Sequential Circuits Prophet-5 Service Manual - Audiofanzine

the same CV and trigger pattern to the Prophet, which digitizes the SEQ CV IN for control of Voice 5.iWith CNTRenabled by CNTR EN at U332-5. OUTO (U315-10) goes low at the first oscillator edge fromthe TUNE CPR. U321-1 inverts this to a high which gales CNTR 2. CNTR 2 isclock cycles, so it decrements at the rate of 2.5 MHz. When CNTR reaches its terminal count, that is,programmed to count CPUwhen one pulse (iri addition to the fake clock) has been counted, OUTO goes high, stopping CNTR 2.CNTR 2's 16-blt register now represents the number of CPU clock cycles occuring during one period ofOSC lA's sawtooth. This number is compared to a reference. An oscillator that is too sharp will have alower count than the reference, and vice versa. As long asthe count and reference don't match, theCPU adjusts the OSC S/H CV and samples the sawtooth again. Successive approximation starts with theDAC MSB, setting each bit which does not cause the oscillator pitch to overshoot the reference.After tuning OSC 1 A at C3, the reference count ishalved to tune CA, since one cycle at C4 should takeexactly half the number of CPU clock cycles than at C3. For octaves C5 - C9, the cycle count in CNTRdoubled instead (2, 4, 8.. .32).the C3 -The CO- C2 biases are actually extrapolated from the curve suggested byC9 bias table, rather rhnn found by dir{?ct measurement because counting such low-frequencypulses would take an inconvenient amount of time.same way.The remaining oscillators are tuned in the4isWhen enabled by U332-12, CNTR 1 simply divides 2.5 MHz by 5682 to produce the 440-Hz square wavesummed into the AUD OUT stage (see SD430). To prevent noise, the A-440 input is grounded by U460-9when not inuse. by inverter-connected U461-9.2-14 SEQUENCER INTERFACEThe sequencer interface is intended for use with SCI's Model 800 Digital Sequencer, although thedesign is flexible enough to interface with many types and qualities of analog inputs. For sequencerecording the Prophet outputs the keyboard CV and a trigger pulse for the most recently played note inpolyphonic mode, and for the lowest note played in UNISON mode. To playback, the sequencer sendsThe Prophet's sequencer output (record) circuitry isthe same as the synthesizer output circuitry. SEQICV OUT appears at S/H U350-7 before buffering by voltage follower U348-6 (see schematic SD333). TheSEQ TRIG OUT Isa bit latched by port -CSOH5, U340-2.The sequencer input (playback) circuitry is a bit more complex (see schematic SD332). ConnectingSEQUENCER.GATE IN closes a switch on J3 grounding U323-10. When port CSI1 is input, this low notifiesthe computer to process the two SEQ inputs. SEQ CV IN is assumed to be an analog voltage which slewsbetween its various values. The slew rate will vary with the type of sequencer (or other accessory).Ideally, the input device only produces its GATE once its CV output has fully stabilized. However, in theworst case some accessories will continue to slew after producing the GATE. For this reason the Prophetdelays the SEQ GATE IN te allow SEQ CV IN time to settle,the CPU.by using the interrupt (-INT) feature ofIDuring initialization the Z-SO's interrupt mode programmed isso that a low on the -INT pin forces theprogram to restart at memory location 38(H) (when the interrupt has been software-enabled). This is thestarting address of the SEQ HANDLER subroutine. Initialization also clears the -INT through port-CSOL5. U331-12 inverts this -CS to +CLR INT which resets U330-4, making -Q high. U330-13, Q remainslow, being held reset by U331-4. When a GATE appears. U331-4 goes low and U331-2 goes high after a2-ms delay through R311/C316. This high clocks U330-3, producing the -INT pulse at U330-2.When it receives the -INT, the CPU completes its current instruction, placing the next instructionaddress and register statuses on the "stack", then enables the SEQ CV IN by latching EN SEQ throughport -C50H4, U339-2 (see schematic SD333). This bit closes switch U371-9, connecting the SEQ CV IN tothe ADC CPR. The SEQ CV is then determined through successive-approximation and stored in aScratchpad table for later addition to Voice 5. After the voltage isinterrupt. However, this time U330-13 goes high, gating voice 5, because U330-10 isprocessed, the CPU again clears thethe SEQ GATE IN through U331-4. When SEQ GATE IN turns off, U330-13 is again reset.being held low by2-20