programming with max/msp - Virtual Sound

Chapter 4T - Control signals
4.1 CONTROL SIGNALS: STEREO PANNING
As we have seen throughout this book, envelope generators can be used to vary
the parameters of a sound, such as frequency or amplitude, in time. Signals that
come from envelope generators – signals that are expressly produced to vary
parameter values rather than to produce sound – are called control signals.
To this point, we have used line segments and exponential curves to describe
control signals. These techniques are efficient because they require only a few
values to describe parameter changes; think of the four segments that we use
for ADSR envelopes, or of the numbers that define an exponential curve, which
can completely describe a glissando. Other control signals, however, may need
to vary in more complex ways. Take the vibrato associated with a string instrument
as an example: there is a continuous change in the frequency of a note
played with vibrato that can best be described as an oscillation around a central
pitch. To simulate such a vibration using an envelope, we might need to use
tens, or even hundreds, of line segments, which would be both tedious and
impractical. Instead of resorting to such primitive methods, we might instead
choose to use a control oscillator, an oscillator whose output is produced for
the sole purpose of providing parameter values to audio oscillators or other
parameterized devices used in sound synthesis and signal processing.
Control oscillators are usually Low Frequency Oscillators (LFOs); their
frequency is usually below 30 Hz. They produce continuously changing control
values that trace waveforms in the same way that audio oscillators do. Every
instantaneous amplitude of a wave generated by a control oscillator corresponds
to a numeric value that can be applied to audio parameters as needed.
Here is an example demonstrating the use of an LFO: in Figure 4.1, you see a
graphic representation of an LFO controlling the position of a sound in space.
This LFO generates a sine wave that oscillates between MIN (representing its
minimum value) and MAX (its maximum value).
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Fig. 4.1 An LFO for controlling the position of a sound in space
The minimum and maximum values, or the depth of the oscillator, define the
limits of the amplitude values being produced, while the rate of the oscillator
is a value that defines its frequency.
from “Electronic Music and Sound Design” Vol. 1 by Alessandro Cipriani and Maurizio Giri
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