AKSO for video?

#1

HI @nsm

Thanks for all your hard work. I’ve been following the progress for a long time. Just saw that ordering was open today and ordered 2 boards.

Can you tell me which op-amps are used for the I/O buffers? I’d like to experiment with Akso as a video oscillator/processor (something I did a bit with Axoloti). Axoloti is ultimately too slow to be of much use for video, but I’m hopeful for Akso. It could be groundbreaking.

A potential issue is that TL07xx op-amps that are typically used for Eurorack buffers have a slew rate that’s way too slow for video. Luckily, depending on the circuit, there are possible drop-in replacements. Could be tricky, as wide bandwidth op-amps sometimes require some things that slower chips don’t (some resistance in the feedback loop for instance). I’m not sure if the schematic is published or not, but I’m curious to look at it and see if this modification is possible without having to wire in a daughterboard.

Another constant frustration I have with Axoloti is the inability to tune oscillator objects to precise frequencies, which could also be helpful for audio people. Also, the frequency ranges of objects seem to be arbitrarily limited. I’ve managed to stumble around the code enough to make small object modifications, but my skills are squarely in the analog domain. I haven’t had any success in either of those departments. Maybe there’s someone out there that can fix these things?

Thanks again, and really looking forward to exploring this new territory.

Jonas

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#2

@JonasBers This is a fascinating question. You’re talking about analog video? What range of frequencies do you need to process?

The full schematic is not yet published. The Akso hardware does indeed use standard TL07xx op-amps. The programmability of the analog signal chain depends on some analog switches as well. The codec (the ADAU1961) is limited to the audio range.

My gut feeling is that it will be tough to directly modify the standard audio IO path to support video directly. An expansion board might be the way to go. For you previous video experiments with Axoloti were you actually using the codec?

Another interesting idea: the H7 includes some integrated programmable analog peripherals including an opamp that can be driven from the internal dac:

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ST claims that the internal dac runs up to 1MSPS. See the H7 reference manual for details.

The other interesting thing is that the integrated ADC channels are 16-bit on the H7.

#3

Ah, didn’t realize the codec was limited to 20kHz. It would be relatively easy to modify those buffers, (just replace the op-amps and kludge in a few resistors), but there’s no point if that’s the maximum frequency.

To answer your questions:

  • yes, analog video, mostly
  • frequencies from sub audio up to a few MHz are all useful. The tricky range is horizontally synced, which is 15k and above. There are still some useful things that can be done between 15 - 20k, but processing video isn’t one of them. It could make a decent open source LZX Diver: https://lzxindustries.net/products/diver, but it would be limited in the horizontal range. Diver also uses an STM board btw.
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#4

Wow! Diver is beautiful. This is the first I had heard of it. Let me meditate on this a bit and see what I can come up with.

Have you looked at my FPAA work? That tech can work up to that low MHz range. It might make sense to keep video processing in mind for an FPAA expansion. Doing an FPAA expansion for Akso has been one of my plans from the beginning.

My rough guess is that such an expansion will have about four FPAAs. It will probably also be usable standalone away from Akso.

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#5

I haven’t seen your FPAA work, and in general, I’m out to sea with anything digital or programming related. I was a beta tester for the LZX Memory Palace, which is a digital video frame buffer and effects processor, but only from the perspective of end-user.

I’ve done quite a bit with DIY modular video synthesis, but heavily influenced and in some cases directly assisted by LZX. It would be super cool to see an LZX compatible FPAA expansion for Akso. Lars, who runs LZX, is pretty generous with knowledge and might even be open to working with you, or at least providing some insight.

Glad I got you thinking about it. Video synthesis is such a rewarding and eye-opening world – it’s totally broadened my perspective on what a signal is and how it can be worked with.

#6

By the way, on the frequency tuning question, there are definitely some quirky aspects of the legacy system that I think stem more from design choices than from true technical limitations.

My goal with the software longer term is to expose the true hardware capability as much as possible, in a way that is still as simple as possible for non-technical users.

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#7

This is super interesting. I’ve been accumulating CRTs for this kind kind experiment for a couple of years now, but have been intending to use ESP32s to do the video stuff, using its 2xDACs as X and Y (or whatever) as outputs and taking CV as inputs.

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#8

Nice! I tend to favor analog displays as well. You may find this interesting though – it’s made by sending x&y from my video synth into a digital stroke to raster converter connected to an old VGA monitor tube. These were used by the military for a while and are unfortunately very rare, but I’m sure someone could make something similar with FPGAs or something.

#9

I do! Followed!

Also, apparently that was already liked by a college roommate of mine, cl0aca!

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#10

Nice! He came to my last DIY video synth workshop, just prior to the COVID lockdown. Small world!

#11

@nsm It’s so good to hear your comments re: exposing the capabilities. There’s nothing more frustrating than running into design-imposed limitations. It’s one of the reasons I started hardware hacking and eventually just making my own stuff.

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#12

Wait, so do we live near each other? I’m in Western Massachusetts.

#13

I’m in NY, about 2 hours north of the city in the Hudson Valley. I have some friends in western mass though, and I’ve been playing X-Fest for a few years, so I’ve met lots of people in that area through the great noise/experimental/improvised music scene you have over there.

#14

Awesome! If we’re ever able to go places and do things again, be sure to ping me! I’m on all the social media things.

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#15

By the way, I’m based in Minneapolis right now, but I’m picking up and moving to Pittsburgh in the next few weeks for work. I’ve never lived east of Minneapolis. Not super close to you all but closer than now.

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#16

It’s far, but maybe someday things will be ok again and we’ll be able to get together at some East Coast music thing!

I’ve never been to Pittsburgh but Pittsburgh Modular does really great stuff.

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