KVR Audio

massafagga · Post by **massafagga** » Sun Jul 11, 2004 8:23 pm

hi,

what is the best way to do pitch shifting in real time? i want to program an octaver for bass.
i thought about doing an fft, halve the spectrum, and transform it back in time domain. but i expect that either i get a big latency when using too much samples or that tracking and acuracy is not very good because of too less samples (especially for low frequencies)

thx tobias

texture · Post by **texture** » Sun Jul 11, 2004 11:10 pm

One thing you might be able to do is slice the audio into small fragments and play back each fragment at a different speed. For increaing the pitch, this would mean looping the fragments. You would also need to apply some sort of envelope to the fragments. This is kind of like granular synthesis I guess.

I doubt this is the best way though!

Kim Lajoie · Post by **Kim Lajoie** » Mon Jul 12, 2004 2:52 am

If you only want to make a sound an octave lower, a really sneaky way of doing it is to flip the polarity of every 2nd cycle, and maybe lopass filter to taste.

Only sounds decent on simple (monophonic) sounds.

Forever,

Kim.

Miles1981 · Post by **Miles1981** » Mon Jul 12, 2004 8:35 am

You can interpole the samples with a zero, and then filter it at your original sample frequency.

The problem with FFT is that you suppose the signal is periodic, and that's not the case.

Paul_FX · Post by **Paul_FX** » Mon Jul 12, 2004 9:51 am

Take a look at Perry Cooks STK. There is a delay based pitch shifter in there that sounds sweet..

Paul

massafagga · Post by **massafagga** » Wed Jul 21, 2004 11:38 pm

first, thanks for all the answers.

but I think the fft is the best way to do what i want. if anyone is interested, i found a good article with additional c++ code on that theme by Stephan Bernsee:
http://www.dspdimension.com/
look under articles or downloads for "Pitch Shifting using the Fourier Transform"

@miles1981: but you can expect the signal to be periodic for short times, so you can do a fft.

stefancrs · Post by **stefancrs** » Thu Jul 22, 2004 6:58 am

If you just want a standard bass-pedal octaver I'd saying doing it in time-domain is the best method. You can easily play around with different methods then. Adding a filtered "square wave" that changes it's polarity for instance every 2nd time that the input sounds crosses zero. Or every time. Etc. You could expand this into a small "bass synth" (as in "guitar synth" etc) by adding envelope followers, filters and so on.

Happy coding :)