectrum of
human voice inflection and lowers the actual quality of the audio. For example, if you've
ever listened to talk radio, you can always tell the difference in quality between the radio
host and the telephone caller.
Nyquist believed that you can accurately reproduce an audio signal by sampling at twice
the highest frequency. Because he was after audio frequencies from 300-4,000 Hz, it
would mean sampling 8,000 times (2 * 4000) every second. So, what's a sample? A sample
is a numeric value. More specifically, in the voice realm, a sample is a numeric value that
consumes a single byte of information. As Figure 1-12 illustrates, during the process of
sampling, the sampling device puts an analog waveform against a Y-axis lined with nu-
meric values.
127
2
1
-1
-2
1 Second
-127
Figure 1-12
Converting Analog Voice Signals to Digital
This process of converting the analog wave into digital, numeric values is known as quan-
tization. Because 1 byte of information can represent only values 0-255, the quantization
of the voice scale is limited to values measuring a maximum peak of +127 and a maximum
low of -127. Notice in Figure 1-12 that the 127 positive and negative values are not evenly
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