This is a sub-circuit of Rene Schmitz' VCO3 that I've been working on, but I suppose it could be added to any VCO with a hard sync input, or put into a different module if there's some spare panel space, to provide soft sync where it's not otherwise available. The circuit requires the triangle output of the VCO that is to be synced and the pulse output of the VCO that is the 'master'. It has a level control so that you can control the level of the pulse signal. It provides a pulse signal dependent upon the coincidence the peak of the two wave forms, which is fed to the slave's hard sync input.

It 'captures' when the frequencies are fairly close together, but also can sync at intervals such as octaves where the waveforms become consistently coincident. The capture range can be widened by increasing the level of the pulse waveform, and I suppose you could set it to where it was purely hard-syncing.

The trimpots could be replaced by resistor dividers once you figure out the levels you want for your system.

Sound Samples of the Soft Sync Circuit Using Two VCO3's

Soft Sync Capture and Lock

This first example demonstrates the capture and lock of the soft sync. The recording begins with a mix of sawtooths from each VCO. Initially, they are un-synced, and you can hear them moving in phase. Shortly after the sample starts, I connect the pulse output of the master to the pulse input of the soft sync circuit, the output of which is connected to the hard sync input of the slave VCO. You can hear the two VCO's audibly 'lock' together abruptly 12 seconds into the sample. BTW, the further apart two VCO's are within the lock range, the quicker sync is attained.

As the sample progresses, I adjust the fine tune of the master VCO up and down - you can hear that the slave VCO tracks it, and the timbre changes the closer to the ends of the lock range it gets. Then I increase the swing of the fine tune down past the lock range and you can hear the VCO's lose sync 29 seconds into the sample - it's very obvious, because the two VCO's lose their frequency relationship. Shortly after that, I tune the master VCO back up, the VCO's lock, and I increase the tuning until once again the lock range is passed, and again the two VCO's 'detune' at around 34 seconds into the sample. Then I swing it back into sync and peck a few notes.

Soft Sync Capture and Lock (409 KB)

Sweeping the Slave VCO Frequency During Soft Sync

A happy artifact of this process causes the slave VCO to 'step' as its frequency is radically shifted in relation to the master VCO. This comes in handy for easily setting intervals between the two VCO's.

In this sample, the master VCO is the 'high' tone, and the slave is the initially low tone. In the first part of the sample, I am simply sweeping the coarse tune of the slave VCO back and forth - notice the 'stepped' response from it.

Then, as the sample progresses, I move the coarse tune of the slave a small bit, play a few notes, move the coarse tune a bit more, and play a few more notes, etc. This is to demonstrate the ease at which intervals can be tuned using the circuit.

Sweeping Slave Frequency (465 KB)

Sweeping the Master VCO Frequency During Soft Sync

Sweeping the master VCO has quite a different effect. In this sample you can hear the slave lock and unlock as the master VCO is swept. This could be used for setting the frequency of the master oscillator in relation to the frequency of the slave. As mentioned before, fine adjustment of the master VCO within the lock range results in different timbres.

At the very end of the sample, I disconnect the sync pulse, and you can hear briefly the VCO's unlock to show the unsync'ed frequency relationship of the two VCO's.

Sweeping Master Frequency (305 KB)

Sample of Soft Sync In Use

Just me pecking at the keyboard. The sample starts out with two synced oscillators. The sawtooth from the master oscillator and pulse wave of the slave are mixed together.

27 seconds into the sample, I add the divide by two suboscillator output of the master, put some vibrato onto both VCO's, and run it through my improvised BBD circuit to give somewhat of an impression on how cool a *real* ensemble chorus would really treat a sample like this. Oh yeah, as with all of my recent samples, they're going through the 291 core VCF (just too lazy to disconnect it).

A few other examples of use can be found in my 'Birth of a Synth' section.

Using the Soft Sync Circuit (494 KB)

Using FM with this circuit is pretty interesting. Here are a couple of samples that feature only the triangle wave of the synced oscillator. It's being modulated by the triangle of the master oscillator, which is being modulated by a triangle LFO.

Slave FM'ed by Master 1 (646 KB)

Slave FM'ed by Master 2 (476 KB)

Schematic and Theory

Zoom on Schematic