MATRIXSYNTH: Who becomes silverware first? - 10 minute LFO filter sweep and semi-generative Eurorack jam


Tuesday, December 05, 2023

Who becomes silverware first? - 10 minute LFO filter sweep and semi-generative Eurorack jam


video upload by thesrabbit

"This sonic experiment was inspired by the Dreadbox Erebus LFO having a whopping 10-minute long cycle time. Therefore, I thought it might be fun to run that LFO into every single filter I have with a CV input, including big analog polys with a VCF control input on the back intended for expression pedals. The honorary VCFs include: 2x Roland System-500 521, G-Storm Electro SH-2 and SH-5, Dreadbox Erebus, Oberheim OB-X8 (OB-X SEM LP mode), Sequential/Oberheim OB-6, Roland Juno-6.

Interestingly the filters on the big polys don't open all the way. I assume this has something to do with the inputs being calibrated for expression pedals rather than Eurorack CV. According to my oscilloscope, the Erebus LFO range is -5v and +5v. This is pretty typical for Eurorack. I set the cutoff at about 50% for all the filter modules. This seemed to work perfectly. But I was not able to do that with the polys. With the cutoff at 50%, the filters would open up all the way, but they wouldn't close nearly enough. I had to bring the cutoff completely down, and as a result, the filters on the polys wouldn't open up all the way. I did alternate takes where I walked around and tried to smoothly nudge each one up in order to get the filters fully open, but that was a disaster. In retrospect, I could have set up a one-shot MIDI LFO to do this and triggered it once at the right moment, but I already had enough things to worry about.

One of my audio interfaces is dead or dying, and it keeps ruining takes and sometimes crashes Logic. I finally turned it off and went with my lone Focusrite Scarlett 8i6, which has always been solid. But now I had an extra synth without an input and I wanted to have the polys on their own tracks so that I could blend them in with the Eurorack stuff to taste. And there's no possible way I could overdub since the whole point is to have one LFO sweeping all of the filters at the same time. I ended up resorting to an old 1989 JVC cassette deck to record the OB-6. So that shot at the opening of the video is legit. There's such a big wall of sound here that it's impossible to tell if it ended up adding any saturation. It definitely added some hiss and crackles but that's fine in this context. The cassette itself is ancient, cheap consumer grade, 60 minutes per side. I ended up recording over some 90's black metal that had been there since, well, the 90's.

To add to the grit and grime, I intentionally ran the filters from the Eurorack straight into the audio interface, bypassing the typical flow through VCAs. Ok, fine, I totally forgot about the VCAs, but it sounded extra dirty, so I didn't stress out too much. I certainly wasn't going to power everything back on, tune everything up and do another take because of it.

Obviously when the filters are closed or nearly closed for several minutes, there needs to be something to fill that gap. That's where the generative stuff comes in. There's 3 different sample and hold circuits feeding different things, including back into themselves in one case. Some voltage is running through different logic gates in conjunction with other, steady LFOs. Since I'm out of filters, the sound consists of pink noise through a VCA, triggered at random by a very short decay envelope. This runs into a recovery tape delay module, that gives sort of a slapback effect.Then there's a single oscillator, pitch controlled by another S/H circuit run through a quantizer and then running into the spring reverb tank. It's also triggered by the same random voltages triggering the noise hits. Then there's the Mutable Instruments Plaits, making the plinky plonky sounds, again everything triggered at random. It's running into the majestic Calsynth Typhoon.

The last thing to note is that the Erebus LFO also acts like an automated fader to increase and decrease the level of the VCA where all the rhythmic sounds are running through. I simply inverted the signal of the LFO output to do this. As the filters open up, the VCA level diminishes. When the filters fully close, the VCA level increases to maximum.

Last last thing. That knob twisting near the beginning isn't for sci-fi FX, though that's an added bonus. I was simply trying to get the LFO back to its "starting position." I didn't quite succeed. I don't believe the Erebus has any sort of LFO triggering mechanism to help with this. All good though.

FX: Audiothingies DoctorA delay and reverb, Boss DM-101 delay

No post processing other than a 30Hz low cut and slight limiting on the master"

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