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 Forum index » DIY Hardware and Software
Frequency to CV module
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prgdeltablues



Joined: Sep 25, 2006
Posts: 222
Location: UK
Audio files: 12

PostPosted: Fri Oct 12, 2012 1:01 pm    Post subject: Frequency to CV module Reply with quote  Mark this post and the followings unread

Well, I'd bought a frequency to voltage converter chip, so I thought I'd have a go at a frequency to CV converter module.

HEALTH WARNING: This isn't a pitch to CV unit - though the 1V/Oct tracking isn't too bad. It also isn't a 'play your guitar through your synth' unit, though you can use a guitar as a CV source. There are far more complex designs out there that aim to achieve those goals, and from what I've read, this chip is not the best choice for that more precise job.

My main purpose was in fact to build a reasonably simple circuit so that I could take the audio out from a theremin app on my tablet and convert both frequency and envelope into CV's, to turn the synth into a sort of theremin, controlled by the tablet. It's not too bad at that. It also works reasonably well with a Native American flute, again deriving a CV to control an oscillator. It can be got to work with an electric guitar, though it's... eccentric.

I'm using a polyfusion envelope follower to extract the envelope.

A particular feature [flaw, depending on your point of view] is that it takes a clearly audible time for the frequency to change, especially from no signal - that leads to each note starting with an upward swoop.

Schematic attached. It's basically three subcircuits: a wide bandpass filter followed by a comparator to condition the signal into a square wave, hopefully of the fundamental (this derived from the Slacktave), then the frequency to voltage chip, from the datasheet, tweaked for best performance for this purpose, then a log amp from National Semi application notes (followed by a times ten amp and offset trimmer (the scale trimmer in the log amp circuit is adjust to about 4.6k to yield 100mV per octave, which is then multiplied up to 1V/Oct.

There's no temperature compensation in the log amp, and in my breadboarded circuit I haven't even matched the transistors: the app note says how to provide temperature compensation.

Any suggestions for improvement would be welcome!

I'll post some audio clips next.

Peter


Freq to CV.jpg
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Frequency to CV schematic
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Freq to CV.jpg


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prgdeltablues



Joined: Sep 25, 2006
Posts: 222
Location: UK
Audio files: 12

PostPosted: Sat Oct 13, 2012 2:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Some audio clips (which appear below in reverse order).

The synth pretending to be a theremin, driven by the Leon app on the tablet. CV in to V/Oct on the VCO, sine wave out, into a VCA driven by the envelope CV.

NA flute - original recording (and no, I can't really play!); synthesised version - CV from Freq-CV into V/Oct, this time with a triangle out (the flute's waveform is a distorted triangle), through VCA controlled by envelope. Then both together,- you can hear some beating, but that's just as likely the flute not being perfectly tuned as it is the Freq-CV accuracy.

Guitar - some noodles for a laugh. Triangle VCO through LP filter, with cut-off frequency also controlled by the CV, then VCA as above. You can hear the module lose tracking - even getting it to this level took some tweaking of pots, both on the module and the guitar.

Peter


Synth Leon theramin.mp3
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 Filename:  Synth Leon theramin.mp3
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NA flute original.mp3
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 Filename:  NA flute original.mp3
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Synth NA flute.mp3
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Both flutes.mp3
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Synth guitar.mp3
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 Filename:  Synth guitar.mp3
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RingMad



Joined: Jan 15, 2011
Posts: 404
Location: Montreal, Canada
Audio files: 4

PostPosted: Sat Oct 13, 2012 5:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting circuit. That "Synth guitar" clip is pretty funny. Can you play "Stairway"? Smile.

James.
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telbonic



Joined: Jan 08, 2010
Posts: 39
Location: uk

PostPosted: Sat Oct 13, 2012 9:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Ever since hearing Goldfrapp's "Lovely Head" I've fancied something that can convert voice to cv, so I'm stoked to see your circuit & really hope some of the regulars take an interest too.

*runs off to check out freq > voltage chips*
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elmegil



Joined: Mar 20, 2012
Posts: 2122
Location: Chicago
Audio files: 16

PostPosted: Sat Oct 13, 2012 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

EAS over on Muff's is selling "mind reader" PCBs which is a redo of the Korg MS-20 input processing module--takes an input and generates pitch CV and envelope.

