my take on a divide down oscillator module

[albiedamned]

My wife got me the Joy Division "Heart and Soul" box set for xmas, so I've been listening to it a lot lately*. That got me to researching what synths they used on songs like Atmosphere and Decades. Turns out it was an Arp Omni 2, which I learned is a member of the "string synth" family. These string synths uses divide down oscillators to provide full polyphony, as in you could literally press every key and have all the notes play at once. Of course there were drawbacks, mostly that the notes all shared the same envelopes, vcas, and filter. But still it's an interesting concept. It was all new to me, but I suspect that this history lesson is not news to many on this board.

I did some digging on the forums here, and I see some mentions of divide down oscillators, but apparently no one has done it. Among other concerns, there is the question of how to tell it what notes to play, since you can hardly have inputs for 61 different notes. You could have a MIDI input, but that's not very modular. So I did some brainstorming, and came up with this approach. I'm not a module maker, so I'd love to hear some input from people who actually know what they're talking about as to whether this is feasible.

The core of the module would be 5 oscillators to play the top octave for 5 different notes simultaneously. You need 12 of course for complete polyphony (in western tuning), but I think 5 is enough to play most common chords. The module itself would have 5 columns of controls, one for each oscillator. The columns would each have the following:

- Knob and CV input for tuning. This is not a V/Oct input. Instead, it lets you scan across a one octave range, to set the note for the top octave oscillator. So you can set it to play anywhere from C to B. By using multiple columns with the appropriate CV's, you can set up chords.

- Knob and CV input for octave selection. Each oscillator has 7 divide down circuits so it can produce a total of 8 octaves. This input controls which octaves are active. It works the way that ADC pattern selection works on the Fonitronik sequencer (full credit to him for this idea). The voltage is converted to an 8 bit digital value, where each bit represents one of the octaves being on or off. So by supplying different voltages, you can change which octaves are playing for each note.

- Output. Each note has a dedicated output so that notes can be processed separately if desired (for those with enough vca's, filters, and envelopes). Of course it's actually a mix of all the octaves for that note, not just a single note.

- Mix Level knob. Each note is also sent to a master mix output, and this knob lets you control the level of that note. Of course, it controls the level of all the octaves at once.

One thing I'm not sure about is waveforms. Can divide down oscillators do waveforms besides square waves? From what I've read I think they can but I don't really understand how. If so, then a waveform selector for each column, or maybe just one overall waveform selector, would be good too.

That's it. Thoughts? My thought is that by setting up the notes where you want them (either manually or through multiple rows of a sequencer), and then mixing in subtle modulations to each note using separate modulation sources, you could achieve a full and animated sound. Then you could modulate the octaves for even more animation.

* I've always been a JD fan and I have all their other stuff but I never got around to getting the box set. There's some really good live stuff on disc 4, for those who haven't heard it

[snufkin]

not exactly what your asking for but
http://erthenvar.com/store/patchchord?m ... urer_id=11

if doing it the old fashioned way divide down ICs are kinda pricey as they are used to repair old stringers and organs

as far as I am aware they only produce square waves but each output can be filtered to produce other wavforms after the fact

[dslocum]

I've wanted to do a good VCO with 12 note outputs for quite a while. The problem is that you usually need to start out with a very high frequency oscillator (over 1 Mhz or so) and divide down by several strange division ratios to get what is called the "top octave", like 216. Top octave generator chips (TOG) used to be available and fairly commonplace, but are no long made - though there are a couple folks making equivalent solutions. (http://www.organservice.com/crm/topdividers.htm)

A 1 Mhz synth quality VCO is not available to my best knowledge. The best high frequency VCO for synths is the LM331
(EDIT: has NOT been discontinued as my original post suggested) that Ian Fritz designed into his Double Deka VCO. The highest frequency of that chip is 100 KHz, which in Ian's design is fine because he only needs to divide by 10 to get the DDVCO stages.

After you have the top octave, you can then divide by powers of 2 to get octave relationships for each TOG output. It quickly gets expensive in parts count and power consumption. Then you need to have a VCA and EG to control amplitude for each output. And we haven't even gotten into anything more than square wave outputs.

The Holy Grail of this type of instrument (I've basically described a simple 70's combo organ) is getting it to sound like something other than a clarinet / square wave. You can combine multiple octave signals in varying strengths and filter, but control of these settings adds a lot of complexity.

Love it or hate it, the PolyMoog was IMHO about the best implementation of this sort of technology in modern hardware. It used a custom IC chip, without which the machine probably would have out weighed a Volkswagen. I'm certainly not forgetting about the first synth, the Hammond Novachord, or exclude the wonderful Yamaha GX-1 (and derivatives), but those really didn't rely on the same sort of TOG technologies we're discussing here.

