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MODEL 1000
S/N 1301 and Above
Revisions 3.0, 3.1 and 3.2
Manual No. TM1000D.2
SYNTHESIZER
TECHNICAL MANUAL
SECOND EDITION
By STANLEY JUNGLEIB
Wine Country Productions, Inc.
1572 Park Crest Court, Suite #505
San Jose, California 95118 USA
Phone: [phone] FAX [phone]
SEQUENTIAL Product Specialists Since 1987
|"1 V

PROPHET-5 SYNTHESIZER
TECHNICAL MANUAL
By Stanley JungJeib
Artwork: Denis Simard
Greg Armbruster
Second Edition
For Revisions 3.0, 3.1 and 3.2
Manual No. TM1000D.2
Issued: October, 1981
Copyright©1981 by
SEQUENTIAL CIRCUITS, INC.
All rights reserved. Printed in USA.
The contents of this manual are the property
of SCI and are not to be copied or reproduced
without our prior written permission.
D.2

IA*
About This Manual and Servicing The Prophet
The Prophet Is asophisticated instrument and SCI issues its technical manual for use by qualified
technicians only. Of course the manual will be read by Prophet owners and others Interested in
synthesizer design. And we realize it will also be used by some to develop modifications for the
instrument. While we support this innovative attitude in spirit, we cannot support it financially:
Modifications or unauthorized service void the Prophet's warranty. They also invariably extend service
time (thus, cost) if factory repair is required. Familiarize yourself thoroughly with this manual before
attempting any work on the Prophet. This will at least help you judge whether you should be working
on it at all. If in doubt, please contact our Service Department.
This edition of the Technical Manual (TM1000D) documents Prophet-5s with serial numbers 1301 and
above. Although great care was taken to ensure the changes would be transparent to players,
technicians will find "Rev 3" quite adifferent instrument from its predecessors. Rev 2(S/N 184-1299, see
TM1000C) was amere refinement of Rev 1(S/N 1-182). Rev 3, however, uses new voltage-controlled ICs
in the analog synthesizer and, in the microcomputer, avastly different ADC, DAC and control voltage
distribution scheme. Although the main purpose of the revision was to remove production limitations
caused by inconsistent supply and quality of the previous synth "chip set," the hardware redesign
encouraged the implementation of more sophisticated editing, tuning and maintenance software while
improving servicability; the number of voice trimmers being reduced from 80 to 45.
The manual is organized as follows:
SECTION 1, MECHANICAL provides aphysical introduction and directions for
disassembling/assembling the Prophet.
SECTION 2, THEORY explains general function and circuit operation, referring to block diagrams and to
the schematics for hardware details.
SECTION 3, DOCUMENTS contains schematics and pictorials identifying all components.
SECTION 4, SERVICE contains procedures for routine tests and trims.
SECTION 5, PARTS cross-references component designators to SCI stock numbers.
SECTION 6, GLOSSARY decodes abbreviations appearing on SCI documentation.
SECTION 7, APPENDIX contains selected data sheets.
Your response to the questionnaire on the next page will help us monitor our publication's usefulness
About The Second Edition
In addition, to the original revision 3.0 data (with afew corrections), the following
material covering later refinements of the instrument has been added. The revisions
occured chiefly in the microcomputer's memory configuration, added PITCH and MOD
CV inputs, and in the addition of aserial interface for communication with the Model
1005 Polyphonic Sequencer or Model 1001 Remote Keyboard.
SECTION 8, THEORY discusses the hardware changes comprising revisions 3.1 and 3.2.
SECTION 9, PROGRAMMING covers use of the serial interface.
SECTION 10, DOCUMENTS contains schematics and pictorials for revisions 3.1 and 3.2.
SECTION 11, SERVICE includes instructions for updating a3.0 or 3.1 instrument to
level 3.2, for using the diagnostic memory tests, and for an added adjustment on
PCB 3.
SECTION 12, PARTS lists SCI stock numbers for 3.1 and 3.2-level assemblies. D.2

