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CPlasmaGuns

Sound fx algorithm

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Given given an array of samples of a PCM-encoded sound, how would I go about slowing the sound down without lowering the pitch? Is it possible? Thank you.

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The best way I can think of would be to duplicate each sample n times to extend the length of the sound. maybe adding some interpolation to the algorithm to get an even sound.
Daniel Holley
Jag_007

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But duplicating samples like that would lower the pitch. I think I''ve heard of an algorithm that can change the pitch of a sound without changing its length. If anyone could tell me how to change the pitch that way, maybe I could combine it with the duplicating-sample method and get what I want . . .?

DUPLICATE EACH SAMPLE ONCE:
Sound is now twice as long with 50% pitch, then
DOUBLE PITCH USING ALGORITHM:
Sound is now twice as long with original pitch

Has anyone ever had to do this before?

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I''m far from certain, but perhaps you could try copying small bits of the sound; something like this:

Original: ABCDEFGHIJKLMNOP
Result: ABABCDCDEFEFGHGHIJIJKLKLMNMNOPOP

Maybe you could use interpolation of some sort. You''d also have to tweak the size of the "blocks." In mine I used a "block" size of 2; it may very well need to be higher, especially for higher frequency sounds.

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TerranFury, I just implemented your Block-duplication idea. Although the results were amusing, I didn't get what I wanted. I can't really explain what it sounded like - - very grainy and the speech that I slowed down became incoherent. I've uploaded the results here:

home.inreach.com/clkawa/sound1.wav (original sound)
home.inreach.com/clkawa/sound2.wav (block-duplication with block size of 4)

Any other suggestions?




Edited by - CPlasmaGuns on February 24, 2002 9:44:44 PM

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You could do some high-res spec analysis (not FFT, something more like auto-regressive analysis) on a set frame size, then regenerate the samples using the same parameters over multiple frames (2:1 for twice the length). If this is all Greek to you, you might be able to find something on the web by looking for "Voice Fonts".

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