Sequential prophet 5 - Tune

U 5 J 6
Figure 2-10
TUN MUX/TIMER
2-1S TUNE
The final circuitry to be discussed at the functional level has only been briefly mentioned before. The
TUNE circuitry is a negative feedback system, using multiplexing and period measurement to mini¬
mize pitch differences between the oscillators. It does this by issuing separate BIAS CVs for each OSC
(see 2-4). At the DAC the BIASES have a full range of about 10V. But this is attentuated 100:1 to provide
a little over a semitone in range at the OSC This resolution allows steps of about 1 cent (1/100 semitone).
The need for a TUNE circuit should be clear by this time. As has been shown, there are many sources
of oscillator control which are summed in two different places to define the actual pitch of the ten
oscillators: 1) Those summed in the COMMON ANALOG circuitry — INIT FREQ, MTUN, P-BND,
W-MOO, UNI CV and FINE on OSC B — are common to each set of five OSC As and OSC Bs. 2) In the
voices control sources include the COMMON ANALOG output, KEY CV, BIAS CV, INITIAL FREQ
trimmer, P-MOD FREQ CV (OSC A only), and on VOICE 5. possibly an EXTERNALLY supplied
FREQ CV. These are of course summed individually for each oscillator. Since all of these sources affect
oscillator pitch, any can detune them. Additionally, each OSC also has a V/OCT and a HI V/OCT
trimmer, which affects the tuning. Finally, any 2030 VCO can be different enough from the others to
be out of tune. They age, and their pitch is affected by temperature (although this effect is reduced
with temperature-compensating resistors).
There are two basic parameters to work with in tuning. One is initial frequency (INIT FREQ) that is,
setting all oscillators to sound the same pitch for a particular CV sum. The other is scaling, or V/OCT,
which desires a predictable pitch change to accompany a specified CV change. In the Prophet, both
parameters are fine-tuned automatically on power-up, or whenever the TUNE switch is hit.
See Figure 2-10. The TUNE circuit consists of the TUNE MUX. a level comparator (LVL CPR), and two
of a three-section programmable timer. The TUNE MUX sequentially connects each oscillator saw¬
tooth output to the comparator, which converts them to pulses to clock the first timer. This counter
is preset to count a specific number of oscillator cycles. OUT 0 goes low when it starts to count cydes,
then high on the terminal count. The low on counter l’s gate (G) enables it to increment at the CPU
clock rate — about 2.5 MHz. The high stops the counter. The resulting 16-bit number represents the
period of counter 0's programmed number of cycles. The number, called REF CNT, is stored in
memory.
2-19