Sequential prophet 5 - Voice Function; Oscillator a (Osc A)

In UNISON mode the KEY CVs are switched off. Instead, the UNISON CV supplies all pitch control
through the COMMON ANALOG circuitry. Also the GATEs are tied together, so all ENV GENs trigger
simultaneously.
Patch CVs and S signals originate from either of two places inside the microcomputer, depending
upon the operational mode. The central processor unit (CPU) implements these modes, as controlled
by the programmer — grey — switches. In MANUAL mode the CVs and S signals directly follow the
programmable — black — knobs and switches. If desired the patch may be recorded into the non¬
volatile program memory (NV PRGM RAM). Then in PRESET mode, the programmer selects data in
NV PRGM RAM which patches the voices.
Below, paragraphs 2-3 through 2-15 will explain the voices, common analog circuitry, control
panel/keyboard, and the microcomputer system in more detail. Paragraphs 2-16 through 2-24 will
then note hardware functions. Throughout these descriptions you will often read that ‘the computer
... (does this or that)", without it being explained why. Very often, the reason for this is that the 'why'’
lies in the microcomputer program itself. And except for the most general remarks, the program will
not be detailed.
NOTE: The Prophet software is proprietary information. Knowledge of it is not required to either
learn how the Prophet operates or to adjust or repair it.
2-3 VOICE FUNCTION
To understand what the microcomputer system does one may first learn what signals the voices
require. VOICE 1 is used for explanatory purposes. (Except for circuitry on VOICE 5 enabling it to be
controlled by an external sequencer, the Prophet's five voices are functionally identical).
NOTE: Throughout this manual OSC A and B stands for OSC 1 and 2 marked on the control panel.
Also the terms OSC and VCO. FILT and VCF. AMP and FIN VCA. are used interchangeably.
Fig. 2-4 diagrams a single voice and should be referred to for paragraphs 2-4 through 2-9 below. Four
types of functions are shown: 1) VC-modules; OSC A. OSC B. VCF. FIN VCA. and the FILT and AMP
ENV GENs; 2) CV summers (SMRs) which combine the various CVs for OSC FREQ or PW, or FILT FREQ;
3) Control VCAs. each shown as a triangle with a “CV" line, used for setting audio or other CV levels;
and 4) Solid-state analog switches, each shown as a box with an “S" line. A binary 1 or HIGH logic
signal on the S line closes the path between the switch's input and output. These switches make it
possible for binary data from the computer to patch the voices.
The only independent CV inputs to a voice are its KEY CV and oscillator BIASs. All other CVs are
common to all voices. For example, the FILT ATK CV is the same for all five FILT ENV GENs.
The Prophet has two distinct and separate modulation systems. W-MOD (WHEEL-, monophonic) and
P-MOD (POLY- or polyphonic). W-MOD uses a single LFO (or noise source) whose output, through
the COMMON ANALOG circuitry, affects all voices identically. On the other hand. P-MOD estab¬
lishes the same modulation "patch" within each voice using the five separate FILT ENG GENs or
OSC Bs. The minute differences between for example, oscillator phasing, creates varied modulation
between the voices. P-MOD is discussed below. W-MOD in paragraph 2-10.
2-4 OSCILLATOR A (OSC A)
OSC 1 A s FREQ is controllable by four CVs. A SUM CV originates in the COMMON ANALOG cir¬
cuitry. It is the sum of the W-MOD, P-BND, MTUN. OSC A INIT, and UNI CVs.
The second signal to OSC A's FREQ SMR is the KEY 1 CV. which carries polyphonic pitch information
for each voice. As mentioned under 2-2. when playing in UNISON mode. KEY 1 CV is switched off and
UNI CV — through A SUM CV — becomes the principal FREQ CV.