OSC B KBD and FILT KBD switches are for keyboard tracking in UNISON mode. UNISON CV is^.In UNISON (monophonic) mode a UNISON CV derived from the lowest note played on the keyboardbecomes the main pitch controi for allvoices through the Common Analog circuitry. The five GATEsoccur simultaneously, but the envelope generators only re-trigger ifno keys are held.The microcomputer system itself consists of the microprocessor or CPU, memory (MEM), andinput/output (I/O) Interface. The CPU executes the program permanently residing in the read-onlymemory (EPROM). This program determines how the various input devices— keyboards, switches, andknobs—are "read", and how "data" generated by them is"processed" to control the synthesizer. TheScratchpad RAM is the CPU's work area. It holds data representing the current status of the keyboardand all controls. Control status depends on the operational mode. In MANUAL mode the control datacorresponds to the current settings of the black, programmable knobs and switches. If desired this datacan 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 forthat sound. The EDIT feature allows one to alter individual control settings inthen be recorded to supplement or replace the original program inNV RAM.Scratchpad, which canThe remainder of this section discusses the <strong>Prophet</strong>'s actual analog and microcomputer circuitry. Eachsub-circuit is described functionally in relation to its block diagram. Principal !Cs and any uncommondesigns are explained. However the function of common linear, TTL, and MOS ICs isnot coveredbecause anyone contemplating technical work on the <strong>Prophet</strong> must already be familiar with, forexample, combinational logic and op amp circuits.2-3 ANALOG SYNTHESIZER DESCRIPTIONThe definitions of the various CV and switch commands in the analog synthesizer must be clear beforethe Voice and Common Analog sub-circuits can be discussed. Referring to Figure 2-4, the CommonAnalog circuitry includes the master summers (MSUMs), Glide, and WHEEL-MOD circuits. The MSUMSproduce five Common Analog output CVs: A SUM, B SUM, PW A SUM, PW B SUM, and FILT SUM CV.These enable simultaneous pitch control and modulation for the voices, as follows.R104 MASTER TUNE (MTUN) and R1PITCH wheel adjust OSC A and B frequency directly—that is,without computer processing—through A SUM CV and B SUM CV. To prevent interference with theTUNE routine (see paragraph 2-13), the switches shown disconnect these controls during the TUNEroutine. In UNISON mode the KBD component of the FREQ CV appears through the UNISON CV. Theprocessed through the Glide circuit which, essentially, delays quick changes in UNISON CV as GLIDECV increases. The OSC B MSUM 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 VV-MOD signal, switchable to any MSUM consists of pink noise or LFO output as determined by theW-MOD SOURCE MIX CV. CV inversion to the LFO VGA allows the NOISE and LFO levels to move inopposite directions for a single 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 terminateat the voices. First, Patch CVs control all five voices identically. For example, all five Amplifier EnvelopeGenerator attack times will be equal for any patch, so the Amplifier Envelope Generator ATK CV is wiredto the same terminal on allfive AMP ENV GENs. Other Patch CVs include MIX OSC A, P-MOD ENVAMT, and FILT RESONANCE. Second, the Individual CVs determine the frequency of a single VCO orVCF alone. Each voice has two OSC S/H CVs and one FILT S/H CV. These signals contain the polyphonicKBD component of the total FREQ CV applied to these devices. The ten OSC S/H CVs also contain INITFREQ control—actually common to the OSC As or Bs—and the TUNE "biases", which fine-tuneeach VCO.D.22-7IP
'IAll processed CVs originate from the computer at a separate Sample/Hold (S/H). This circuit isdiscussedin paragraph 2-12, but for introduction Figure 2-4, Detail A shows a typical S/H used with VC-devices.Detail B adds a voltage-to-current converter (V-C CONV) required to control the operationaltransconductance amplifiers (OTAs—see paragraph 2-5) used as "VCAs" in all places. Twenty-three S/Hson PCB 3 supply the Common Analog and Patch CVs and fifteen S/Hs on PCB 4 supply the IndividualOSCandFILTCVs.Voice 1 is used for explanatory purposes, below. The five voices are functionally identical. Seeschematics SD431 and SD334.2-4 OSCILLATOR A, B, AND LFOThe <strong>Prophet</strong>'s eleven oscillators—two per voice, plus one LFO—are based on the CEM 3340 integratedVCO. The IC isscaled at IV/octave. This means an overall CV change of exactly IV ideally produces apitch change of exactly one octave. So a CV change of 1/12V (83.3 mV) changes pitch by one semitone.In the voices the basic oscillator