of Microprocessors

754 MUSICAL ApPLICATIONS OF MICROPROCESSORS
VERSION generator carries sound file reading even further. With VER­
SION, a particular note recorded in a sound file can be identified in terms of
file positions for the beginning of the attack, beginning of steady-state,
beginning of decay, and end of decay. Then, with additional parameters, the
note can be replayed at a different pitch and with a different duration (by
truncating or repeating the steady-state samples). Transitions from one
envelope segment to the next are smoothed by interpolation, the degree of
which can also be specified.
Many unit generators are quite simple. ADN for example has one
output and any number of inputs that are simply added together to form the
output. LOOKUP has an input, an output, and uses a wavetable to translate
the input to the output. Two parameters also specify the expected range of
inputs. Besides use as a nonlinear waveshaper, it can be used in a myriad of
ways to tailor response curves in instruments more efficiently than using
complex expressions in parameter fields. SEG is an envelope generator that
uses a waveform table for the envelope shape. The table is divided into a
number of segments (usually three), which correspond to envelope phases.
Parameters determine the amount of time spent in each segment according to
either constants in the SEG statement or variables from the NOTE
statement. A very useful feature is that the duration of designated segments
may be calculated automatically from the total note duration.
Complete instrument definitions require an INS statement at the
beginning and an END statement at the end. The INS statement includes an
"action time" field, which can be set nonzero to avoid using memory for the
instrument definition until needed. If the instrument is to be heard, the
OUT unit generator must also be included. OUT accepts the final
instrument output as an I/O block and sums it with other instruments that
may be playing into the C-MUSIC standard output sample stream, which
will ultimately go to a sound file. For stereo or quad output, OUT requires
two or four inputs. If an instrument is to sound in only one channel, the
other inputs are set to zero.
Figure 19-17 shows a complete, generalized, FM-type instrument
block diagram and its corresponding definition. Wavetable F2 is expected to
have an overall amplitude envelope shape, F3 a deviation envelope, and F4 an
envelope to vary the modulation frequency. Fl is just a sine wave for classical
FM, although it could be some other shape for complex FM. The instrument
is controlled by three P-fields in NOTE statements. P5 and P6 are the usual
duration and amplitude parameters, while P7 sets the average ratio of
modulation to carrier frequency. Below is the complete C-MUSIC instrument
definition for this instrument, which is called GENFM. Note that, since FM
synthesis can be adversely affected by table lookup truncation error, the size
of F1 has been set to 8,192 samples.
The function generation score section specifies the content of wavetables.
This section consists of a series of GEN statements, each of which will