i
'1
Table of Contents
i
SECTION 1MECHANICAL
1-0 GENERAL 1-1
1-1 PRECAUTIONS 1-1
1-2 SERVICE POSITION 1-1
1-3 PCB 4VOICE BOARD 1-3
1-4 PCB 3COMPUTER BOARD .' 1-4
1-5 PCB 1/2 CONTROL PANELS 1-5
1-6 KEYBOARD 1-6
SECTION 2THEORY
2-0 GENERAL 2-1
2-1 SYNTHESIZER BACKGROUND 2-1
2-2 THE PROPHET 2-4
2-3 ANALOG SYNTHESIZER DESCRIPTION 2-7
2-4 OSCILLATOR A, BAND LFO 2-8
2-5 MIXER AND AMOUNT VCAS 2-9
2-6 FILTER 2-9
2-7 ENVELOPE GENERATORS 2-9
2-8 AUDIO OUTPUT 2-10
2-9 MICROCOMPUTER SYSTEM DESCRIPTION 2-10
2-10 MICROPROCESSOR, MEMORY, AND I/O INTERFACE 2-12
2-11 CONTROL MATRICES 2-15
2-12 ADC, DAC, AND CV OUTPUTS 2-16
2-13 TUNE AND A-440 2-19
2-14 SEQUENCER INTERFACE 2-20
2-15 CASSETTE INTERFACE 2-21
SECTION 3DOCUMENTS
3-0 DOCUMENT LIST 3-1
3-1 DOCUMENT NOTES 3-1
SECTION 4SERVICE
4-0 GENERAL 4-1
4-1 OSCILLATOR ATEST 4-3
4-2 OSCILLATOR BTEST 4-4
4-3 MIXER AND NOISE TEST 4-5
4-4 UNISON AND GLIDE TEST 4-5
4-5 FILTER TEST 4-6
4-6 AMPLIFIER TEST 4-6
4-7 LFO AND WHEEL-MOD TEST 4-7
4-8 POLY-MOD TEST 4-8
4-9 FINAL TEST 4-9
4-10 POWER SUPPLY TRIM 4-10
4-11 PITCH WHEEL TRIM 4-10
4-12 MASTER SUMMER OFFSET TRIM 4-10
4-13 WHEEL-MOD LFO VCA BALANCE 4-10
4-14 DAC GAIN, ADC GAIN, SEQ INTERFACE TRIM 4-11
4-15 WHEEL-MOD NOISE VCA BALANCE 4-11
4-16 VCO SCALE TRIM 4-12
4-17 POLY-MOD FILTER ENVELOPE VCA BALANCE 4-13
4-18 POLY-MOD OSCILLATOR BVCA BALANCE 4-13
4-19 FILTER ENVELOPE AMOUNT VCA BALANCE 4-14
4-20 FILTER TUNING 4-14
4-21 FINAL VCA BALANCE 4-16
4-22 VOICE VOLUME 4-16

SECTION 5PARTS
5-0
5-1
5-2
5-3
5-4
5-5
5-6
SECTION 6GLOSSARY
SECTION 7APPENDIX
CHASSIS 5.1
PCB 15.T
PCB 25_1
PCB 35-2
PCB 45_4
pcb 5"!!"!.' "!!.'.'!!! "s-b
BILL OF MATERIALS (TOTAL ITEMS) 5-9
SECTION 8
CA3280 Dual Operational Transconductance Amplifier
CEM 3310 Voltage Controlled Envelope Generator
CEM 3320 Voltage Controlled Filter
CEM 3340/3345 Voltage Controlled Oscillator
THEORY
S-0 INTRODUCTION
8-1 REVISION 3.1 COMPUTER and POWER SUPPLY
8-2 REVISION 3.2 COMPUTER and USART
8-3 REVISION 3.2 ANALOG and POWER SUPPLY
8-1
8-1
8-2
8-4
SECTION 9PROGRAMMING
9-0 INTRODUCTION
9-1 Data Format
9-2 Error Checking
9-3 Status Bytes
9-4 STATUS 0: SEND KEYBOARD and PROGRAM
9-5 STATUS 1: ACK, RECEIVE KEYBOARD and PROGRAM
9-6 STATUS 2: TRANSPOSE ON
9-7 STATUS 3: SAVE TO TAPE
9-8 STATUS 4: LOAD FROM TAPE
9-9 STATUS 5: CLEAR TRANSPOSE
9-10 STATUS 6: INITIALIZE SEQ LOWER PROGRAM
9-11 STATUS 9: DISABLE TUNE
9-12 STATUS A: ENABLE TUNE
9-13 STATUS B: RECEIVE PROGRAM CHANGE
9-14 STATUS C: SYSTEM CONNECT
9-15 STATUS E: RECEIVE SHORT KEYBOARD DATA
9-1
9-1
9-2
9-2
9-3
9-3
9-4
9-4
9-5
9-5
9-6
9-6
9-6
9-6
9-6
9-6
SECTION 10 DOCUMENTS
10-0 DOCUMENT LIST 10-1
SECTION 11 SERVICE
11-0 INTRODUCTION
11-1 REVISION 3.2 RETROFIT KIT INSTRUCTIONS
11-2 REVISION 3.0 MEMORY TEST
11-3 REVISION 3.1 MEMORY TEST
11-4 REVISION 3.2 MEMORY TEST
11-5 WHEEL-MOD BALANCE TRIM
11-1
11-1
11-4
11-4
11-5
11-6
SECTION 12 PARTS
REVISION 3.2 COMPONENTS

