Frequency Settings; Envelopes - Yamaha YM3806 Programmer's Manual

OPQ Programmer's Guide V 1.1
Also during release phase Op3 affects on the other operators: if Op3's retrigger is disabled
and it has long release time, it will not retrigger before its output level has decreased near
zero. During that time any other same channel's operator won't retrigger either, even if its
own release time has already ended.

3.4 Frequency settings

In OPQ each channel can have two different frequencies. Frequency is always set to an
operator pair, meaning that two of the channel's operators can use one frequency, while
two other can use completely different frequency. The two frequencies do not need to be in
a harmonic relation, which again opens up interesting possibilities in sound design.
There is also possibility to detune each operator from its nominal frequency. And of course
the frequency multiplication is available as always in FM.

3.5 Envelopes

Each operator has individually controllable envelope curve. In fact, half of the registers are
used for the envelope settings.
OPQ envelope is a traditional ADSR-style envelope with one added parameter, sustain
rate. During sustain phase the envelope curve can also decrease, and sustain rate
controls the decrease rate. If sustain rate is 0, the sustain phase is horizontal (not
decreasing), and the the envelope becomes basic ADSR. The curves are logarithmic in all
phases, but with small values of sustain rate the curve is practically linear, as in the picture
below.
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