Sequential prophet 5 - Oscillator B; Filter Envelope Generator (Filt Env Gen); Filter (Filt)

BIAS 1A is one of the ten BIAS outputs from the Prophet's tuning circuitry. Explanation of the auto¬
matic tuning process must be left until computer functions have been described. For now, it is suf¬
ficient to know that BIAS is a minute CV which corrects pitch deviations throughout the oscillator
range. The BIAS has a little over a semitone in range.
Fourth, OSC A FREQ can be controlled through the P-MOD circuit. A switch applies a CV.mixed
from either the FILT ENV GEN or OSC B outputs to the A FREQ SMR.
The A PW SMR combines the PW A INIT CV. which follows the setting of the OSC A PW knob, with
LFO or NOISE modulation from the W-MOD circuitry. A P-MOD switch parallel to that for FREQ A
functions similarly for PW A.
When activated. OSC A SYNC switch allows OSC A to tune only to harmonic frequencies of OSC B.
or changes OSC A's timbre when the two are not in harmonic relation.
OSC A has positive-going sawtooth and pulse outputs switched to the MIX OSC A AMT VCA. This CV
follows the MIX OSC A knob setting.
2-5 OSCILLATOR B (OSC B)
OSC B function is similar to OSC A except that OSC B can be a CV source, as well as an audio source.
A switch in the KEY 1 CV patch disables polyphonic tracking when OSC B is to be used as a fixed-
frequency oscillator. In such cases a switch in the OSC B MSUM circuit also subtracts UNI CV from
the B SUM CV. LO FREQ operation is also enabled by a switch in the B MSUM circuit.
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OSC B PW is unaffected by P-MOD. so a summer equivalent to the A PW SMR is not required.
PW B CV results from PW B INIT CV and PW B W-MOD CV. combined in the PW B MSUM.
OSC B has a bipolar triangle-wave output in addition to the positive-going sawtooth and pulse
outputs. The P-MOD OSC B AMT VCA sets CV level to the destinations selected by the three P-MOD
switches.
2-6 FILTER ENVELOPE GENERATOR (FILT ENV GEN)
The FILT ENV GEN is a four-stage ADSR transient CV source. GATE 1 is a binary signal which goes
HIGH whenever that key to which the voice is assigned is pressed. GATE is also applied to the TRIG
input through a pulse-forming network containing C402. When GATE first appears, the resulting TRIG
pulse actually starts the ADSR sequence.
The ATK, DEC, and REL periods are exponentially controlled by the CVs shown. The SUS level is
linearly controlled by the FILT SUS CV. The SUS period continues for as long as GATE is present (key
down) after the ATK and DEC periods. The REL period starts when the GATE is turned off (key up).
FILT ENV GEN output to the FILT is normally controlled by the FILT ENV AMT VCA. As mentioned
above, the P-MOD FILT ENV AMT VCA allows the FILT ENV GEN to modulate OSC A FREQ or PW.
Note that the P-MOD FILT switch is a redundant path for the ENV GEN output. This switch is intended
to allow OSC B to modulate the FILT.
2-7 FILTER (FILT)
There are four audio inputs to the FILT: 1) NOISE, whose level is controlled elsewhere; 2) OSC A; and
3) OSC B. whose levels are controlled by their MIX AMT VCAs. Fourth, the FILT output is fed back to
the input by the RES FDBK VCA to sharpen FILT resonance.
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