'I
SECTION 1
MECHANICAL
1-0 GENERAL
This section shows how to remove the Prophet's main assemblies. Not all of the procedures given here
should be necessary at any one time. For some service situations you will only need to separate the top
and bottom panel assemblies and arrange them as shown in Figure 1-0. This configuration, discussed in
paragraph 1-2, allows access to all trimmers on PCB 4and most trimmers on PCB 3. However, for some
PCB 3trims you will have to swing-out or completely remove PCB 4, as discussed in paragraph 1-3.
1-1 PRECAUTIONS
Observe the following precautions when working on the Prophet:
SWITCH POWER OFF AND CHECK 115/230V SWITCH ON BACK PANEL BEFORE CONNECTING
PROPHET TO POWER OUTLET OR AMPLIFIER.
NEVER TOGGLE 115/230V SWITCH WITH POWER ON.
TRIMMING, OF COURSE, MUST BE PERFORMED WITH POWER ON. SO AVOID THE POWER SUPPLY
PRIMARY CIRCUITRY, WHICH CONDUCTS LETHAL VOLTAGE.
SWITCH POWER OFF BEFORE DISCONNECTING OR CONNECTING ANY CIRCUITRY, OR
REMOVING OR INSTALLING PCBs.
IMPORTANT! WHENEVER THE AUDIO CABLE IS DISCONNECTED, PCB 3MUST BE GROUNDED TO
THE BACK PACK PANEL
p*
DO NOT BEND OR STRAIN THE PCBs. OTHERWISE MAY CAUSE TINY BREAKS IN THE PRINTED-
CIRCUIT TRACES WHICH WILL BE EXTREMELY DIFFICULT TO FIND.
i
TO REPLACE SOLDERED COMPONENTS SWITCH POWER OFF, REMOVE THE PCB COMPLETELY
FROM THE INSTRUMENT AND DESOLDER FROM BOTH SIDES. USE AVACUUM SYRINGE OR DIP
DESOLDERER. KEEP VACUUM SYRINGES CLEAN TO PREVENT THEM FROM SPRAYING MOLTEN
SOLDER. DON'T OVERHEAT THE PADS. WORK CAREFULLYI
1-2 SERVICE POSITION
TO PREVENT DAMAGE TO THE TOP PANEL, KEYBOARD, OR WOODWORK, USE ACARPETTED OR
SIMILARLY-COVERED WORK SURFACE WHEN OPENING THE BOX.
fb-set up the Prophet for service first switch power off, unplug power cord, and turn instrument over to
expose bottom panel. Remove two large screws near front feet. Remove eleven screws around
perimeter of bottom panel.
Holding top and bottom panel assemblies together, turn the Prophet right-side-up again. Remove four
screws along top edge of back panel. Slowly slide the top panel assembly forward—about nine inches—
so when raised the control panel will clear the large power supply capacitors.
1-1

TOP
PANEL
ASSEMBLY
Z-80
CPU
NON-VOLATILE
PROGRAM RAM
EPROM
SCRATCHPAD
RAM
BOTTOM
PANEL
ASSEMBLY
MEMORY AND
I/O INTERFACE
PCB3
COMPUTER
OSCILLATOR AND
FILTER CV
DEMULTIPLEXER
SAMPLE/HOLDS PCB4
VOICES
VOICE 1
VOICE 2
VOICE 3
VOICE 4
VOICE 5
FINAL
VCAs
LATCHES COMMON AND
PATCH CV
DEMULTIPLEXER
SAMPLE/HOLDS
P303/
VOICE
POWER
CABLE W2
PCB 3/4
INTERCONNECT
ENVELOPE
GENERATORS POLY-MOD/
FILTER
ENVELOPE
VCAs
3AUDIO
OUTPUT
POLY-MOD
OSCB
VCAs
MIXER P402/
AUDIO
OUTPUT
CABLE
Figure 1-0
SERVICE POSITION

