System information
end. Then, at the far end, use the transmitted information to generate a completely new
audio signal that has the same characteristics as the original. The reproduction is so
good that the human ear can’t tell the difference.
The principal advantage of digital audio is that the sampled data can be mathematically
checked for errors all along the route to its destination, ensuring that a perfect duplicate
of the original arrives at the far end. Distance no longer affects quality, and interference
can be detected and eliminated.
Pulse-Code Modulation
There are several ways to digitally encode audio, but the most common method (and
the one used in telephony systems) is known as Pulse-Code Modulation (PCM). To
illustrate how this works, let’s go through a few examples.
Digitally encoding an analog waveform
The principle of PCM is that the amplitude
‖
of the analog waveform is sampled at
specific intervals so that it can later be re-created. The amount of detail that is captured
is dependent both on the bit resolution of each sample and on how frequently the
samples are taken. A higher bit resolution and a higher sampling rate will provide
greater accuracy, but more bandwidth will be required to transmit this more detailed
information.
To get a better idea of how PCM works, consider the waveform displayed in Figure A-2.
Figure A-2. A simple sinusoidal (sine) wave
‖ Amplitude is essentially the power or strength of the signal. If you have ever held a skipping rope or garden
hose and given it a whip, you have seen the resultant wave. The taller the wave, the greater the amplitude.
Digital Telephony | 601
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