Vidiot (and others): How to create wobbly waves by using audio frequencies around 50 (or 60) Hz

tl;dr:

  • feed a 49,5 Hz (or 59,5 Hz) audio frequency into the Horizontal CV In or the Vertical CV In
  • enjoy :wink:

Longer version:
I use this technique with a Vidiot. I assume it can work with other devices/modules. Maybe it’s a well known thing but I couldn’t see it mentioned anywhere.

You may have already tried routing audio signals into the horizontal or vertical oscillator CV inputs, which usually generates funny waves crazily spinning around. I found out (by chance) that some specific audio signals instead create very predictable and slow behaviours.

First, you need to be able to generate stable audio frequencies around 50 Hz (or multiples) and slightly modulate their pitch (say from 49 to 51 Hz). So get a VCO, or install a free app on your phone (that’s what I do), or properly program your synth (a Volca FM with Osc Mode = Fixed will do). I suggest to start with a basic sine wave. More complex waveforms will produce more chaotic movements.

EDIT: I use PAL, so this happens around 50 Hz. If you use NTSC, go for the range around 60 Hz.

Create a patch that uses the Vertical and Horizontal oscillators. Example:
video
Route your ~50 Hz audio frequency into a CV inputs. If you play exactly 50 Hz, the pattern will get deformed (proportionally to the loudness) but stay still.
As soon as you drift away from 50 Hz, the pattern will move (faster and faster).
Try also playing around multiples of 50 Hz (199-201 Hz, etc).

Example - Horizontal CV In:
video :star_struck:
Example - Vertical CV In:
video2 :crazy_face:
I suspect interferences play a role, but I don’t really know what’s going here, which has been making it even more fun to play with. I look forward to your explanations.

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The scrolling stops at 50 Hz because you are (I assume) using PAL encoding, which has 50 video fields (25 frames) per second. If you had your Vidiot set to NTSC, it would happen at 60 Hz instead, because NTSC uses 60 fields (30 frames) per second. I believe I recall reading somewhere that this frequency was chosen to make it easier to derive sync pulses from the mains power AC waveform in the US, but I don’t have a source for that claim offhand. This means that 50 times a second, if you were hooked up to a CRT display, the sawtooth oscillators driving the deflection solenoids (electrically controlled magnets) for the electron beam would have their phase reset, causing the beam to return to the top left corner of the screen to trace the next video field. The oscillator for the horizontal deflection solenoid would additionally get a reset pulse at the end of every scanline, at an accordingly much higher frequency (15.6+ kHz). This is “sync”, just like how you might sync audio oscillators to each other when you FM them. Digital displays of course do not do all this with electron guns, and can often draw pixels to the screen in any order they like, at least from software’s perspective, but they’re decoding a composite video signal meant to drive analog displays.

So when your audio rate voltage signal is at 50 Hz, that waveform is at the same frequency as the Vidiot’s vertical sync frequency, and the scrolling slows down and stops. When you’re farther away from that frequency, your audio oscillator is running faster or slower than the Vidiot’s vertical oscillator, so you get a scrolling effect as one oscillator’s phase “runs past” the other. If you have an audio rate oscillator with a sync input, you can try syncing it to your video system to get modulation happening without scrolling as the audio and video oscillators stay in sync with each other, though you may need to boost the voltage levels of LZX 1V sync outputs to get some oscillators to register it.

So yes, this is a fundamental property of analog video. And it is frickin’ magic.

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Hi @csboling, that’s right! I updated the post with a note about NTSC vs. PAL.
Detailed and clear explanation, thanks a lot!