AN INSTRUMENT YOU PLAY WITH AN INSTRUMENT
The relationship between a musician and a musical instrument is
beautiful and subtle. Years are spent to learn musical techniques
and musical material. In the best musicians, the instrument becomes
an extension of the body's capacity for expression. Now that electronic
modification of musical sound has become commonplace, it is time
to look at musical electronics as an extension of musical instruments.
The newest additions to the musical electronic "arsenal,"
Instrument Controlled Synthesizers, merit special attention and
an expansion of our concepts of musical expression. Instrument-controlled
synthesizers are musical instruments that are activated by other
musical instruments. In the same way that a guitar is the link between
a musician's body and the outside world, an instrument-controlled
synthesizer is "played" by the output of a musical instrument.
This concept is vitally important if you wish to extract the maximum
musical potential from these new instruments, or to effectively
Many people think of musical effects as pushbutton boxes that simply
"work" when used with a musical instrument. This conception,
which is weak even when applied to musical effects, breaks down
when applied to instrument-controlled synthesizers. It is almost
like expecting a saxophone to just "work" the first time
you pick it up. Even a kazoo is not that accommodating. A musician
who understands that the instrument-controlled synthesizer is itself
a musical instrument, with all the subtleties of any other musical
instrument, will be willing to devote the time and attention to
perfect a new technique in his musical language.
Much of the initial confusion about (and hence, resistance to)
instrument- controlled synthesizers centers around the notion that
you have to "play into" them. But you even have to "play
into" a fuzz-tone, and you certainly have to "play into"
your native musical instrument. That is one of the beautiful aspects
of making music: the musician's skill in controlling the instrument,
or basic musicianship. So I would ask you, if you have a notion
that "playing into" any musical device is a drawback,
alter your thinking into the much more useful notion of simply "playing"
the device. As with other musical instruments, the identity and
charm of instrument-controlled synthesizers will derive as much
from their peculiarities and limitations as from their inherent
EXTRACTING CONTROL SIGNALS FROM MUSICAL SOUNDS
If you've read this far, you are probably curious about how one
musical instrument can "play" another. It's a fascinating
subject. The human ear can discern the many aspects and subtleties
of musical sounds with great precision and in many dimensions. But
how does an electronic "box" decipher the complex information
in a musical signal, and turn it into control voltages and parameters
for a synthesizer? Moreover, what kinds of control information can
we hope to derive from a musical signal?
If we consider a single note played by a musical instrument, we
can characterize it by a few different aspects: time length; loudness;
pitch; tone color. Obviously, there are more aspects than these,
and there are subtleties in defining each aspect. We have to come
up with working definitions that can be translated into "sense-able"
quantities: reduction of a musical quality to a number or a voltage.
Let's look at the four aspects just mentioned.
Starting with a note's time length: The basic question is, "Is
a note being played or not?" With some instruments, particularly
wind instruments, the question is fairly easy. Wind instruments
have a limited dynamic range and a fairly quick decay after a note
is released. But with a guitar, the note fades gradually into inaudibility.
When can we say it is "off"? Let's say if the signal goes
below 1 millivolt we will call it "off." Well, wouldn't
you just guess that in the process of dying out, the guitar note
sometimes gets a little louder before it gets a little softer! So
a note enable detector, as we call the note "on " or "of"
sensing device, must be sophisticated enough to take into account
the subtle amplitude variations in a musical signal without producing
"false" outputs. To roll this all up into a definition:
A note enable detector senses the amplitude of a musical signal
and produces an on output if a note is audible and an off output
if a note is not audible. This is a "digital" output signal.
The loudness of a note is also related to the amplitude of the
musical signal. Loudness is a relative or "analog" quantity,
and is approximately measured by the musical signals "amplitude
envelope." The "amplitude envelope" is essentially
what is read by a VU meter on a mixing console: a continuous indication
of the "average" AC signal voltage. Instead of using a
meter, we convert the average AC signal into a corresponding DC
signal. The envelope voltage is then used as a control parameter
relating to signal loudness.
There is another way to measure loudness. In a guitar, the loudness
is determined by the magnitude with which a note is "struck."
All of the information about loudness comes at the beginning of
the struck note. We need a "transient detector" to determine
when a note is struck, and something like a "peak follower”
to determine how high the peak value is before it dies down. So
we have two new pieces of information-gathering hardware about the
note: the attach detector (a digital signal that records the moment
of a new note's attack) and the peak follower which records the
amplitude at the time of attack.
might also assign the attack detector to the function performed
by the enable detector, since it tells when a new note has been
initiated. Actually it does function in conjunction with the enable
detector. Imagine the case of slurred but accented notes in quick
succession. The enable detector would indicate that only one note
was played, because the signal amplitude never reached the "off"
level. But the attack detector would record the beginning of each
new note. Together, the attack and enable detectors accurately specify
note time-lengths. The attack detector, peak follower and envelope
follower specify the note's loudness dynamics.
Pitch, of course, is simple. You merely have to find the fundamental
frequency of the note being played, and produce a control voltage
relating to it. Unfortunately, the fundamental frequency is often
masked by other overtones of the musical signal, attack transients,
wind noise, or interfering signals from other notes. Other notes!!
You mean you can only extract the pitch of one note at a time? Well,
yes. With presently available technology, only one "pitch"
can be extracted at a time, if a "single input" system
is used. A single input system is a system that accepts the musical
instrument input- whether the instrument is polyphonic or not-through
one input jack. Instrument-controlled synthesizers which are polyphonic
must essentially use a separate isolated "pickup" for
each voice employed. These concepts and instruments (notably, specialized
polyphonic guitar synthesizers) would greatly expand the scope of
this article, so I will deal with only the theory and implementation
of "single voice" synthesizers. We will define a pitch
follower as a device which tracks a musical note's fundamental frequency
and converts it to an appropriate control voltage. A pitch follower
is not simple.
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