Sequential prophet 5 - Microcomputer System

2-11 MICROCOMPUTER SYSTEM
It should be clear from the above that operation of the Prophet’s analog circuitry requires only some
CVs, GATE signals, and logic signals for the switches.
You don't need a computer to sound the voices. In a previous synth generation the required signals
might be provided by patch cords, mechanical switches, potentiometers, and a voltage-divider
keyboard. But such control would not usually be programmable or polyphonic, and that is why the
Prophet has a microcomputer system. “System" refers to the combination of electronic hardware and
computer software, or, program.
Figure 2-6 fills out the main elements of the microcomputer system and its input and output (I/O)
circuitry. If you are unfamiliar with microcomputers you might find it helpful to focus on the DATA
BUS (DB). Physically, this is a set of eight parallel wires carrying digital signals between the Z-80 cen-
trol processor unit (CPU), memory (PROM. RAM), and (I/O) devices.
Obviously, not all of the information transferable between data-bussed circuits will be able to be
communicated simultaneously. This would be like trying to hold a telephone conversation over a
party line with a dozen others. Control signals from the MEM ADR. IN PORT, and OUT PORT
decoders allow “conversation" between the CPU and any one other data-bussed device by, in effect,
inhibiting devices not involved with the conversation. In short, the decoder signals tell the MEM and
I/O devices when they are supposed to place data on the bus. or when data is intended for them.
Let's see what takes place to sound a single key. The CPU, of course, runs everything according to
unalterable program instructions contained in ^rasable/programmable read-only-memory (EPROM).
(The program is actually a continuous "loop".) The CPU places data and addresses on those busses,
and activates the -IOREQ and -WR signals. This produces CSS from the OUT PORT DCOD, which
latches the data into the keyboard, switch, and LED matrices. The matrix converts (digitizes) key
strokes into binary data. The address changes. -IOREQ and. this time -RD are activated. This produces
-CSE from the IN PORT DCOD, driving matrix data representing the key held on to the bus. The CPU
performs calculations on the matrix data to determine the actual position of the key pressed. It then
places the key number into the SCRATCH PAD RAM by supplying an appropriate address and -MREQ
to the MEM ADR DCOD, and -WR.
To sound the note the key number is read from the SCRATCHPAD and latched to the digital-to analog
converter (DAC) when the OUT PORT DCOD issues CSC VDAC, an analog voltage corresponding to
the latched key number, is distributed by the CV DMUX to, in this case. KEY 1 CV. CV DMUX ad¬
dresses are also latched by command from the OUT PORT DCOD. Finally, the GATE signal is latched
off the bus.
All of this activity is yet a minute portion of the whole loop. Of course, the program must be constantly
read from EPROM. In PRESET mode, the CPU also reads patch CV and S data from the NV PRGM
RAM. In MANUAL mode, the loop also includes a scan of the black switches and knobs. In either
mode, data output also operates the LEDs.
Although these operations are sequential, you perceive no delay between, for example, the key¬
stroke and the note produced because of the speed at which the program is executed. The program
loops about 100 times each second in MANUAL mode, and about 140 times in PRESET. (This amounts
to about 350 to 475,000 DB operations per second.)
Paragraphs 2-12 through 2-15 describe output and input circuit functions in more detail.
2-12 CONTROL VOLTAGE (CV) and SWITCH (S) SIGNAL GENERATION
The analog circuitry and microcomputer system have been outlined. This paragraph concentrates on
the form computer data must assume in order to control the voices.
2-12