Sequential Circuits Prophet-5 Service Manual - Audiofanzine

'i2-13 TUNE AND A-440The synthesizer and microcomputer circuits employed for polyphony and programmability having beencovered, this paragraph explains the third main feature resulting from the Prophet's A/D hybridtechnology. The TUNE system has only been briefly mentioned before, but it is responsible for theentire performance of the instrument in the sense that without its corrective influence, the Prophet'sten voice oscillators could not be expected to stay in tune over their nine-octave range. The reasons forthis are several.There are two basic parameters to work with in tuning. One is initial frequency (INIT FREQ) that is, thespecification that all ten oscillators sound the same pitch given the same, steady CV. The other isSCALE(or, V/OCT), which desires a predictable pitch change to accompany a specified CV change. INIT FREQerrors can be introduced by any of the many sources of oscillator control. While summing resistors inthe Common Analog circuitry are precision-matched, errors may stillMTUN. P-BND, W-MOD, UNISON, and FINE (on OSC B).result from the combination ofIn the voices the A SUM and OSC S/H CVsumming resistors and P-MOD FREQ A circuits are error sources. SCALE errors generally occur in theVCO itself.For example, while a CV change from IV to 2V may produce an exact octave interval, achange of 5V to 6V might cause a pitch change of greater or less than an octave. All VCOs have this errorto some degree somewhere in their range. For an IC, its associated components, such as the SCALEtrimmer, can contribute error. Chips age, and their parameters will change with changingtemperature—although this effect is significantly reduced with on-chip temperature compensationcircuitry (see Appendix).Actually, TUNE has two stages, only the first of which occurs when the TUNE switch ispressed. In lessthan ten seconds the microcomputer samples each VCO frequency at octaves, while using successiveapproximationto determine the exact 14-bit CV required to tune the VCO to a reference. The systemmeasures the period of each oscillation in terms of CPU clock cycles. The difference between the ideal,83mV-step CV which should produce the reference frequency and the 14-bit, 651 -uV CV which actuallydoes so is called bias. Biases for each oscillator are determined at the ten C's (CO - C9) across the VCOs'full range. The biases are placed in a 200-byte table in Scratchpad RAM (1 bias/oct x 10 oct x 2bytes/bias). The second stage of TUNE occurs while playing. Whenever an OSC S/H CV is to change inresponse to a keystroke, the computer determines the new note's position between octave bias-points,and calculates the exact bias for that semitone.^, -? A'''The first stage functions as follows. First the CPU disables the PITCH wheel, MTUN, and UNI CV, byopening switches in their signal paths. The TUNE circuit consists of 1036^/40 TUNE MUX (see Figure 2-5or schematic SD430), U435 TUNE CPR, and Counter (CNTR) and 2 of U315, a three-section 8253Programmable Interval Timer. (CNTR 1 is used for A-440, discussed below). When activated, the TUNEMUX sequentially connects each oscillator output to the TUNE CPR in the same way the POT MUXsamples pots for the ADC CPR. The TUNE CPR converts the sawtooth to pulses. With no input at U435-3,R4158/R4159 hold U435-2 at 1.36V, so U435-7 isso that when it(hysterisis).high. U435-3 increases with the positive-going sawtooth,crosses 1.36V, U435-7 goes low. R4170 feeds-back the transition to prevent oscillationThe TUNE flip-flop (FF) must be explained in conjunction with the 8253. The PIT contains threeindependent, multi-mode, 16-bit presettable down-counters addressed as memory locations 1800-1802(H), plus a control word register at 1803(H). TIMER -CS must be low for all write or read operations.-WR or -RD indicate the CPU is writing a control word or count, or expecting a count. Duringinitialization (on power-up) each counter's mode is programmed by a byte written into the controlword register. CNTR operates in ''one-shot" mode. It is programmed to, at first, count one oscillatorpulse. But the 8253 actually requires an extra clock pulse—which we call a "fake clock"—to accuratelybegin the count. Therefore each oscillator sampling is preceded by the fake clock pulse from the TUNEFF,under control of U332, port -CSL04 (see schematic SD332). After the fake clock pulse, FFD goeshigh, allowing the TUNE FF to invert TUN MUX through -Q (U322-6).The TUNE FF iscleared in preparation for each new oscillator pulse.then constantlyD.2 2-19