Raise the top panel assembly to service position shown. For best stability both top panel side back edges
should rest on the bottom panel. Actually, the top panel upper edge rests on the back panel cable. This
is normal; but asmall (half-inch) prop may be added to improve stability.
The instrument can operate normally in this position. Of course its operating temperature will be far
below normal. This may slightly affect adjustments or the performance of certain ICs.
REMEMBER THAT IN SERVICE POSITION THE TOP PANEL ASSEMBLY BALANCES ON ITS REAR EDGES.
DON'T UPSET THIS BALANCE BY PUSHING TOO HARD WHEN PROBING OR TRIMMING.
When reassembling the Prophet, turn instrument over and first start two large screws near front feet.
Then start bottom and back panel screws to obtain overall alignment before tightening all screws.
1-3 PCB 4VOICE BOARD
PCB 4may be swung-out and operated under power. For this set-up, switch power off, remove screws
identified in Figure 1-0 in the order suggested, and position board on insulation as shown in Figure 1-1.
nxmrrm**
INSULATION COMMON
ANALOG
Figure 1-1
PCB 4SWINGOUT
TO PREVENTGROUND LOOPS, PCB 3IS GROUNDED ONLY THROUGH THE AUDIO CABLE TO THE
BACK PANEL. THEREFORE, WHENEVER THE AUDIO CABLE IS DISCONNECTED, PCB 3MUST BE TIED
TO THE BACK PANEL
CAREFUL! DON'T ALLOW PCB 4TO SHORT AGAINST STANDOFFS FROM PCB 3OR AGAINST THE
BOTTOM PANEL.
To remove W2, completely switch power off and disconnect voice power cable at P303 on PCB 3(see
Figure 1-0). Also remove PCB 3/4 interconnect at P401 on PCB4, with agentle "see-saw" motion. Detach
audio output cable from P402.
When reconnecting voice power and audio output cables, be sure tabs interlock. Also be sure the PCB
3/4 interconnect is correctly mated and firmly seated at P304 and P401.
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1-3
**

1-4 PCB 3COMPUTER BOARD
PCB 3is held by the screws identified in Figure 1-2, and by two direct connectors to PCB 2, behind it. To
remove PCB 3first remove PCB 4. Disconnect back panel cable from P302 and wheel cable from P301,
remove screws, and pull while rocking the board to minimize stress.
P301/
WHEEL
CABLE 5
PANEL
CABLE
Figure 1-2
PCB 3MOUNTING
When replacing be sure all connector pins are correctly mated with PCB 2before fastening screws.
Alignment is insured by machine screw holes with their standoffs. Remember to reconnect the wheel
cable, too, before replacing PCB 4. When reconnecting wheel and back-panel cables, be sure tabs
interlock.
1-4

1-5 PCB 1/2 CONTROL PANELS
Once PCB 3and PCB 4have been removed, control panel removal involves pulling off all knobs,
unscrewing all potentiometer mounting nuts—using ahalf-inch nutdriver—and removing screws
identified in Figure 1-3.
J201/W1
KEYBOARD
CABLE
PCB 2
LEFT
CONTROL
PANEL
W201
PCB 1/2
INTERCONNECT
PCB1
RIGHT
CONTROL
PANEL
Figure 1-3
PCB 1/2 MOUNTING
When replacing, check keyboard cable, that all pots fit correctly through the front panel, and that
switches operate freely before tightening pot nuts.
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1-5

