Can I convert audio frequency to CV?

[starkaudio]

I'm an upright bass player who also loves modular synthesis but I never considered combining the two until now. I'd like to generate CV from my upright bass, but I'm not sure how feasible it would be.

Check my logic here:

I think if I can convert the frequency to CV, and the amplitude to CV (like using an envelope follower perhaps) I can get interesting analog CV from my upright bass. So I'm looking for either modular synth vendors who could support this or maybe software for my mac that could do this. Worrisome things like tracking and latency I'll worry about on my own...I'm just looking for ideas and pointers to start with.

Suggestions? Has anyone seen modules from any vendor to do this? Or modules I could build upon? Or software? (I must admit I lean towards hardware.)

[kkissinger]

The basic hardware method to do this would be to:

1) low-pass filter the input signal (that is, make it contain as much foundation tone as possible

2) Compress this signal by a factor of 100%

3) Convert the signal to a square wave with a comparator

4) Integrate the square wave -- (you can find op-amp integrator circuits online) The integrator will produce a DC voltage that corresponds to the frequency of the square wave

5) Amplify the DC voltage -- trim the gain to 1v/octave

Note that if you hit two strings at once or if there are resonances from the open strings, this may confuse your converter. This is a characteristic of pitch-to-voltage converters -- for better or worse!

Now -- another possibility -- that would be easier --

Simply build a module that contains front-end processing for your bass signal -- a wave-shaper and/or a comparator would be an excellent choice. You would use the resultant (bright) waveform to run through filters, etc. With this method, resonant vibrations won't cause any big problem -- you can still build an envelope follower to control other circuitry.

OK -- if you want that PHAT sound of two detuned oscillators, you need to convert your waveform to a sawtooth wave then run it through a Sawtooth Shifter -- Yusynth and CGS have such circuits -- that allow you to get that phat sound with only one input signal (you will need to build an LFO to drive the shifter).

All the best! Will be curious to hear what others have to say and what you finally decide to do.

[kybertech]

http://www.expert-sleepers.co.uk/es1.html

They sell a module, and fairly cheap considering...

Keep in mind though that this is most likely works best for sine waves.
For every other waveform I would use a FFT on a microcontroller or pc. If you combine it with other techniques you can even get usable results but FFT alone usally has a drift a few hz. But if you compare that to the out-of-the-box 8 bit midi/cv converters you are still fine.

[kx]

Hi,

A wee while ago I built a circuit using the LM2917 frequency to voltage converter IC : http://www.national.com/mpf/LM/LM2917.html#Overview

It uses a very simple circuit and I got it as a free sample from National semi

I was using it as a means to convert a frequency output from my audio interface to a CV, before I ended up getting a MOTU 2408mk2 which can output DC. I imagine it should work with your double bass arrangement though.

Cheers,

Jason

[monomania]

Analogue Systems do one
http://www.analoguesystems.co.uk/index2.htm