The third attribute of a sound, timbre, depends on the presence or absence of overtones,
or harmonics. Any complex waveform is actually a mixture of sine waves of different
amplitudes, frequencies, and phases (the starting point of the waveform on the time
axis). These component sine waves are called harmonics. A square waveform, for
example, has an infinite number of harmonics.
In summary, all steady sounds can be described by their frequency, overall amplitude,
and relative harmonic amplitudes. The audible equivalents of these parameters are pitch,
loudness, and timbre, respectively. Changing sound is a steady sound whose parameters
change over time.
In electronic production of sound, an analog device, such as a tape recorder, records
sound waveforms and their cycle frequencies as a continuously variable representation of
air pressure. The tape recorder then plays back the sound by sending the waveforms to
an amplifier where they are changed into analog voltage waveforms. The amplifier sends
the voltage waveforms to a loudspeaker, which translates them into air pressure
vibrations that the listener perceives as sound.
A computer cannot store analog waveform information. In computer production of sound,
a waveform has to be represented as a finite string of numbers. This transformation is
made by dividing the time axis of the graph of a single waveform into equal segments,
each of which represents a short enough time so the waveform does not change a great
deal. Each of the resulting points is called a sample. These samples are stored in memory,
and you can play them back at a frequency that you determine. The computer feeds the
samples to a digital-to-analog converter (DAC), which changes them into an analog
voltage waveform. To produce the sound, the analog waveforms are sent first to an
amplifier, then to a loudspeaker.
Figure 5-2 shows an example of a sine wave, a square wave, and a triangle wave, along
with a Table of samples for each.
NOTE
The illustrations are not to scale and there are fewer dots in the wave forms than there
are samples in the Table. The amplitude axis values 127 and -128 represent the high and
low limits on relative amplitude.
- 132 Audio Hardware -
To Next Page
Loading...
