Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013

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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.