Sequential prophet 5 - The Prophet

Figure 2-2
Prophet Block (Diagram
THE PROPHET
The Prophet is a subtractive, performance and studio analog synth. It provides instantaneous patch
repeatability and polyphonic capability without the limitations of organ technology or fixed presets.
Fig. 2-2 shows the Prophet at the most general level. Instead of controlling the synth directly, the
keyboard and most controls are processed through a microcomputer system. The microcomputer
system provides a way to store all of the switch and knob settings which form a patch, and solves the
problem of generating five sets of independent CVs and GATEs from a single keyboard. Common
analog circuitry mixes the few controls not processed by the system with processed control signals
from the voices. These patch the five voices to sound fundamentally alike (homophonous) with,
typically, pitch differences corresponding to (at most) five simultaneously held keys.
Fig. 2-3 specifies the principle functions of the four main blocks. Beginning with the voices, their
outputs are combined and overall volume set by a VCA controlled directly from the front panel.
Each voice is a complete synth with two VCOs. a MIXER. VCF. FINAL VCA. and two ENV GENs but they
are patched homophonously by common switch (S) signals and CVs. Each voice also has its own
KEY CV which provides polyphonic pitch information, and GATE, which signals that the key is being
held. All S signals and most CVs are generated by the computer. The common CVs include master
tuning (MTUN), pitch-bend (P-8ND), wheel controlled modulation (W-MOD), which are mixed in
the COMMON ANALOG circuitry. The COMMON ANALOG circuitry also requires a few S signals.
The microcomputer performs the tasks of voice assignment. It decides which held keys sound which
voices through their KEY CVs and GATEs. The computer assigns VOICE 1 to the first key held, VOICE 2
to the second key. and so on. After the five initial assignments the system is “last note priority’'. The
earliest used voice is reassigned to each new note played. Repeated notes key the same voice. For
example, holding C. D, E, F, and G. sustains voices 1-5, respectively. Adding A "steals” VOICE 1 from
the C which "disappears”, even though it may still be held.
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