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Repitching Analysis

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Repitching Analysis

Postby Elephone » Thu Jan 31, 2013 7:17 pm

Just curious: Is it possible to determine, by spectral analysis or something (not just by ear) whether something has been repitched from its original recorded note. Furthermore, is it possible to determine what the original pitch was? Is this possible to do unequivocally, so it would be permitted in a court of law say?

I mean, for instance, if someone slows down a guitar part, won't the picking noise be incorrect (unless the player is using a huge plectrum, in which case the playing might be impossible)?
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Re: Repitching Analyses

Postby desmond » Thu Jan 31, 2013 7:19 pm

James101 wrote:I mean, for instance, if someone slows down a guitar part, won't the picking noise be incorrect (unless the player is using a huge plectrum, in which case the playing might be impossible)?

Not necessarily, because pitch/time shifting algorithms could repitched detected pitch components but leave transients and noise-based plosives unpitched, and then there's the whole whether formants have been shifted or not as well.

I think it would be pretty difficult to conclusively prove, but then I'm not a DSP expert by any means...
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Re: Repitching Analysis

Postby alexis » Fri Feb 01, 2013 2:07 pm

James101 wrote:Just curious: Is it possible to determine, by spectral analysis or something (not just by ear) whether something has been repitched from its original recorded note. Furthermore, is it possible to determine what the original pitch was? Is this possible to do unequivocally, so it would be permitted in a court of law say?

I mean, for instance, if someone slows down a guitar part, won't the picking noise be incorrect (unless the player is using a huge plectrum, in which case the playing might be impossible)?

I would venture a "Yes" for a court of law, that it could be identified as repitched (do things have to be unequivocal for courts of law, or just preponderance of evidence?). These repitchers in the end are simply algorithms that perform mathematical functions to transform one signal into another.As a result their output I would think would have a mathematical "finger-print", unique for each algorithm.

And if someone wrote their own repitch algorithm and used it, even though the "fingerprint" might not be recognized as a "Melodyne" or "Auto-tune" one, I'd go out on a limb and say the repitched audio would be recognized as un-natural. Sort of the equivalent of a fingerprint at a crime scene having only right angle turns.

My 2 cents!
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Re: Repitching Analysis

Postby Elephone » Sat Feb 02, 2013 5:43 pm

Thanks. Are there algorithms for simple re-pitching? I didn't necessarily mean any time correction would be involved. If we slow a tape machine down (to digital) I suppose the dynamic range might give the game away.
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Re: Repitching Analysis

Postby The Red Bladder » Sat Feb 02, 2013 6:09 pm

The answer is no, because -

1. Most commercial recordings are re-pitched (Autotune, Melodyne, etc.) anyway, especially lead guitar, bass and vox.

2. The old method of cutting the audio into little packets and layering them on top of one another (invented by Eventide, I believe) is no longer used, except in the cheapest pedal effects, so you no longer get those warbling/flanging effects that came with pitch-shifting.
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