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Pitch shifting creates echo/reverb

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Pitch shifting creates echo/reverb

Postby dschnabel » Fri Jan 10, 2020 9:51 pm

Sorry for this noob question but I'm pretty lost. Maybe someone here can point me in the right direction.

I'm trying to pitch shift a recording of a spoken computer voice to make it sound like a chipmunk.

This is how it sounds after I pitch shift: https://vocaroo.com/1ANYSlHjWi2
There's some weird echo/reverb going on but I can't identify what causes this. The words are hard to understand.

I want it to sound more like this: https://vocaroo.com/oSHlsqTWTDv (created using https://lingojam.com/ChipmunkVoiceChanger).
While there's more "grainy-ness" to this version, the words are much easier to understand.

So my question is: what filter should I apply to get rid of the echo/reverb so that the words are easier to understand?

(for completeness, the original recording is here: https://vocaroo.com/eXRI32uWd0I)
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Re: Pitch shifting creates echo/reverb

Postby James Perrett » Sat Jan 11, 2020 6:16 pm

The simplest way for the software to create this effect is probably going to be to split the file up into small chunks, double the speed of each chunk and then repeat each chunk to fill in the gaps. So you'll hear a slight echo as each chunk is repeated.

There are more sophisticated algorithms around which would be worth trying - I use Reaper which gives you a choice of a few with its built in pitch shifting. It may also be worth decreasing the pitch shift - rather than an octave shift try 8 semitones and see if that helps.
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Re: Pitch shifting creates echo/reverb

Postby The Elf » Sat Jan 11, 2020 8:15 pm

An octave is a long way to expect pitch shifting to preserve the diction of a voice. Back in the day those chipmunk voices were done by speaking at half speed, then speeding the tape up - and I'd look at a similar technique using sample playback speed, rather than pitch-shifting. You may find it gives you something more akin to what you're expecting.
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Re: Pitch shifting creates echo/reverb

Postby dschnabel » Sat Jan 11, 2020 9:24 pm

Very interesting, James Perrett. I think what you describe (split file up into small chunks) is exactly what this algorithm is doing that I'm using. This means I can't really fix the audio by applying additional filters.

I've tried different libraries and rubberband (https://github.com/breakfastquay/rubberband) seems to give me the best result in terms of pitch shift (no echo/reverb, no grainy-ness). Probably they use one of the more sophisticated algorithms that you've mentioned.

Thanks for helping me understand of what's going on!

Also thanks to The Elf for suggesting the speed up option. In my case, I'm working with computer generated voices that don't allow me to slow down the speed more than a few percentages. But for real voices this would definitely be an option.
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