Why not use the standard VCO sync?
The simple answer is that all the standard SynthEdit oscillators use what is called “Hard Sync”, which is an “all or nothing method. Using this method of “soft sync” allows control over how much sync is applied to the VCO. What we do is take a portion of the Master (Sync) Oscillators wave form as it crosses zero, and use this to Phase Modulate the Slave (Audio) Oscillator.
Third party modules:
There are two third party modules used for prefab the ED Zero Crossing Detector, and the ED Flip Flop (Audio).
The Oscillator HD is one of the community SE modules available from the SE website.
The Soft Sync Master/Slave oscillator.
This consists of our two oscillators (master and slave) the output of the master is fed to the ED Zero Crossing Detector, which generates a very short (1 sample long) pulse everytime the output of oscillator 1 crosses 0 volts. We can set the detector to trigger on either the rising voltage, the falling voltage or both rising and falling. This is fed to the ED Flip Flop (Audio) which is used as a bistable so the first pulse triggers a “High” output, then the second resets the output to a “Low” output. This allows us to gate a one cycle long burst of the Master oscillator signal of variable ampliture to feed to the Slave Oscillators Phase Modulation plug.
What we are effectively doing is using the output of the Master Oscillator to gently (or not so gently) change the phase of the Slave oscillator, thereby changing its output shape to a greater or lesser degree.
Note: If you are intending to use large levels of Sync, or waveforms (such as ramp) with fast rise/fall times you will introduce aliasing, so it’s good policy to include oversampling in the Master/Slave Sync Oscillator’s container to limit this.
The 1 Pole HPF is included to cut out any “LF rumbles”, or possibly DC from any aliasing produced by very fast phase changes.
Settings and values.
Settings for the ED Flip Flop are:
V Threshold: 0.01 V. This is the input level at which the flip flop triggers and switches state.
V Out: 10V.
Both slave and Master Oscillator HD:
Reset Mode: DCO (Sync).
1 Pole HP:
Freq Scale: (Properties) 1kHz per Volt.
Pitch: 0.014 (14Hz)
Sync Gain:
Level Adj Input 2: You can use the default of 0 to + 10V, but you can also go from -10 V to + 10 V which allows the sync to use phase reversal in the modulation.
Master 1 octave above Slave.
As you can see below changing the edge detection also affects the resulting output signal from the slave oscillator.
Rising edge zero crossing detection.
Falling edge zero crossing detection.
Using both rising and falling edge zero detection.
Increasing the amount of Sync fed to the PM Plug.
