OCTOVOLT//Eurorack Modules for Audio and Video

Eurorack Prototype of Two Comparator Effect

I have been working on a version of cyberboy666's two_comparator_effect, which I call TCE. The prototype is working, and I have used it in some of my live video art performances in support of live bands and DJs at shows and parties at Light Rail Studios in San Francisco. It's been a great addition to my video set up.

You can see it working in the video below, on the Octovolt YouTube channel.

This circuit was originally designed by Rob Schafer and later improved by cyberboy666. I talked with cyberboy666 about whether it would be okay for me to create this Eurorack version, and he said that it's open hardware so I should feel free to do so. If you are interested in his design, which leaves the electronic components exposed and operates on a dedicated +5v wall wart power supply, you can get his version on the underscores website, where you'll also find a lot of other excellent circuits for video art.

I hope I have contributed to the lineage of this circuit's design by converting it to a Eurorack module. In the spirit of this, I decided to put the open hardware logo on the front panel rather than my normal octopus logo. I also have the KiCAD files published on GitHub.

The front panel holes were a little bit too small for Alpha 9mm potentiometers, so I had to use tall trim pots instead. I have revised the front panel to allow the Alpha pots, and I was able to improve the PCB a bit as well. As a result, it should be a little bit easier to build. And with the Alpha potentiometers offering more connections between the PCB and the front panel, the whole thing will be a lot more sturdy.

Currently, as I write this post, I'm waiting for the new PCBs and panels to come back from the fabrication house. When I get them back and test them, assuming everything works as it should, this will be the second Octovolt module. As you might imagine, I'm pretty excited about it.

I plan to create a different version of TCE (still thinking about the name, but maybe Thresholds) which will be designed for 0-1v RGBY signals rather than composite video, and it will offer CV control and some alternate outputs.

Working on this module has led me to think a lot about how comparators can be used in video circuits. After studying some of the Sandin Image Processor circuits on scanlines.xyz, while also thinking about comparators, I started developing an "edge rendering" circuit. You can take a look at my progress on that circuit on my personal Instagram account, especially this post, and this post, and this post. I built a prototype of this circuit on David Haillant's Eurorack Stripboard, and the documentation for that is also up on Github. But a better, more polished version of it is coming soon.

Another thing that has emerged through my work on TCE, and the other circuits where I use comparators, is the fact that the comparator in TCE (and the original two_comparator_effect) is actually a little bit too slow for video. The comparator in question is the LM339, which is cheap, widely available, and comes in a through-hole quad package. Unfortunately, it has a response time of about 1.3 microseconds, and for accurate video work we would want a response time under 83 nanoseconds (according to my calculations for NTSC 720×480 video -- please contact me if I'm wrong). The resulting image from the LM339 is shifted very slightly to the right. You can see this pretty clearly in this post, where the blue and red signals are going through the LM339, while the green is not. Still, it has a cool aesthetic. TCE's slow comparator will not be perceivable unless one mixes the output with the original signal (or an accurate version of the original), like I did in that post. Regardless, with the circuits I'm currently developing, I'm going to use a faster comparator. I'm still working out which one to use.

If you're interested, please check back soon for the PCB/panel sets and full DIY kits, or check out the GitHub repository for TCE.

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