of Microprocessors

SOUND" MODIFICATION METHODS 45
cutting quality of the voice disappears and the result sounds more like a
well-tuned belch. Although the exact explanation for this will be detailed
later, it is one case in which the phase relationship among harmonics in a
tone makes a big difference in timbre.
All kinds of percussive sounds are completely altered by backward
playing. In most cases, if the percussive sound is pitched at all, the sense of
pitch is heightened by being played backward. This is probably due to the
fact that for a short period following a sharp attack transient the ear is
recovering and is less able to perceive pitch.
Another possibility is splicing the same sound back to back to eliminate
the sudden cutoff that occurs when percussive sounds are played backward.
Using the attack portion for both the attack and decay, using the decay
portion for both, or even using the attack from one sound and dissolving into
the decay portion of another are all possibilities.
Speed Transposition
Another trick that nearly everyone who owns a reel-to-reel tape recorder
has tried at least once is playing the tape at a different speed than it was
recorded. A speed doubling changes speech into familiar monkey chatter,
while a speed halving produces drawn out groans. More useful effects are
created if, when producing the source material, the undesirable speedup or
slowdown effects of the processing are compensated for. Such an example is
the production of the once popular "chipmunk" records in which the original
vocals were sung much more slowly than usual in order to have a reasonable
tempo after the speed doubling.
More severe speed changes usually distort a sound beyond recognition.
Male speech when slowed by a factor of 16 or more comes out sounding like
rhythmic beating on a constantly changing drum. Other common, complex
sounds take on a new dimension when slowed substantially. The fine structure
that usually emerges is reminiscent of examining a pond water droplet
through a microscope.
Although standard tape speeds are usually powers of two times the base
speed of 15 inches/sec, many professional recorders can be easily run at intermediate
speeds. These machines usually have synchronous motors whose
speed is precisely determined by the power linefrequency rather than voltage.
Ifthe motor circuit is separated from the rest of the electronics and connected
to a power amplifier driven by a variable-frequency oscillator, the tape speed
may be continuously varied over a fairly broad range. The newest professional
recorders use a dc servo motor system whose speed is proportional to a
reference voltage, which is normally tightly regulated. Disconnecting the
reference and connecting a variable voltage source can give a very wide speed
range.
With precisely variable speed, it is possible to record a single sound or
note and convert it into all of the notes ofthe scale. A Christmas song "sung"