Page 199 - Asterisk™: The Future of Telephony

sound and send that information to the far end? The original waveform wouldn’t get
there, but all the information needed to reconstruct it would.
This is the principle of all digital audio (including telephony): sample the characteristics
of the source waveform, store the measured information, and send that data to the far
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 principle 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 spe-
cific 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 ac-
curacy, but more bandwidth will be required to transmit this more detailed informa-
tion.

To get a better idea of how PCM works, consider the waveform displayed in Figure 7-2.
To digitally encode the wave, it must be sampled on a regular basis, and the amplitude
of the wave at each moment in time must be measured. The process of slicing up a
waveform into moments in time and measuring the energy at each moment is called
quantization, or sampling.
The samples will need to be taken frequently enough and will need to capture enough
information to ensure that the far end can re-create a sufficiently similar waveform. To
achieve a more accurate sample, more bits will be required. To explain this concept,
we will start with a very low resolution, using four bits to represent our amplitude. This
will make it easier to visualize both the quantization process itself and the effect that
resolution has on quality.

‖ 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.

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