The downside of course is that it requires two four-ganged pots and while EAS is supposed to be getting those as well, they have not turned up yet. But I'm very much interested in what results I can get to use it to play my synth with my Bass.
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LFLab



Joined: Dec 17, 2009
Posts: 493
Location: Rosmalen, Netherlands

PostPosted: Sun Oct 14, 2012 4:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Been eying those ic's and the burr brown ones, looks nice!
The earliest incarnation of the analogue sytems input module also uses this ic. Apparently it delivers a linearly proportional cv.
Also, since most of the f-V ic's are aimed at control, you might get (even) better results by turning the signal into a squarewave using a schmitt trigger/ comparator.
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telbonic



Joined: Jan 08, 2010
Posts: 39
Location: uk

PostPosted: Thu Oct 18, 2012 1:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
EAS over on Muff's is selling "mind reader" PCBs which is a redo of the Korg MS-20 input processing module--takes an input and generates pitch CV and envelope.

The downside of course is that it requires two four-ganged pots and while EAS is supposed to be getting those as well, they have not turned up yet. But I'm very much interested in what results I can get to use it to play my synth with my Bass.


Is there a circuit diagram for EAS's board? I couldn't see one but I'm probably just being blind. Just wondering if there's an easier answer than a quad pot if they're made from unobtainium -I do have one idea, but it would help to see the original circuit.

EDIT: Wait, it's a straight copy of the MS-20 ESP circuit. Ignore that q, I'll go find it ^^

EDIT 2:
Ok I've found this: http://www.cykong.com/Synths/Korg%20MS-20/Images/MS-20-ESP-Schematics.gif

It's not very clear so if anyone else has a better copy it'd be a real help.

Anyway, here's what I'm thinking: replace the unobtainable pot with a cheap digital version. Pros: You get a buildable version of the circuit Cons: Might be a PITA to find the right digipot at an acceptable price, unless you go 8 bit on the low cut freq control.
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elmegil



Joined: Mar 20, 2012
Posts: 2122
Location: Chicago
Audio files: 16

PostPosted: Thu Oct 18, 2012 3:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

He hasn't published one himself, but it's the external processing circuit from the MS-20.

You can find it here:

http://www.synthdiy.com/files/2008/KorgMS20-ServiceManual.pdf

Page 5 at the bottom.
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telbonic



Joined: Jan 08, 2010
Posts: 39
Location: uk

PostPosted: Fri Oct 19, 2012 1:49 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
He hasn't published one himself, but it's the external processing circuit from the MS-20.

You can find it here:

http://www.synthdiy.com/files/2008/KorgMS20-ServiceManual.pdf

Page 5 at the bottom.


Brilliant thanks. Sorry OP I appear to have somewhat hijacked this tread in my enthusiasm Embarassed
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elmegil



Joined: Mar 20, 2012
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PostPosted: Fri Oct 19, 2012 5:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Are there digital versions of a 4 way pot, or would I simply have to use 4 digipots and assume that they were close enough to work?

(and actually looking at the circuit it'd be a total of 8...)

And I answer my own question:

http://www.mouser.com/Semiconductors/Digital-Potentiometer-ICs/_/N-4c498?P=1yzn3jhZ1yzotm4Z1z0vkozZ1yzxat0Z1z0jdigZ1z0z63x
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masterofstuff124



Joined: Sep 21, 2015
Posts: 23
Location: Miami, Florida

PostPosted: Mon Sep 12, 2016 8:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Anyone have Any luck converting this schem or adapting the digital potentiometer chips. I'm very keen on getting pitch to voltage and the mindreader pcbs are all gone.
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Moosgummi



Joined: Feb 01, 2015
Posts: 14
Location: Hiltown, Germany

PostPosted: Tue Sep 13, 2016 7:29 am    Post subject: Reply with quote  Mark this post and the followings unread

You can get four-ganged pots on aliexpress really cheap. Don´t know about their quality, but might be worth a try...
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LFLab



Joined: Dec 17, 2009
Posts: 493
Location: Rosmalen, Netherlands

PostPosted: Fri Sep 23, 2016 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

I do think you are better off using the burr brown f to V IC's, especially the more expensive ones have pretty good accuracy.
You will need to do some pre processing (convert to square, as the IC's are aimed at tacho readout, and are really expecting square waves).
And perhaps do some post processing to convert to log, V/Oct.
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Fri Sep 23, 2016 4:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nice project prgdeltablues. I enjoyed the guitar. I really wasn't expecting it to track that well. Very nice in my opinion
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