1-6 KEYBOARD
After removing PCB 3and PCB 4, turn the top panel assembly over and disconnect keyboard cable at
J201 (see Figure 1-3).
Figure 1-4 details keyboard mounting. At both ends, remove keybed supports by first removing keybed
screws then rear keyboard mounting screws. Remove front keyboard mounting screws. The entire
keyboard will slide out of the case—and back in—as shown in Figure 1-5.
CAREFUL! DURING THIS OPERATION THE J-WIRES FOR THE LOWEST AND HIGHEST KEYS ARE
VULNERABLE TO DAMAGE FROM COLLISION WITH THE KEYBOARD MOUNTING BRACKETS.
WHEN REPLACING CHECK J-WIRES AT EACH END OF KEYBOARD FOR CONTACT WITH BUS BAR.
Also, remember to attach the wheel ground lug.
PITCH
WHEEL
DETENT SHAFT
SLOT
R1
PITCH
WHEEL
R2
MOD
WHEEL
FRONT
KEYBOARD
MOUNTING
SCREW J-WIRES
SET
SCREW
BUS
BAR
WHEEL
GROUND
LUG REAR
KEYBOARD
MOUNTING
SCREW
KEYBED
SCREW KEYBED
SUPPORT KEYBOARD
MOUNTING
BRACKET
Figure 1-4
KEYBOARD MOUNTING
1-6

Figure 1-5
KEYBOARD REMOVAL
1-7/1 -8

SECTION 2
THEORY
2-0 GENERAL
This section explains the Prophet's theory of operation. First ageneral description relates the Prophet to
the basic analog synthesizer and introduces the concepts central to the Prophet's advances over
conventional performance instruments: programmabtlity and polyphony. Then the analog synth and
microcomputer functions are covered and their sub-circuits detailed. Throughout it is assumed the
reader is already familiar with Prophet-5 operation (see Operation Manual CM1000B).
2-1 SYNTHESIZER BACKGROUND
The synthesizer is anew instrument but the urge to set technology in search of music is quite old.
Consider the pipe organ; of traditional instruments, most like the synthesizer. The organ achieved its
broad dynamic, pitch and timbral ranges through intricate mechanical arrays; for generating steady
wind, transferring key action to pneumatic valves for each note, activating ranks of pipes, and to couple
pedalboards and keyboards. Later, the bellows tremolo and pedal-operated swell shutters were added
for dynamic and timbral expression. As the organ expanded into larger halls pneumatic and hydraulic
devices were developed to isolate the keyboard from the force required to key more pipes at higher
wind pressure. With the advent of electricity the keyboard and drawknobs, formerly levers, became
switches. Elaborate relay banks now drove solenoid valves for each pipe. More recently, electronic
organs—in many variations—have relied on motor-driven tone generators or switch/relay banks for
oscillator frequency division or waveform addition or shaping. Another influential technology, the tape
recorder captured natural, instrumental, and electronic sounds for new uses and stimulated the idea of
composing music not performable "in real time" on conventional instruments. Afew keyboard
instruments have been directly based on tape mechanisms.
Now, what is asynthesizer? It can be distinguished from the organ or tape recorder by reliance on
electronic rather than mechanical devices to generate and modify sound. Except for performer's
controls asynth need have no moving parts. (Even this exception may one day disappear.) Thus it is far
more flexible and controllable than an organ. In contrast to organ or tape technology, voltage control
(VC) allows the immediate and precise adjustment of the basic parameters of an audio stage such as
oscillator (VCO) or filter (VCF) frequency, or amplifier (VCA) gain, by asignal from another VCO, an
envelope generator (ENV GEN), keyboard (KBD) or sequencer (SEQ). Most music is analyzable into
elements replicable by afew VC-modules. For example, if asynth imitates adrum or piano it is not by
being a(mechanical) percussion instrument, but by simulating the dynamic envelope accompanying
percussive sounds with aVCA under control of an ENV GEN.
2-1
h

TO '
ADDITIONAL
VCOs
TO
ADDITIONAL
ENV GENS ADDITIONAL
HARMONICS
JVW
VCO B
INIT FREQ
KEYBOARD
1760 Hz
ENV GEN
®®(D ®
880 Hz
ENV GEN
®®®®
J
*
*MIXER
>
440 Hz
"OBOE 11
Figure 2-0
MONOPHON1C ADDITIVE SYNTHESIS
2-2

