Frequency modulation of one audio signal by another

JeanFran · Joined: Jun 20, 2011 Posts: 2 Location: France

Hi,

I'll gladly receive some help here . I consider myself not to be totally helpless in the field of modular synthesis but i feel stucked at the moment .

I want to modulate the frequency of an audio signal A by an audio signal B and hear the modulated signal A'. That's all ! It's basic FM synthesis except that i want to choose the carrier and the modulator freely . And i just don't find any piece of gear to do that .

Any advice or hint ?

Take care,

Jean-Francis

Edit : I fear not being specific enough .
Consider a DX7 . A, the carrier, is modulated by B's frequency, the modulator . B's amplitude give the modulation's strength . A', the modulated carrier, is heard . I just want to be able to choose any audio signal for carrier and modulator . Clearer, isn't it ?
Tell me there's an issue !

abstraktor · Joined: Apr 27, 2009 Posts: 189 Location: glasgow, scotland

Modular Synthwise-if one audio signal is patched through a VCA or Filter and then the other is patched into a modulation input of that module then in theory you should get it.
You might have to boost or attenuate the modulating signal before it goes into the CV in though....

ian-s · Posted: Mon Jun 27, 2011 2:21 pm Post subject:

The system in the DX7 is called FM but it is the oscillator phase that is being modulated. Some vco have linear FM input which gets you closer to what you want but still not a direct equivalent. Synthia's zero oscillator does proper DX style FM (and a lot of other things).

ndkent · Joined: Jan 03, 2006 Posts: 63 Location: new york

Using an audio signal as a modulator is do-able in any system that lets you configure the signal path. After all, it's functionally just exchanging an external signal for one generated internally by a an oscillator or something.

If you think about it, what happens in a traditional carrier is the modulating signal tells it to change pitch. An oscillator is designed to be told what pitch to play. So it's built to be modulated in frequency. An existing audio signal has no pitch input whatsoever. There are a few options but they all are a huge compromise. One could sample the sound, because once sampled the pitch can be changed by simply playing back faster or slower. The price paid of course is the speed changes and you are dealing with sound that already happened and was stored in digital memory. There are analog delay devices either using BBD chips or magentic tape. When the delay time is modulated the byproduct is a pitch change, but the nature of it is a change has to alternate between up and down.

The other kind of compromise is a pitch shifter. Simplified they work by constantly duplicating or throwing out tiny amounts of sound and smoothing the pieces really quickly. That way it's a realtime pitch change with a tiny amount of processing delay. "Autotune" effects also connect here. You are sending commands to repitch stuff. You pay a price that the sound is sort of constantly re-assembled. Compare to an oscillator that simply produces a wave at the pitch you tell it, hopefully you can understand the faults in trying to modulate existing audio.

I guess frequency shifters are worth mentioning. They sound useful here but it's a red herring. I'd call them very complex amplitude modulation devices. Which brings up Amplitude modulation in general. Definitely not the same as frequency modulation. It's not mentioned all that often because it's sounds tend to lose their harmonic structure, in other words the can lose a clear pitch but do create different waveforms and is quite easy to do with existing signals and a basic VCA. A VCA has no problem adjusting the amplitude of what's going through it. And so long as the range is right, you can modulate it with anything positive.

I'm still learning the ins and outs of phase modulation myself. As I understand it, by changing the phase a byproduct of the change in frequency. I don't think there is a practical way to let this kind of modulation happen to an existing sound as a synthesis method.

Eric G · Joined: Mar 07, 2007 Posts: 19 Location: Sweden

Eric G · Joined: Mar 07, 2007 Posts: 19 Location: Sweden

I see you want to FM any signal, so a VCO won't do it.
An FM modulator isn't a basic module, really. A pitch shifter might do the job, if it can handle through zero modulation.

A cheat is however to use an analog delay. You will get phase modulation, but the audio effect will be very similar to FM.

stefanv