System information

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 4 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.
Figure A-3 shows the information that will be captured when we sample our sine wave
at 4-bit resolution.
Figure A-3. Sampling our sine wave using four bits
At each time interval, we measure the amplitude of the wave and record the corre-
sponding intensity—in other words, we sample it. You will notice that the 4-bit reso-
lution limits our accuracy. The first sample has to be rounded to 0011, and the next
quantization yields a sample of 0101. Then comes 0100, followed by 1001, 1011, and so
forth. In total, we have 14 samples (in reality, several thousand samples must be taken
per second).
If we string together all the values, we can send them to the other side as:
0011 0101 0100 1001 1011 1011 1010 0001 0101 0101 0000 1100 1100 1010
On the wire, this code might look something like Figure A-4.
When the far end’s digital-to-analog (D/A) converter receives this signal, it can use the
information to plot the samples, as shown in Figure A-5.
602 | Appendix A: Understanding Telephony