Yamaha DX7 II FD Reference Manual page 18

Pmd There are two ways to produce vibrato by applying LFO to pitch: "automatic" (LFO is
applied to any played note) and "controlled" (LFO is applied by operating a continuous
controller such as the mod wheel). The PMD (pitch modulation depth) parameter controls
the amplitude or intensity of the LFO signal as it is applied to pitch "automatically." This
parameter acts like an output gain control for the LFO signal routed to modulate the pitch.
(The intensity of "controlled" vibrato is determined by the Pmod parameter, which is
described in the next section.)
0 No LFO is applied automatically to pitch.
99 LFO is applied automatically to pitch at maximum amplitude.
It is largely up to your discretion whether or not a voice should include automatic vibrato.
It depends on the musical context and your playing style.
Amd As with vibrato, the application of LFO to the operator output levels can be "automatic"
or "controlled." The AMD (amplitude modulation depth) parameter controls the
amplitude or intensity of the LFO signal as it is applied to output level "automatically."
As with Pmd, this parameter acts as an output gain control for the LFO signal routed to
modulate the operators' level. (The intensity of "controlled" tremolo or "wah-wah" is
determined by the Amod parameter, which is described in the next section.)
0 No LFO is applied automatically to output level.
99 LFO is applied automatically to output level at maximum amplitude.
As you may recall, application of LFO to the output level of a carrier modulates the
volume of the voice, producing a tremolo effect. Application of LFO to the output level of
a modulator produces a "wah-wah" effect as the timbre of the voice is modulated.
Sync The Sync (key synchronization) parameter performs a function similar to that of the
Osc.sync parameter. In this case, it determines whether or not the waveform produced by
the LFO will begin at phase 0 every time a key is pressed.
off LFO waveform does not start at phase 0 for each keypress.
on LFO waveform starts at phase 0 for each keypress.
Turning the Sync on is useful at slow LFO speeds for producing an identical effect during
the attack of each note. Turning the Sync off produces more natural chorus or rotary
speaker effects, since the LFO cycle is not influenced by the keys being played.
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