Optical disc controlled FV-1

[mark2]

This was just a fun experiment, taking the idea from this Make magazine article with an analog tremolo.

• A pot connected to the uC (an arduino nano) controls the PWM speed for the fan, which spins a CD with light and dark markings.
• an IR LED beams IR light up
• an IR receiver diode reacts to the reflected light, and is read on an analog pin of the uC
• the uC "learns" the dynamic range of the reflection and scales a PWM signal that it writes to a digital pin
• a LPF smooths out the PWM, and a voltage divider steps it down to 3V3 (ideally I'd run the uC at 3v3 but that was inconvenient at the moment)
• the 3v3 signal is fed back into my frankenstein FV-1 test pedal which has been modified to let you switch whether pot2 reads from the potentiometer or a wire sticking out of the enclosure (ignore those blue, white, and brown wires which are from other unrelated experiments)

It was a fun experiment, but it's not very practical for obvious mechanical reasons.. but moreover, interesting patterns on the disc don't translate to interesting sounds. Even at very low speeds everything sounds quite uniform and no better than a simple regular pattern. That was the most disappointing part. I thought you could get some interesting beats but the ear (at least my ear) averages it out and it still sounds like a normal lfo.

[ElectricDruid]

I think you might be running up against the digital filtering on the pot inputs inside the FV-1. I can't remember the details (Maybe Frank will pop up) but they're pretty well smoothed to prevent noise creeping in. I do the same thing in my own PIC designs, for the same reason. For knob twiddling, you really don't need a fast response. Something smooth is much to be preferred, and the output from the digital filter can even provide *more* bits of resolution than the actual ADC (since it can smooth between ADC values). So it's a win-win, really - unless you decide you want to feed a fast modulation signal into the input instead of a slow pot!

[mark2]

I'm not sure if you're referring to something internal on the hardware... but as for the patches there's no RDFX filtering or anything.e.g. that tremolo is literally just 3 lines (rdax, mulx, wrax)

[ElectricDruid]

I'm not sure if you're referring to something internal on the hardware... but as for the patches there's no RDFX filtering or anything.e.g. that tremolo is literally just 3 lines (rdax, mulx, wrax)

Yes, I was referring to internal hardware filtering. In this thread Frank says the TC is about 100msecs:

http://spinsemi.com/forum/viewtopic.php?f=2&t=15

Given that 4TC is about 98% to the final value and 5TC is about 99%, that's a rise time around 0.4 - 0.5 sec. I don't know if that's significant for your optical disc, but it's fairly slow for modulation waveforms. Slow enough to turn a 1Hz square wave into a triangle!

[mark2]

Ahh,  that's definitely going to make a noticeable difference. Thanks!

[mark2]

If anyone's curious, I found this trick that may counteract the lag: http://www.spinsemi.com/forum/viewtopic.php?p=984#p984

Haven't yet tried it but presumably it will make the effect more pronounced and the transitions less sloppy.