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 Forum index » DIY Hardware and Software
polyphony......
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bubzy



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PostPosted: Tue Jan 21, 2014 3:29 am    Post subject: polyphony...... Reply with quote  Mark this post and the followings unread

ok, so, I've got a bunch of oscillators, and would like to get maybe 3 note polyphony from a keyboard, but I cannot seem to find a decent starting point, or even get my head around the theory. ive done a quick search here, and a google search, but I always find it best to ask, so that im not bogging myself down in irrelevant material (as usual). just some pointers on how to get started would be great, thanks Very Happy


oh, its gonna be a CV keyboard, not midi. no touch sensitivity, just keys acting as buttons I guess. Smile

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AndyR1960



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PostPosted: Tue Jan 21, 2014 6:14 am    Post subject: Reply with quote  Mark this post and the followings unread

I'll be honest with you Richard I tried this several years ago and gave up... sorry to be such a harbinger of negativity Wink

I ended up using a hacked MIDI controller via a Paia 4 channel MIDI to CV converter - but you need 4 VCOs for it to work correctly, as the Paia's CV output is rotational (only using 3 VCOs will cause a gap every 4th note!).

There may well be circuits out there (I've not found any), but will probably incorporate quite a bit of digital encoding/decoding.

Andy
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analog_backlash



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PostPosted: Tue Jan 21, 2014 7:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

I built the ACXSynth MIDI2CV from a HexInverter kit recently, see:

http://electro-music.com/forum/topic-60751.html

I have managed to get pseudo-polyphony from it, but I haven't recorded any samples yet. It's pseudo, because I have 3 VCOs, but not 3 VCFs, 3 VCAs etc. This means that if I play a 3 note chord, it sounds fine, but if I then play single notes, you get those notes plus two of the notes from the last chord Sad . It also needs some fine tuning as it seems to really sound bad at higher frequencies (the oscillators are basically MFOS Ultimate VCOs, by the way, so perhaps not the best). At some point in the distant future, I'd like to make 4 of everything and try it out, as THeff did here:

http://electro-music.com/forum/viewtopic.php?highlight=chords&t=59728

I'll record a few chords soon and post them here for you to hear (it won't be as good as THeff's playing...)

Gary
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richardc64



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PostPosted: Tue Jan 21, 2014 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

I've been wondering why none of the rocket surgeons in Microcontrollers and Programmable Logic have tackled polyphonic midi2cv.
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gdavis



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PostPosted: Tue Jan 21, 2014 11:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm working on a polysynth project including an arduino based poly midi-CV converter. Only 2 voices CV and gate so far but I'm eventually shooting for 8 and possibly midi PB/CC to CV. Right now it's just on a bread board but I'm about at the point of doing up a schematic and PCB.

The whole story is in my blog (http://gndsynth.blogspot.com/).

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analogmonster



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PostPosted: Wed Jan 22, 2014 1:19 am    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
I've been wondering why none of the rocket surgeons in Microcontrollers and Programmable Logic have tackled polyphonic midi2cv.


Well, I did... I developed an 8-Voice MIDI2CVGATE module for my Formant Modular System:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

My description is a bit euphoric (it was my first microcontroller based module), but anyway, see details on http://www.analog-monster.de/ucvm_en.html

The absolute minimum of equipment is one VCO, one ADSR and one VCA per voice, followed by VCFs and so on. Better is (of course) to hire a complete monophonic synth per voice and define modular polyphony as a set of parallel driven monophonic synthesizers.

One experience I made with this kind of polyphony is that you need an intelligent strategy for voice assignment. If you use "round robin" the interface must have an input parameter for the number of voices. Disadvantage is that the mono synths have to sound absolute identically because if you press a chord sequence each chord might strike each note a few microseconds earlier or later, so each chord might sound different in its spectrum due to key reassignment. Other problems are releasing a chord key (key / note reassignment) and so on. A lot of things to consider.

