Hearing Mixed Sounds and Clipping
Sounds can be mixed together by adding together the numbers that represent
the sounds. In order to control their relative loudness, we can change the
amplitude of the individual sounds. In this applet, we mix two SineOscillators
using an AddUnit.
Important: the sound card in your computer can only handle a limited
range of numbers. That range typically corresponds to a range of –1.0
to +1.0 in JSyn. If you add together oscillators and their sum goes beyond
that range, then the result will be "clipped" to fit within
this range. This can introduce severe distortion and can sound very bad
if you are trying to produce a smooth tone. If you are trying to produce
a nasty sound, then this can be a useful technique. Guitar distortion
boxes use a similar technique, but they do not clip abruptly at –1.0
and +1.0. They have a gradual distortion that imitates the saturation
of a tube amplifier. To prevent clipping, set the amplitudes of your sounds
so that they cannot sum to over 1.0. For example, if you have N
oscillators, set the amplitude to (1.0/N).
To do:
- Set the amplitude of each oscillator to about 0.4.
- Change the frequencies of each oscillator so that you can hear each
of them distinctly.
- For safety, turn down the volume of your stereo speakers until it
is at a soft level.
- Set the amplitudes of each oscillator to about 0.6. See and hear the
clipping that results.
- The scope shows the color-coded output of both oscillators and the
mixer. Uncheck the Auto box to stop the scope. Convince yourself that
the mixer output is the sum of the two oscillators. (It may appear to
be very slightly delayed.)
- Set both the amplitudes to 0.4. Set the frequencies almost the same
but not quite. The wavering effect is called "beating." Notice
that the mixed signal is large when the oscillators rise together, but
small when they move in opposite directions. The beat frequency is the
difference in frequencies of the two oscillators.
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