Volca Sync DIY

A few weeks back Cameron (yes - the Cameron who started all this synthesizer nonsense) came into the office with a neat little toy. It was the Volca Keys Analog Synthesizer, made by Korg. It was great for annoying other occupants of the office (especially Tom) but it couldn't really do anything my modular couldn't already do (apart from in the sequencer department and I already had plans to put that right).

Last week, I found myself outside a music shop and I went inside and got myself another member of the Korg Volca family, which could do some different things; the Volca FM, which is a Digital FM synthesizer...

I spent quite a few happy hours over the Easter weekend putting right the deficit on my modular synthesizer by building the sequencer which I've been plotting for the last few weeks. Once I had that running, I thought it would be neat to synchronize the little Volca up to the modular, so the two can play along together...

The Volca FM (in common with the other units in the Volca series) has a couple of sync connections on its front panel: SYNC IN and a SYNC OUT, both on 3.5mm TRS jacks. The user manual explains their use and enough to deduce the 'format'...

The new sequencer for my modular synth is controlled by an Arduino (NANO) and has 5V clock signals bouncing around all over the place. But there isn't a  15ms pulse / trigger. Of course, I could generate one in software and output it over a GPIO but

1. that would take up resources in the code and, more importantly, 
2. I have already assigned all the IO resources of the NANO to other functions. 

So - I decided to generate the sync pulses from the existing clock pulses using hardware.

All that's needed is a simple one-shot, triggered at the leading edge of each of my existing clock pulses. Sounds like an easy job for a venerable 555.

Here's my circuit:

I put the trimmer on the resistor which sets the time constant as I didn't know just how sniffy the Volca would be about the precise length of the 15ms pulses (the total resistance should be about 137 kOhms for a 0.1uF capacitor). As it happens, I suspect a 100k and a (fixed) 33k in series would do fine.

Here's the circuit in development on a solderless breadboard, with my sequencer off to the left:

Adjusting the trim pot allowed me to set the specified 15ms pulse length:

and here are a train of pulses, generated when my sequencer is stepping at its fastest rate (as currently programmed):

Here's the circuit at the bottom of my (working, but not-yet completed) sequencer controller module:

The whole thing clocks the Volca FM perfectly:

I can't test to confirm that the same circuit will also drive the other Volcas (because I don't have any) but I'm sure it would. Perhaps I'll ask Cameron for a loan of the Keys, just to be sure.

Now I need to arrange for a clock input to my sequencer to respond to the SYNC OUT of the little Volca - it only seems fair.

...-.- de m0xpd