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Pitch Ratios with a Global Ratio

June 19, 2013 | 4:39 am

Hello All,

I’ve had a little search but can’t come up with much from the forum.

I have a few different granular polys~ which all work using a pitch ratio (0.5 = -12 semitones, 2 = +12). I want to be able to change these pitches with a global control, which also works on ratios. After a bit of rudimentary fuddling I had something which sounded correct by adding the global pitch ratio to the current pitches, but this doesn’t work when the main pitches are in a high ratio, as the global pitch is limited in my patch to -1.

Here’s a patch to demonstrate the problem:

<code>

– Pasted Max Patch, click to expand. –

</code>

Obviously this is broken, but my little monkey brain doesn’t know the maths to make it work.


June 19, 2013 | 4:53 am

You want to multiply ratios, not add them (and no need for a funky offset to the global ratio).

Just think: if your global transposition as an octave and the local transposition is zero, you get ratios of 2.0 and 1.0 respectively. Multiplying the two values gives you a net transposition of one octave; adding them directly would give you a transposition of a perfect twelfth (octave plus fifth). (Your funny offset would actually give you the correct result in this particular case, but that’s just bad luck—or good luck, depending on how you look at it). Run through the maths for a couple of simple examples and you should be able to convince yourself that multiplication is the correct operation.

<code>

– Pasted Max Patch, click to expand. –

</code>


June 19, 2013 | 6:23 am

Thanks Peter, that makes perfect sense.


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