At the end I was a bit disappointed. All software stragegies had advantages and disadvantages, so I still prefer real polyphony like in organs. In my opinion modular synthesizers should keep its monophonic behaviour because you always will be confronted with module tolerances.
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JovianPyx



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PostPosted: Wed Jan 22, 2014 1:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've read about some folks doing analog polysynths - many are lackluster because of low voice count and the voices are relatively simple, often not patchable. Being polyphonic increases the complexity of patching (which wasn't mentioned by the OP, but it may still be in his mind). In order for the synth to sound like a synth and not a toy, you will need a rather good deal of "matching" between voice cards. Ideally, the cards would be interchangeable without changing the sound of the synth. This will mean choosing stable oscillators and stable filters (since differences in how Fc responds will change how that card sounds). Tracking from card to card is essential for this.

I've designed a rather large number of digital polysynths. Even in a digital world, polyphony is complex. My own advice would be to simplify things by use of a hybrid approach. Namely, use a microprocessor to be a MIDI controller and a voice assigner. Of course, I did read what the OP wants and he did say "not MIDI", but MIDI simplifies a lot of what needs to be done. You can still have the voicing analog even with MIDI as a controller. Using MIDI allows you to employ a simple cheap digital keyboard. Without MIDI, you will likely still want a microprocessor to absorb the state machine required to do keyboard scanning and voice assignment. Either "Keys acting as buttons" or a simple keyboard designed for digital use can be "scanned" using the microprocessor. Another bright point about MIDI is that it's easy to sequence a MIDI synth.

In my view, a large amount of planning is necessary for doing all of this. A part of this would be to list out all of the features you think you want and then start computing the cost. If I assume that you want to stick with real expo CV VCOs, and since you probably don't want drifty pitch, you will be required to use matched transistors and tempcos. The adjustment of the CV tracking is going to be essential (and likely a bit maddening). And again, consider a hybrid approach by using DCOs instead of VCOs. A DCO (Digitally Controlled Oscillator) is a design that maintains frequency digitally, but it's output circuit is analog. This is done by using a digital circuit to reset an integrator. The output is a nice smooth ramp (not DAC produced) and you get digital pitch accuracy without the expense and tempcos and match transistors.

None of this is meant to discourage. But I do think that a good deal of thought ought to be put into it before you start drawing schematics. Be prepared for the project to take a good deal of time. And above all, do have fun with it.

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bubzy



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PostPosted: Wed Jan 22, 2014 3:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

schematics are for girls
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analog_backlash



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PostPosted: Wed Jan 22, 2014 6:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Reading the JovianPyx post made me feel like this hanged ( Wink ).

I know that you're right. It's much easier (and possibly cheaper) to buy a commercially available digital (analog modelling) synth, but where's the fun in that!

I know what you mean though. On my experiements, I can play a chord 3 times and it sounds different each time, depending on the delay between each note. This is down to differences in the 3 VCO's tones (and that's without worrying about filters, VCAs etc). DCOs are the way to go, but I've not yet built one of those (I haven't found many schematics for them to tell the truth).

As for the microcontroller control of the whole beast, I think that it's beyond my capability Crying or Very sad .

Gary

P.S. I will post a few chords (as I stated earlier) just to show how it sounds currently - been tied up doing things in the real world Beyond the Synthdome.
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analog_backlash



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PostPosted: Thu Jan 23, 2014 8:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi all.

Here's a sample, as promised. Sorry for the bad tuning/playing, but it's an example anyway. It does also sound a bit thin as Scott suggested.

Gary

P.S. My apologies to Lou Reed (RIP).


Polyphonic Sample.mp3
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PHOBoS



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PostPosted: Thu Jan 23, 2014 9:23 am    Post subject: Reply with quote  Mark this post and the followings unread

o I like that, got more ? Laughing
(somewhere at the start it reminds of a clockwork orange)

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analog_backlash



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PostPosted: Thu Jan 23, 2014 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks PHOBoS Very Happy . That was just a sample that I recorded today (very quickly). I'll try to create something a bit longer for you to work your magic on Cool .

Gary
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