Snufkin's link above about Erthenvar's Patchcord is about as good an idea as I've seen in this regard in a very long time. I haven't yet tried one, but look forward to the day I do. Note that it still only produces rectangle waves, but interestingly, it will follow the input waveform pulse width ratio - and there's nothing wrong with the variety of sounds you can get out of variable width rectangle waves!

Sorry if I've been long winded.

[frijitz]

The best high frequency VCO for synths is the LM331 (also discontinued)...

Hmmm ... where did you hear that? Still active, as far as I can tell from their website:
http://www.ti.com/product/lm331

Ian

[albiedamned]

The approach I described doesn't use a single supersonic oscillator at the top, instead it uses 5 oscillators that can produce the top audible octave, and then divide down 7 times each. The oscillators are voltage controllable but don't have to track 1 V/Oct, so presumably this means they can be a little more inexpensive.

[Just me]

All those electronic Hammond organs were top octave divide down machines. Instead of a chip, they had a small pcb with all the components for a single note OSC potted in goo and it would fit in a row of sockets on he main board. The electronics weren't that big but the key wiring was, uhm, complex. Yep, that is the word. Complex.

[Just me]

The insides of an Italian TOD string/horn/organ/piano synth with an ARP lable.

[Why Adapter]

One thing I'm not sure about is waveforms. Can divide down oscillators do waveforms besides square waves? From what I've read I think they can but I don't really understand how. If so, then a waveform selector for each column, or maybe just one overall waveform selector, would be good too.

You get an approximation of a sawtooth by mixing a few octaves of squares, with decreasing amplitude the higher you go, to get a 'Staircase Wave'.

[dslocum]

The best high frequency VCO for synths is the LM331 (also discontinued)...

Hmmm ... where did you hear that? Still active, as far as I can tell from their website:
http://www.ti.com/product/lm331

Ian

Huh!? I stand corrected. I'll update my post. Thanks for setting me straight (as usual!) ;-)

[Chiefy11]

I've got this, it does the job quite nicely with the drawbars. One set of square (under the scope, not so square, but sounds all the same) and one set of sine (yes, it is heaven). It sounds great through a leslie, and also gets great calliope or "nintendo" type sounds. Still wish I had some sort of divide down saw.

[clusterchord]

i cant stress enough the importance of having accurate, warm ensemble effect (BBD). you can variations of it in all string machines, including Omni.

your modern day options are either JH triple chorus DIY...(TDA chips) or Elkorus (NOS Solina chips).


cheers

[robotmakers]

Another divide down implementation can be found in the Korg PS 3100, 3200, 3300 series, where 12 top octave oscillators (independently tunable) are divided down to create the lower octaves. I'd imagine there are some clever circuits in there to create waveform variations in the most parsimonious manner.

J Haible's Korg clone

Cheers,
Roger

[Christopher W.]

I have very little to add in terms of technical expertise, but as someone who recently discovered the joys of a (Mark 1) Omni I can say that they are brilliant instruments. It took me two days before I realized there was no need to tune it. The onboard chorus/phaser is "okay" but any external chorus - from boutique megabucks analogue to built-down-to-a-cost cheap digital - turns it into a pure syrupy goodness.

Someone really needs to do a modern iteration.

[albiedamned]

So no one has really commented directly on my idea for a modular divide down implementation. My initial post was a little long winded, so I'll try to summarize it here. Five top octave oscillators, each of which can be tuned manually or via CV over a one octave range (not V/Oct response), and each of which has 7 divide down octaves. Each octave output can be turned on or off by means of a CV input that is converted to an 8 bit digital value, where each bit represents the on/off state of one of the octaves (I stole this idea from Fonitronik's new sequencer).

You can only have 5 notes at a time (each in up to 8 octaves), so it's not fully polyphonic, but 5 is enough to do most common chord voicings. The VC tuning of each oscillator lets you program in chord sequences via sequencer rows and/or animate the voicings with LFO's or other modulation sources. And the octave control provides further animation.

[MR-808]

My synth-building buddy has a raging hard-on for the Freeman String Symphonizer Prototype #4. He's especially enamored with the idea of 3 top oscillators. Approximating it would be a pretty big project.

Seems like XR-2206s could be used to make patchable VCOs in the right frequency range for the top oscillators. Divide down using KS-87s? Jürgen's PolyKorg clone would be a good basis for the rest of it. And his Solina Chorus for the final output stage, of course.

As for turning notes on and off, I'd want MIDI for flexibility. Though your approach sounds like it could be fun.

I'm going to have to think about this a few more years.

[LoveHertz]

a raging hard-on

whats that?