The Yamaha DX7 made its sounds by wiring six sine oscillators into each other.
Valis has the two parts that needs, and the wiring is written in the circuit like
any other arc. Three examples show it: examples/dx7-epiano.ttl,
examples/dx7-bass.ttl and examples/dx7-bells.ttl.
Background: frequency modulation synthesis and the DX7.
val:FMOperator is a sine oscillator whose phase is driven by an audio input,
pm. The signal on pm is the modulator. How far it bends the phase is its
amplitude times val:depth, called the modulation index. An operator with nothing
on pm is a plain sine, which is how a carrier is made. Several arcs on pm are
summed.
Strictly this is phase modulation rather than frequency modulation, and so is the DX7. The two produce the same sidebands, and phase modulation does not drift in pitch.
| port | meaning |
|---|---|
frequency |
connect val:MidiPitch here |
ratio |
multiplies the frequency. A whole number is a harmonic of the note; anything else is inharmonic |
fixed |
above 0, ignores the keyboard and sounds this many hertz |
detune |
in cents |
level |
output level on the DX7's 0 to 99 scale |
amp |
0 to 1, for an envelope to drive |
depth |
radians of phase shift at full scale on pm |
feedback |
0 to 7, the operator's own output fed back into its phase |
val:RateLevelEnvelope is the DX7's envelope. On key down the level goes to
l1 at rate r1, then to l2 at r2, then to l3 at r3, where it stays until
the key comes up. Then it goes to l4 at r4. Levels and rates are the DX7's 0 to
99. Levels are linear in decibels, so each step down is about 0.75 dB. A rate is a
speed, so a segment takes longer the further it has to travel.
Connect its out to an operator's amp. On a carrier that is the loudness. On a
modulator it is the modulation index, so a modulator whose envelope falls makes a
note that starts bright and softens, which is most of what FM sounds like.
velocity and velsens make harder playing louder. Connect val:MidiVelocity to
velocity. The modulators usually take more of it than the carriers, so a hard
note is brighter as well as louder.
The DX7's 32 algorithms are fixed ways of wiring its six operators. Here nothing
is fixed. A modulator's out goes to another operator's pm, and a carrier's out
goes to the mix. The examples use two of the DX7's:
2 -> 1, 4 -> 3 and 6 -> 5. Used by the electric piano.2 -> 1 and a chain 6 -> 5 -> 4 -> 3. Used by the bass and the bells.Each example defines one :Op subcircuit, holding an operator, its envelope, and
the pitch and velocity sources that feed them. A voice is six instances of it with
their ratios and envelopes set, and the circuit is that voice stamped out as a
pool with val:voices. See writing circuits.
Some DX7 algorithms feed an operator's output back into an earlier one, through
more than one operator. Separate val:FMOperator elements cannot do this. An arc
that returns to an earlier element is a cycle, and val:UnitDelay, the only thing
that breaks one, delays a loop by a control slice and a sample, 33 samples, which is far too long
for FM. A loop has
to be closed a sample at a time, so it has to be inside one element.
val:FMMatrix is that element. It holds six operators and a modulation
matrix: m<d><s> is how much operator s modulates operator d, as a fraction of
depth. The entries on the diagonal, m11 to m66, are the DX7's own feedback
control from 0 to 7. carrier1 to carrier6 choose which operators are heard, and
amp1 to amp6 are for envelopes, one val:RateLevelEnvelope each.
Operators are evaluated from 6 down to 1, as on the DX7. A modulator numbered higher
than its target is used on the same sample. One numbered lower is used as it was a
sample ago. So the usual wiring costs nothing, and a loop has one sample of delay on
the arc that runs against the order. An algorithm is a choice of matrix entries and
carriers: algorithm 5 is m12, m34 and m56 at 1 with carriers 1, 3 and 5.
examples/dx7-brass.ttl uses it for a loop between operators 4 and 6. The matrix
keeps the whole voice in one element, so it is also cheaper than six separate ones,
and an operator whose level has fallen to nothing is skipped.
val:LFO can be wired to frequency for vibrato.The patches are written in the spirit of the factory sounds and are not copies of them.