of Microprocessors

676 MUSICAL ApPLICATIONS OF MICROPROCESSORS
a fixed argument array size of, say 32 bytes will be used for all Level 1
routines.
Figure 18-9 shows one possible way to arrange the five arguments into
fields within the 32-byte sound control block. With the 6502,68000, and most
other microcomputers, indexed addressing can be used to quickly access the
fields in the sound control block. One simply loads the address of the control
block passed by the caller into a register and then uses fixed offsets to access
the data. In a machine without indexed addressing, such as the 8080, the
control block can be copied into a fixed memory area, the samples computed,
and those descriptor elements changed by the execution (such as the table
pointer) copied back. Alternatively, the order of the fields within the block
can be carefully sequenced so that increment index instructions can be used
to scan through them.
The initial byte of activity flags is used by the upper level routines to
keep track of which control blocks are actually generating sound. The sound
10 byte identifies which Level 1 routine the control block is formatted for.
The wave table address is the address of the first entry of the waveform table
being used. The wave table pointer is shown as a 32-bit value, which consists
of a 22-bit fractional part and a lO-bit integer part. The integer part is added
to the wave table address to actually get a sample. The frequency parameter is
also shown as 32 bits and is the waveform table pointer increment. The
amplitude parameter gives the desired waveform amplitude, while the sample
buffer address indicates where the 30-sample buffer is located.
Figure 18-10 shows a flowchart for the tone generator subroutine.
Essentially, it goes through a sample computation loop 30 times and returns.
Note that the argument giving the address of the sample buffer is
incremented during execution. This is desirable and minimizes the overhead
associated with building up large sample blocks for mass storage devices,
such as 3,000 samples for IBM-style magnetic tape. The other calculations
perform standard waveform table lookup without interpolation as described
in Chapter 13.
The percussion generator routine is less straightforward than the tone
generator. In Chapter 15, it was seen that a wide variety of mechanisms
produce percussive sounds. Covering all of these in a single subroutine is not
practical. Figure 18-11 shows a model that can be used to approximate the
majority of common percussive instruments as well as numerous others. The
pulse-excited bandpass filter is used to provide a damped sine wave, while
the white noise source and multimode filter provide filtered noise. Three
gain controls are shown, which allow a noise envelope, the ratio of noise to
damped wave, and overall output amplitude to be independently controlled.
For maximum efficiency, amplitude control of the damped wave is controlled
by varying the intensity of the strike pulse, which only has to be done once,
rather than doing a sample-by-sample multiplication of the generator output.