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 Forum index » Clavia Nord Modular » G2 Building Blocks
FM to PM converter
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Tim Kleinert



Joined: Mar 12, 2004
Posts: 1118
Location: Zürich, Switzerland
Audio files: 7
G2 patch files: 227

PostPosted: Thu Mar 10, 2016 3:11 pm    Post subject: FM to PM converter
Subject description: Phase modulation for all stock G2 oscillator modules with FM input
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I just noticed this thread and had a quick go at the subject.

The building block presented here expands the functionality of the G2 by providing true oscillator phase modulation (PM) for oscillator modules that have an FM input.

Mathematically speaking, PM is the first derivative of FM. For an arbitrary unknown external audio signal used as modulation source, it thus suffices to differentiate it (using this super-cheap building block), amplify the result to a usable level, and patch it to the FM input of the carrier oscillator.

In the common scenario of a synthesizer voice architecture where one oscillator phase-modulates another, one however has to take into account that the amplitude of the modulator derivative is also relative to the modulator frequency (since higher modulator frequencies result in higher amounts of momentary change within the waveform). Thus, in order to keep the PM modulation index uniform across the pitch range, the amplitude of the PM signal has to be scaled with a -6dB/octave slope controlled by the pitch of the modulating oscillator. I implemented this with an OscMaster module that both controls the pitch of the mod osc as well as the LevelScaler module performing the required roll-off. Any pitch modulation of the mod osc thus should be patched into this OscMaster module.

I leveled out the circuit so that the PM mod range matches the FM mod range pretty accurately. (As test reference I used a sine modulator. The derivative of sine is cosine, so in a free-running oscillator scenario FM and PM should sound identical.) I had to boost the final level by a whacking factor of 512x, (9 bits) which means that quite a bit of dynamic resolution is lost in the process. However, I couldn't detect any audible differences in the sine modulator PM/FM comparison.

This also means that when modulator waveforms have very steep flanks, clipping is imminent and the PM scenario doesn't perform correctly. However, the meters of the cascaded mixers used for boosting give you a clear visual indication of this happening, and their pad switches are a convenient way to back down the signal.

PM with clipping-prone steep-flanked classic analog modulator waveshapes isn't that interesting anyway. Using a filtered oscillator or ShapeOsc as modulator however does some really nice things.

cheers,
-t


FM-PM-convert_TK.pch2
 Description:
Building block that provides phase modulation (PM) for all stock G2 oscillators that feature an FM input.

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 Filename:  FM-PM-convert_TK.pch2
 Filesize:  1.63 KB
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BobTheDog



Joined: Feb 28, 2005
Posts: 3935
Location: England
Audio files: 32
G2 patch files: 15

PostPosted: Tue Mar 22, 2016 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oooh interesting stuff, will have a play with this tomorrow...
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