Sequential PROPHET-600 - 5-2 PLAYBACK

2
THEORY
2-0 GENERAL
This section explains the Prophet-600's theory of operation. First a general discussion
provides a functional overview, then the microcomputer and synthesizer circuits are
detailed. Throughout it is assumed that the reader is both familiar with operation of
the instrument (see Operation Manual CM600) and with the functions of common
linear, TTL, and CMOS integrated circuits (ICs). Anyone contemplating repair or
modification work on the Prophet-600 must already be familiar with op arnp circuits
and digital logic.
The Prophet-600 is a programmable polyphonic hybrid synthesizer. The term "hybrid"
refers to the combination of digital and analog circuitry. The digital side features a <#-
MHz 2-80A microcomputer system with an 8K EPROM operating system and 4K of
non-volatile RAM. This system processes keyboard and control panel inputs into
suitable control voltages (CVs) and switch signals which control the sound produced by
the analog synthesizer voices. See Figure 2-0.
SYNTH
Figure 2-0
GENERAL BLOCK DIAGRAM
Converting all synthesizer control signals into digital form allows sets of control data
which comprise an instrument or effect to be stored in memory as "programs," and be
instantly available as a control selection. Sets of real-time keyboard data are also
stored in memory as "sequences." So the microcomputer primarily functions as a
controller (rather than as a synthesizer).
But while the Prophet-600 emobodies this basic design philosophy of the Prophets -5
and -10, its computer system differs by participating more directly in the synthesis
itself. Most significantly, there are no separate, integrated analog envelope generators
(as in the -5 and -10). Instead, the envelopes are calculated by the computer and
appear as dynamic CVs (at the sample/holds) for each voice filter frequency and
amplifier gain. A noteworthy side-effect of this is that the familiar TRIGGER and
GATE signals for each note no longer appear in the synth. And, for POLY-MOD, since
the filter envelope is already "in" the computer, it is through software integrated into
the CVs for selected POLY-MOD destinations.
TM600A 7/83
2-1