of Microprocessors

68 MUSICAL ApPLICAnONS OF MICROPROCESSORS
would restore the original signal unaltered. Under certain conditions, however,
the effects of these devices are interesting and useful.
Envelope Modifiers
Although spectrum modification of sounds can have a profound effect
on their timbre, altering the overall amplitude envelope can also have a
significant effect. Perhaps the simplest envelope modification is flattening of
the decay in order to give a steady sound amplitude that eventually cuts off
suddenly. Thus, a plucked string instrument such as a guitar comes out
sounding more like an organ.
The oldest and simplest method of doing th.is is to use a clipping circuit
with a low clipping threshold. As long as the input amplitude was significantly
above the clipping point, the output amplitude remained relatively
constant. Of course, such a device aiso severely distorts the spectrum.
Later, envelope follower circuits that generated a voltage proportional
to the amplitude of an input signal were developed. This voltage could then
be used to control the gain of a separate amplifier in any desired manner.
Thus, the guitar sustain device would process the evelope follower signal
such that the amplifier gain would increase enough to cancel the decrease in
input signal amplitude and give a constant output. By suitable processing of
the envelope signal, the attack, sustain, and decay times ofthe output can be
adjusted relatively independently of the envelope characteristics of the input.
Electronic Reverberation
Even util.izing all of the modification techniques mentioned so far,
many purely electronic sounds have a "lifeless" quality that is often undesirable.
Live sounds from an orchestra, on the other hand, have a depth and
richness that cannot be easily explained in terms of frequency, amplitude,
and spectrum.
The concert hall itself adds considerably to the texture of orchestra
music. In fact, an otherwise "dry" recording of electronic sounds is considerably
enhanced by playing through loudspeakers scattered around the stage of
a good concert hall. The difference, ofcourse, is the presence ofreverberation
in the hall in which a combination of direct and reflected sound reaches the
listener. The reflected sound can come from any direction and with different
time delays with respect to the direct sound. Reflection can also occur several
times, each with diminishing amplitude.
Because of the multitude of delay times, concert hall reverberation and
tape reverberation are considerably different. The latter consists of distinct
echoes, while the former has no perceivable echo at all. A multitude of
techniques and devices has been developed to electronically simulate concert
hall reverberation in tape recording.