has ‘oscillator
sync’ on oscillator
2. It seems when
this is turned on oscillator 1
somehow controls the pitch of
oscillator 2 so they stay in
tune. How is this used?
Doug Kraul replies:
A Oscillator sync
is a synthesis
technique where
one oscillator, the
master, controls the operation
of the second (the slave) so
the two are both in tune, with
a fixed phase relationship
(also known as ‘phase
locked’). If you modulate the
second oscillator’s frequency
control when the oscillators
are locked in this fashion it
produces a timbre change
instead of a pitch change.
The principles behind
oscillator sync are quite
straightforward. Every time
the master oscillator
completes its waveform cycle
it causes the slave oscillator
to restart its waveform cycle,
regardless of where the slave
oscillator is in the cycle.
If the two are close in
frequency, or the ratio of their
frequencies is a simple
number (for example the
second oscillator is tuned to
twice the first’s frequency)
then the second’s waveform is
close to its original nature.
However if there is a
significant frequency
difference then the waveform
is altered, sometimes
dramatically, producing a
different timbre.
One typical use is to tune
the oscillators so they are ‘in
tune’, then modulate the
frequency control of the slave
oscillator with an LFO or
envelope. This sweeps the
slave oscillator’s frequency
and the resulting dynamic
timbre is similar to the sound
of a high resonant filter
sweep, except it has a bit
more ‘edge’. The prototypical
‘sync lead’ sound is this
technique using one of
the envelopes to sweep
the pitch.
Oscillator sync has been
used in synthesizers for a long
time. One of the first widely
available synths that featured
it was the ARP Odyssey, and
it became one of its signature
sounds. The Prophet 5 was
also well known for its ‘sync
lead’ patch.
Now let’s look at some
waveform pictures of
oscillator sync. The following
screengrabs are recordings of
two analogue modular VCOs
whose outputs were directly
recorded. The master VCO’s
output was the left input, the
slave the right input. In all of
these screengrabs the top
waveform is the master.
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