440 Hz
A
KBDCV
.440 Hz
*H0B0E"
ENV GEN
®®<D ®ENV GEN
®®<D ®
GATE/TRIG
Figure 2-1
MONOPHONIC SUBTRACTIVE SYNTHESIS
Besides adynamic envelope amusical voice usually has apitch and timbre which consists of a
fundamental and anumber of harmonic frequencies—all of varying relative strengths. Pitch and timbre
synthesis raises adistinction between two techniques, diagrammed in Figures 2-0 and 2-1. The first
technique, additive synthesis, might create atimbre by summing the output of several sine-wave VCOs
for the fundamental and each harmonic. In contrast, subtractive synthesis can start with one sawtooth-
wave VCO generating the fundamental with extensive harmonics, then obtain the desired timbre by
subtracting unwanted harmonics with alow-pass VCF.
The additive and subtractive techniques have encouraged the development of two types of instruments,
roughly, "studio" and "performance." Actually most organs use additive synthesis, but since with a
synth one can individually control the level of each harmonic over time additive synthesis may be
potentially more accurate for synthesizing aparticular sound. Whether additive or subtractive, studio
synths may be configured from dozens of modules interconnected by patch cords. The modules have
knobs to establish the initial settings of VC-parameters such as initial frequency (FREQ), pulse width
(PW), and resonance (RES). But the flexibility and complexity of modular synths has discouraged their
live use on stage because significant sound changes often require repatching modules and precisely
checking knobs. Favorite, complex sounds take along-time to create, and almost as long to recreate on a
modular synth. So these monophonic (one-voice) synths instead feed multi-track recorders on which
polyphonic interpretations or compositions are actually orchestrated.
Acomparison of the number of modules and interconnections depicted in Figures 2-0 and 2-1 shows
why the subtractive configuration has become the popular technique for performance synths.
Subtractive synths may be smaller, so, more portable. Their patches will not be as elaborate, so, easier to
change. Originally, performance synths were monophonic. Or they exploited organ technology so
more than one note could be played at atime. "Preset" switches that select fixed patches supplanted
many modular controls. Though one could certainly change sounds quickly by using them, many
players have found preset synths unsatisfactory because they eliminate an essential part of synth
musicianship—control of the sound itself. Some manufacturers have offered partially-programmable
instruments. But before the Prophet appeared it was not possible for akeyboardist to instantly select his
or her own customized synth sounds and play them polyphonically.
2-3

2-2 THE PROPHET
The Prophet is asubtractive, analog synthesizer suitable for performance or studio use. It provides
instantaneous patch repeatability and polyphonic capability without the limitations of organ technology
or fixed presets. The term "digital-analog hybrid" is often used to describe the Prophet. It means the
digital computer controls the analog audio sources and modifiers. The computer itself generates no
sound (except for the A-440 reference).
Figure 2-2 diagrams the Prophet at the most general level. Instead of controlling the synth directly, the
keyboard and most controls are processed through amicrocomputer system. The system provides away
to store all of the switch and knob settings which form apatch, and solves the problem of generating
five sets of oscillator (OSC) and filter (FILT) CVs and GATEs from asingle keyboard. The Common Analog
circuitry mixes the few non-processed controls with processed signals for the voices. Although only one
voice is depicted on the control panel, the black knobs and switches patch the five voices identically.
This makes the voices homophonous—they sound alike—with pitch differences corresponding to (at
most) five simultaneously-held keys.
DIGITAL ANALOG
/7\
CONTROL PANEL/
KEYBOARD ;7MICROCOMPUTER
SYSTEM
COMMON
ANALOG
^^N,
7
5-VOICE
SYNTH
Figure 2-2
PROPHET GENERAL BLOCK DIAGRAM
Figure 2-3 shows the principle functions of the four main blocks. Beginning with AUDIO OUT, the voice
outputs are combined and overall volume set by the VOL VCA controlled directly from the control
panel. Each voice is acomplete synthesizer with two VCOs, aMIXER, VCF, FINAL VCA, and two ENV
GENs. Each of the ten VCOs has its own FREQ CV, which allows the computer to individually fine-tune
them and allows the voices to follow separate keys. Each voice receives its own GATE, which triggers the
two envelope generators with each keystroke.
All switch commands and most CVs are generated by the computer. Non-processed CVs include
MASTER TUNE, PITCH-bend, and WHEEL-MODulation which are mixed in the Common Analog
circuitry. The Common Analog circuitry also requires afew switch commands.
The microcomputer performs the task of voice assignment. It decides which held keys sound which
voices through the OSC and FILT CVs and GATEs. Voice 1is assigned to the first key hit. Voice 2to the
second key, and so on. After the five initial assignments the system is "last note priority." The earliest-
used voice is reassigned to each new note played. Repeated notes key the same voice. For example,
holding C, D, E, F, and G, sustains Voices 1, 2, 3, 4, 5, respectively. Adding A"steals" Voice 1from the
C-key, whose pitch disappears even though the key may still be held.
2-4

