Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
4
number of discrete levels. The range of the
integers possible is determined by the bit depth,
the number of bits used per sample. A sample’s
amplitude must be rounded to the nearest of the
allowable discrete levels, which introduces error
in the digitization process.
When sound is recorded in digital format,
a sampling rate and a bit depth are specified.
Often there are default audio settings in your
computing environment, or you may be prompted
for initial settings, as shown in Figure 5.3. The number of channels must also be specified
mono for one channel and stereo for two. (More channels are possible in the final production,
e.g., 5.1 surround.)
Figure 5.3 Prompting for sampling rate, bit depth, and number of channels
A common default setting is designated CD quality audio, with a sampling rate of
44,100 Hz, a bit depth of 16 bits (i.e., two bytes) per channel, with two channels. Sampling rate
and bit depth has an impact on the quality of a recording. To understand how this works, let’s
look at sampling and quantization more closely.
5.1.2.2 Sampling and Aliasing
Recall from Chapter 2 that the continuously changing air pressure of a single-
frequency sound can be represented by a sine function, as shown in Figure 5.4.
One cycle of the sine wave represents one cycle of compression and rarefaction
of the sound wave. In digitization, a microphone detects changes in air
pressure, sends corresponding voltage changes down a wire to an ADC, and the
ADC regularly samples the values. The physical process of measuring the
changing air pressure amplitude over time can be modeled by the mathematical
process of evaluating a sine function at particular points across the horizontal
axis.
Flash
Tutorial:
Sampling
and Aliasing
Aside: It's possible to use real numbers
instead of integers to represent sample
values in the computer, but that doesn't get
rid of the basic problem of quantization.
Although a wide range of samples values
can be represented with real numbers,
there is still only a finite number of them,
so rounding is still be necessary with real
numbers.
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