viva Analog “HARD-MOD” // jcm(c)2019

Binary RING MODULATION, Nyquist Aliasing, BOOLEAN 1bit A/D Distortion, … they’re in that same league …

A recent invention of mine, this one could be regarded as a Ring Modulator, but with a (0/1) square wave (carrier) operating at one of the inputsthe name “Binary” Ring Modulator could apply here …

the similarity is conceptual only as the circuitry takes on a vastly different design approach than any (balanced multiplier) “Ring” modulator circuit I’ve seen … as far as sound quality goes, it has an arcade game quality to the sound that would annoy any Randy Rhodes wannabe – that’s fer sure … (rest easy though, it’s more of an academic exercise) … the challenge would be to find a spot in a song where something unique could be done with this thing.

A little background …

This thing came together quite randomly. A few years ago, at some strange insistence, I spent a day or two mucking with Bob Moog’s famous RING MODULATOR circuit. In the end I found that things there only got interesting when the oscillator input going into the multiplier was driven very hard. Unfortunately the circuit at that point started acting up and the carrier would feed-through horrendously – but otherwise the combination sounded good. That gave me an idea …

The challenge was is finding a way to produce the desired multiplication so to speak – in this case by zero’s and one’s if you like – and NOT have the clocking signal come through at the output in any substantial way (or at all really) which is easily perceived in absence of input processing signal. Anybody who’s tried building the OTA version (hardware analogue) of this idea using the the LM13600 or LM13700 datasheet multiplier example knows what I’m talking about. This is the main reason why we don’t see too many of these sorts of circuits out there … as it is utterly useless when the carrier feeds through at perceivable levels …

Of course, in the Digital domain it would be quite easy to program a much “cleaner” working analogue (AVR) of this FX processor idea, and it would be interesting to compare the outcome for each approach. The same could be said for my Nyquist Aliaser (jcm(c)2005)

Indeed, this is a “known” design challenge – one that is associated with the name Barry Gilbert. Of course, it was a challenge I thought I wouldn’t be able to pull off without using precision IC’s and the like … but SPICE seem to show promise once this new design was set. After a fair bit of tinkering I was able to achieve “minus 147db” S/N rejection on the simulator (TINA-TI version 9) – this gave me the go ahead to build …

Believe or not this circuit, my solution to the problem, was inspired by by the VCS3 “envelope Generator” design. The “HARD-MOD” FX circuit is another reward I got for reversing the EMS synth.

What is known: In a well designed linear Ring Modulator achieving good “carrier rejection” is not impossible to do, especially with access to balanced multiplier IC’s (LM1495/LM1496) and well behaved signals. But, when switched completely on and off, to the point of shifting internal bias of circuits, this application begins to pose a particularly difficult challenge …

Coincidentally, it is a problem whose “large-signal” nature is not unlike the one I encountered in another (“Nyquist Aliaser“) design of mine from 2005. As it turns out, this second problem was solved using a similar technique as I did the first time. In fact, I plan on using the very same idea in a third “challenge” fairly soon to see if I can get lucky again (*!*)

With preliminary trimming, and the circuit operating in open air (un-shielded), the carrier feed-through is quite good against the weak signal of an electric guitar. Pay attention to what happens when the carrier is set to an octave related to the root note in the music, and then not. Doesn’t quite behave exactly like a conventional Ring Mod might … for one, chords are still very perceptible – so, in a sense, the output is still quite a bit relevant musically (if that makes sense) …



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