Figure 2-3
PROPHET FUNCTIONAL BLOCK DIAGRAM

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Figure 2-4
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In UNISON (monophonic) mode aUNISON CV derived from the lowest note played on the keyboard
becomes the main pitch control for all voices through the Common Analog circuitry. The five GATEs
occur simultaneously, but the envelope generators only re-trigger if no keys are held.
The microcomputer system itself consists of the microprocessor or CPU, memory (MEM), and
input/output (I/O) interface. The CPU executes the program permanently residing in the read-only
memory (EPROM). This program determines how the various input devices—keyboards, switches, and
knobs—are "read", and how "data" generated by them is "processed" to control the synthesizer. The
Scratchpad RAM is the CPU's work area. It holds data representing the current status of the keyboard
and all controls. Control status depends on the operational mode. In MANUAL mode the control data
corresponds to the current settings of the black, programmable knobs and switches. If desired this data
can be recorded from Scratchpad to the Non-Volatile Memory (NV RAM). Then, in PRESET mode,
selecting recorded programs moves them from NV to Scratchpad, reprogramming the synthesizer for
that sound. The EDIT feature allows one to alter individual control settings in Scratchpad, which can
then be recorded to supplement or replace the original program in NV RAM.
The remainder of this section discusses the Prophet's actual analog and microcomputer circuitry. Each
sub-circuit is described functionally in relation to its block diagram. Principal ICs and any uncommon
designs are explained. However the function of common linear, TTL, and MOS ICs is not covered
because anyone contemplating technical work on the Prophet must already be familiar with, for
example, combinational logic and op amp circuits.
2-3 ANALOG SYNTHESIZER DESCRIPTION
The definitions of the various CV and switch commands in the analog synthesizer must be clear before
the Voice and Common Analog sub-circuits can be discussed. Referring to Figure 2-4, the Common
Analog circuitry includes the master summers (MSUMs), Glide, and WHEEL-MOD circuits. The MSUMS
produce five Common Analog output CVs: ASUM, BSUM, PW ASUM, PW BSUM, and FILT SUM CV.
These enable simultaneous pitch control and modulation for the voices, as follows.
R104 MASTER TUNE (MTUN) and R1 PITCH wheel adjust OSC Aand Bfrequency directly—that is,
without computer processing—through ASUM CV and BSUM CV. To prevent interference with the
TUNE routine (see paragraph 2-13), the switches shown disconnect these controls during the TUNE
routine. In UNISON mode the KBD component of the FREQ CV appears through the UNISON CV. The
OSC BKBD ar>d FILT KBD switches are for keyboard tracking in UNISON mode. UNISON CV is
processed through the Glide circuit which, essentially, delays quick changes in UNISON CV as GLIDE
CV increases. The OSC BMSUM is similar to OSC A. FILT SUM CV mainly consists of the FILT CTF CV
(which follows the FILT CUTOFF KNOB). An external FILT CV IN can be added through the back panel.
The W-MOD signal, switchable to any MSUM consists of pink noise or LFO output as determined by the
W-MOD SOURCE MIX CV. CV inversion to the LFO VCA allows the NOISE and LFO levels to move in
opposite directions for asingle CV. The MOD wheel sets this mixture's output level to selected MSUMs.
Besides the Common Analog output CVs there are two types of computer-output CVs which terminate
at the voices. First, Patch CVs control all five voices identically. For example, all five Amplifier Envelope
Generator attack times will be equal for any patch, so the Amplifier Envelope Generator ATK CV is wired
to the same terminal on all five AMP ENV GENs. Other Patch CVs include MIX OSC A, P-MOD ENV
AMT, and FILT RESONANCE. Second, the Individual CVs determine the frequency of asingle VCO or
VCF alone. Each voice has two OSC S/H CVs and one FILT S/H CV. These signals contain the polyphonic
KBD component of the total FREQ CV applied to these devices. The ten OSC S/H CVs also contain INIT
FREQ control—actually common to the OSC As or Bs—and the TUNE "biases" which fine-tune
each VCO.
D.2 2-7
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