Synth DIY: Synthesis with logic concepts

Synth DIY: Synthesis with logic concepts18.01.2010 11:35:11

Greetings; your lord Ruin here. There has been a lot of ideas being stewed around here at Ruin & Wesen. Some of these ideas come from the act of using digital logic as a sort of analog synth, patching between modules to create flexible signal flow. A lot of these modules have coalesced into what I've called the 'Cashbox', read more about it later. Technically it's not an analog synth but the way you work with it is sort of, well, analogous to... analog. This synth was inspired by what is known as a Lunetta synthesizer. What I've found with the Lunetta style of synth, where I feel my usage strays from is that the Lunettas seem to be made to be played by themselves, without human interaction; as well 'pure' Lunettas are to use nothing but CMOS logic. I've found that interfacing this to an analog synth is a great thing. They are great for sequencing and processing. When you consider that there are essentially what are octave dividers on a chip, with very little external components, or trigger sequencers (shift registers), you see where all this stuff opens up. Of course, these concepts aren't really new. These modules have been used in analog synths for decades; but when you make each block patchable thats where things get interesting. You can design a sequencer however you want. For DIY synth nerds it brings in a new wave of thinking about things, all signal flow and switching essentially, opposed to waveforms and filtering. Sounds generated by these circuits are quite brash, ballsy and gritty. If you've played around with 555's or some really crusty square waves you know the sound. Don't expect nice from these. Expect your ears to hurt.

One simple, but fun module is the humble AND gate. You can use an AND gate as a ringmod type processor. Keep in mind that these concepts are used for digital signals, by that I mean off and on, 1 and 0, aka square waves (I don't mean mp3's you big dummy). It seems our humble friend the AND gate has made an appearance in one of the most popular analog keyboard synths, the ARP Odyssey.

Another great module is the shift register, which can be used as a simple step sequencer, noise source, random bit source, trigger delay and in larger shift registers, as a soft of audio buffer/glitch effect, very interesting things can come out of these. What a shift register does is it takes an input bit at the D(data) input each time a pulse is sent to the CLK(clock) input and moves the data bit over 1 space each clock pulse. With a 4-bit shift register it moves it through 4 spaces, once it reaches the 4th, it either disappears into the void of dead bits or can be sent back into the data input for some feedback, which is where the sequencing comes in. If you load the shift register with a pattern, it will repeat it. There is longer lengths of registers, of course, 8-bit, 16-bit, I have used a 64-bit and I am sure there are sizes in between and beyond.

This digital noise/random bit source would seem perhaps a bit complicated to people just getting into synth DIY, but if you want to break it down into its parts it becomes a lot easier, there is 2 oscillators, consisting of IC1A and B and surrounding components, and the shift register, IC2A. When you make three or so of these random bit sources, all clocked from the same oscillator, you get a 3-bit generative sequencer. Send the bits to a multiplexer or divide by N counter, something with 3 control inputs.

Now, In the cashbox, it is a miniature logic playground. 6 oscillators, 2 binary counters, 2 4-bit shift registers, an input module, used for bringing in external signals; such as a microphone or drum machine, a 64-bit shift register which is great for the aforementioned buffer/glitch effect, a VCA/Attack Hold ReleaseEnvelope combo (Voltage Controlled ATTENUATOR used for digital signals) and a 4-bit D/A converter. Since this is a modular there is a ridiculous amount of possibilities to be explored with just this unit alone, but when used in conjunction with a separate modular system more an more options are revealed. Some examples; patching a square wave oscillator into the binary counter and patch all outputs of the binary counter into the 4-bit D/A and out you get a gritty little saw wave. Or how about using those shift registers like I said before to trigger some drums in your modular system? Hey, anyone want this synth? You know who to call. You want some sounds to listen to?

Here:

This is a 'pure lunetta' tune, as I didn't use any external sounds. so all the generation is going on within the units. some shift register noise for the 'hihats'. and gate doing the bass note and a somewhat complex patch doing the melody, just a bunch of shift registers being clocked fairly slowly with some asynchronous clock inputs. there the VCA with an envelope creating the dynamics on the melody.




This one has input coming from an Er-1, providing the drums. It also serves to sync up the drones and beeps. This one is much more grating and brash compared to schrang. I am using a 64-bit shift register in here to do a soft of buffer override on the Er-1. has the same random melody patch as schrang doing the low bass stuff.

The hum in these demos is due to poor recording. This demo has input in the form of a microphone being sent through the different modules. Mostly some glitcher and other stuff, mixed in with the orriginal signal, providing some interesting chorus/octave effects and general crudiness.




Some more Er-1 being processed by the modules. Still sounds like balls.

I hope this has given some inspiration to you; when I first learned of the concept of simple CMOS for synths, I nearly shit myself; srsly. The realms of possibilities this ushers in for me are incredible. Oh, like I said before, this is nothing new, but I think it deserves a deeper look, a lot is possible when you give yourself limitations. I would also implore you to visit the electro-music forums on this topic, the lunetta forums, for some good discussion: http://electro-music.com/forum/index.php?f=160.

Good day to you and happy tinkering, you filthy animals.