of Microprocessors

SOUND MODIFICATION METHODS 47
playback signal so the original source merely passes through the mixer and is
recorded. A split second later, this signal is picked up by the playback head,
mixed with the continuation of the original, and recorded again. Two split
seconds later, it is passed around the loop again and so on. If each rerecording
of the echo is weaker than the previous one, the echos eventually
die out like normal echos. However, if the re-recording is at the same or
greater amplitude, the echos continue unabated or build up until distortion
is produced.
Of course, the signal is degraded a little every time it is played and rerecorded.
Ifthe feedback factor is just a little beyond unity (re-recorded just a
trifle louder than the original), the middle frequencies will continue to build
up while the low and high extremes will slowly die out. If the original signal
is an isolated sound shorrer than the delay time, the result is an interesting
change in timbre each time the sound repeats. If the prQcess is allowed to
continue long enough, the sound will eventually degrade into a howl with a
periodic amplitude envelope. The user may also change the echo re-record
volume in response to what he hears and improvise. Many sounds in the
category of "the saucer lands" have been produced in exactly this way.
Tape echo in conjunction with other tape-editing tricks can also lead to
interesting results. For example, if a recorded sound is played backward, is
re-recorded with echo added, and the new recording is again played backward,
the echos wind up preceding the sounds, a very strange effect. "Preverb"
would be an apt name for such a process. Different effective echo times with
fixed head spacing may be accomplished by a speed transposition prior to the
addition of reverberation followed by an inverse transposition.
Tape-echo techniques are so powerful that specialized machines have
been built exclusively for that purpose. Multiple playback heads with irregular
spacing was one attempt to provide enough multiple, random echos so
that the result resembled concert hall reverberation rather than Grand Canyon
reverberation. Another refinement was easily movable playback heads to
vary the echo rate. A particularly interesting effect is produced if the head is
moved while the recording is made. The echos would then be of a different
pitch than the original sound!
Multitrack Recorders
Finally, tape equipment can be, and usually is, used to combine sounds
together for the final result. Unfortunately, direct recording of one sound on
top of another on the same tape track cannot be done very well. The highfrequency
bias signal necessary for high-fidelity tape recording tends to erase
previously recorded sounds. Two recorders or a single multitrack recorder,
however, can be used to combine sounds onto one track.
In a typical multitrack setup, the separate sounds to be combined are
each recorded on a separate track of the same tape. When played, the signal