of Microprocessors

78 MUSICAL ApPLICATIONS OF MICROPROCESSORS
depression of a key. Its primary function is ro mark the beginning of a note.
The other signal is called a gate and is used to mark the duration of the note.
The gate signal is present as long as a key is pressed. Thus) these two timing
signals qualify the control voltage output from the keyboard.
Generators are similar to transducers but generally produce a predefined
type of output signal that can be influenced with mechanical and control
inputs but not completely determined in detail as with a transducer. A good
example of an audio signal generaror is a voltage-controlled oscillator. Typically,
several outputs are provided, each one supplying a different but fixed
waveform at a fixed amplitude. The voltage level at the control input directly
affects thefrequency ofthe multiple waveform outputs, but the waveshape and
amplitude, which are fixed by design, remain constant. A good example of a
control voltage generator is the envelope generator. This device supplies a
predetermined voltage contour in response to the trigger and gate signals
mentioned earlier. Mechanical controls generally specify details about the
conrour generated, although rarely a control voltage might be able to specify
some of these.
Modifiers typically accept signal inputs and control inputs and produce
a signal output. Modification of one or more parameters of the input signal is
performed in accordance with the voltage levels at the control inputs. A
voltage-controlled amplifier is a good example of a modifier. Typically, the
signal input is of constant amplitude, but the amplitude of the output is
determined by the control input.
Interconnections
In order to perform a useful funerion, the synthesizer modules must be
interconnected. A true general-purpose synthesizer provides only mechanical
mounting and operating power to the modules; otherwise they are completely
independent. Probably the most popular connection method involves
the use of patch cords similar to those found in old telephone switchboards
but with both ends free. The cords and standard 1/4-inch phone plugs are
quite flexible and rugged and allow fully shielded connections that minimize
noise pickup. A particular arrangement of patch cords is called a patch. A
complex patch may involve so many cords that a ratts-nest appearance results
that may even obscure the front panel of the synthesizer. In such a situation,
it may become difficult to follow a particular cord through the maze without
pulling on one end of it. Even so, most users love the patch cord concept and
on occasion can be seen to point proudly at the "jungle" that represents the
creation of a new sound.
Another popular interconnection technique uses a pinboard matrix.
The matrix is divided into rows representing module outputs and columns
representing module inputs. Each row and column is clearly labeled with
module number, type, and signal name. A connection from an output to an
input is made by inserting a pin at their point of intersection. An output may