range is nine octaves; resulting from 0-5V supplied by the KBD and 0-4Vsupplied by the OSC FREQ knob through the OSC S/H CV (see Figure 2-4). While most CVs in the<strong>Prophet</strong> are quantized by only the most significant seven bits of a fourteen-bit DAC into 128 83.3-mVsteps (128 X 0.0833V = 10.67V), the OSC S/H CVs are quantized by the full fourteen bits into 16,384 651-uVsteps (16,384 x 0.000651 = 10.67). This finer resolution allows the TUNE circuitry to tune the oscillators towithin 1/128 semitone. Italso allows the SCALE MODE feature, where each note can be raised orlowered by up to a semitone. (For further information, see under DAC and TUNE, paragraphs 2-12 and2-13). In UNISON mode the KBD CV is routed through the Glide circuit and the OSC MSUMs.Additional OSC 1A FREQ control is created on Voice 1 itself through the POLY-MOD circuit. TheP-MOD FREQ A switch applies a CV mixed from the P-MOD FILT ENV and P-MOD OSC B AMT VCAsto the OSC 1A CV SUM input. OSC lA's pulse width iscontrolled by two CVs at the OSC A PW SUM.First PW A SUM CV originates in the Common Analog circuitry as the sum of OSC A PW and W-MODPW A (ifON). Second, the P-MOD PW A switch can add a CV mixed from the FILT ENV GEN and/orOSC B. When on, the OSC A SYNC switch tunes OSC A to harmonic frequencies of OSC B or changesOSC A's timbre when the two are not in harmonic relation. This occurs independently of whatever OSCB waveforms are switched on.OSC B is similar to OSC A except that besides being a pitch source it can be a modulation sourcethrough POLY-MOD, and can operate as an LFO with or without KBD control. In the OSC B MSUM, theOSC B LO FREQ switch adds a -7.5V offset through B SUM CV. To provide adequate control range, theINIT FREQ control doubles its range (to 9V) when OSC B LO FREQ is ON.The LFO itself is not controlled by the keyboard, and its pulse width is fixed at 50% (square wave). Itsoutput isa modulation CV effective through the five MSUMs.Details of the 3340 and associated components can be found on its Data Sheet in the Appendix. All 3340outputs are positive-going. But using the LFO and OSC B as a modulation source requires their trianglepeaks travel as far negative as positive for smooth vibrato. On Voice 1, circuitry containing U451-1 (seeSD431) shifts down the dc-level of the triangle to be symmetrical about ground.While creating a symmetrical signal for modulation, the triangle level-shifting poses a slight problem forthe 4016, which isbasically designed to pass only positive voltages. Figure 2-4, Detail C shows the basiccircuit used for switching allbipolar signals in the <strong>Prophet</strong>. Instead of being grounded, the Vss pin isbiased slightly-negative, allowing the switch to pass slightly negative voltages. Diodes at each switchinput protect the switch by clamping negative voltages to not exceed the diode drop, which is also thenegative bias value (0.6V), Where a bipolar voltage (such as P-MOD OSC B with OSC B TRIANGLE on) isbeing switched, the level must be sharply attenuated, where a current (such as allbeing switched, the only voltage developed isop amp node following the switch isa virtual ground.sunriming nodes) isby the 4016's ON resistance (300 ohm, typical) since the2-8
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- Page 11 and 12: 1-5 PCB 1/2 CONTROL PANELSOnce PCB
- Page 13 and 14: Figure 1-5KEYBOARD REMOVAL1 -7/1 -8
- Page 15 and 16: TOADDITIONALVCOsTOADDITIONALENV GEN
- Page 17 and 18: 2-2 THE PROPHETThe Prophet is a sub
- Page 19: COMMON ANALOGVOICE(VOiCES e-5 ARE S
- Page 23 and 24: ajMim—2-8 AUDIO OUTPUTAs shown in
- Page 25 and 26: .'2-10 MICROPROCESSOR, MEMORY, AND
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- Page 29 and 30: 2-12 ADC, DAC, AND CV OUTPUTSThe DA
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- Page 35 and 36: SECTION 3DOCUMENTS3-0 DOCUMENT LIST
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- Page 58 and 59: Inpreparation for service, set up t
- Page 60 and 61: 4-2 OSCILLATOR B TESTSTEP MODULE CO
- Page 62 and 63: 4-S FILTER TEST.STEP'MODULE CONTROL
- Page 64 and 65: 4-8 POLY-MOD TEST—A.STEP MODULE C
- Page 66 and 67: i4-11 PITCH WHEEL TRIMThis trim rem
- Page 68 and 69: A)I4-16 VCO SCALE TRIMBE SURE RECOR
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4-19 FILTER ENVELOPE AMOUNT VCA BAL
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4-21 FINAL VGA BAL1. Switch PRESET
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11 IJSECTION 8THEORY8-0 INTRODUCTIO
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IiSECTION 9PRCX^RAMMING9-0 INTRODUC
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SECTION 13PROPHET-5 REV 3.3 UPDATET
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