Sequential PROPHET-600 - PROGRAMMING

The independent modulation LFO is also "synthesized" by the computer. When
modulation destinations are enabled, the LFO is digitally summed into, for example,
the twelve separate oscillator frequency CVs. two pulse width (PW) CVs, or six filter
frequency CVs. The PITCH and MOD wheels are also fully digitized. The great benefit
of these changes is that there is no need for Common Analog circuitry with a dozen op
amp summers and matched resistors.
In the conversion of the formerly analog envelope generator, LFO, VCA , Glide, and
summer functions into digital form, one witnesses a specific example of the predicted
influence on design of the decreased cost of digital memory, compared to analog
hardware. But the -600 also benefits from the higher level of integration of quality
synth functions offered by some new analog ICs. The -600 has far fewer analog
components and adjustments than earlier Prophets. This all means lower price, lower
power consumption, less heat, better reliability, better oscillator and filter stability,
and easier service.
Figure 2-1 looks at the general functions more closely. Tracing backwards from the
AUDIO OUT jack, the six (polyphonic) voice outputs are summed and the overall level
set by the Final Voltage Controlled Amplifier (VCA). This VCA is controlled by the
MVOL CV which usually follows the setting of the VOLUME knob. During TUNE the
computer sets MVOL CV to zero so the process is not audible. In this case the TUNE
COMPARATOR routes selected oscillator or filter pulses to the TUNE circuitry.
Each voice contains voltage controlled oscillators A and B and a voltage-controlled
combination mixer, 24 dB/octave resonant low-pass filter, and amplifier. Connections
between these ICs are made by seven digitally-controlled analog switches. (The voice
signal flow is described in general in the Operation Manual and in more detail below.)
The computer provides CV and switch signals either from its memory (Edit mode) or as
set on the control panel (Edit/Manual modes).
All CVs which control the synthesizer originate in the voltage demultiplexer (DMUX).
The demultiplexer can be thought of as a spinning rotary switch which momentarily
routes a specific analog voltage from the digital-to-analog converter (DAC) to each
sample/hold (S/H). These devices sample the DAC voltage and hold it until the next
sample. Each S/H voltage value is latched by the LO and HI -order DAC LATCHES.
The S/H selected of the 32 depends on the data latched by the CV DMUX ADDRESS
LATCH. The seven switch control signals are simply bits latched by U330/31.
The microcomputer system (the term "system" emphasizes the combination of
hardware and software) consists of the CPU, program memory (EPROM), non-volatile
memory (NV RAM), and input/output (I/O) interface. The Timer is used in the TUNE
system and the UART handles MIDI communication. The CPU executes the program
permanently residing in the EPROM. This determines how the various input devices—
switches and knobs—are "scanned," how the data resulting from these scans are
processed, and how the output devices—LEDs, latches, and S/Hs—are "strobed" with
the processed data.
The three kinds of mechanical switches in the Prophet-600—program/mode membrane
panel, synthesizer slide switches, and keys—are all wired into one switch matrix. The
mebrane panel is used for mode select and control operations which (besides their
internal functions) only appear as outputs to their accompanying LEDs through the
LED matrix. The slide switches, however, control the synthesizer. So do most of the
knobs, which are read through the potentiometer multiplexer (POT MUX) and analog-
to-digital comparator (ADC CPR). The current physical status of the control
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