FFT Phase Vocoder crackling

are you married to using the robin davies fft macros? the ezfft version sounds more to me like what a phase vocoder is supposed to sound like

IIRC, the robin davies don't automatically sync up their indexes at initialization, maybe its got something to do with that... really i'd recommend ezfft for most purposes, and especially if you're still learning your way around. ezfft also discards the top 'mirrored' part of the fft signal that you don't need to worry about and im not sure robin davies does

i mean... the original ezfft libraries were uploaded to the UL by a native instruments employee as a christmas present to the community, so people building their own stuff with them was pretty much the whole idea. a commercial product is more of a grey area, and not something i really know much about but if you're talking about making an updated phase vocoder and slapping it up on the UL no one's going to begrudge the use of FFT macros

i recommend these https://www.native-instruments.com/en/reaktor-community/reaktor-user-library/entry/show/6639/ and if you're nervous about what its okay to use them for the guy who coded them is here on the forum (tr97) you could always just ask him

I suggested looking up overlap-add. It's a common (fundamental?) part of the process for building a phase vocoder using fft/ifft. From one of the examples, I guess that what Blair is trying to do is a phase vocoder... maybe not, but it seems related, so makes sense to try using similar methods.

Anyhoo, overlap add solves the problems you get when you tamper with fft frames in a way that means the phases of the partials at the end of a frame don't line up with the phases of those partials at the start of the next frame causing clicks/glitches (at least that was my interpretation, someone with better fft chops could explain more clearly for sure @Tr97 ?).

You run multiple parallel channels. Each has a windowed frame of audio followed with a frame of silence, then audio, then silence... each channel is offset compared to it's neighbours. when you mix them together at the end, you get continuous audio, but there is no issue with clicks caused by phase discontinuities, because of the blocks of 'zero padded' silence. Something like that anyway.

I used a few vst plugins in the past that were defined as Phase vocoder where you morph from one sound to another using the phase information of one onto another. You could transform a dog barking slowly into a human voice or whatever the sources you used.

But it seems that term isn't used anymore for that maybe? I see they sometimes simply say spectral morphing.

I used to love a plugin called Shapee,phase vocoder...i liked it more then the Zynaptiq one,transients were preserved better to my ears...it's very old though and was never updated to 64 bit.

Never tried the Melda one.

oh! overlap add as in adding an increasing amount of overlaps together. i was thinking of something else.. maybe overlap save? was it overlap save you used in convolution macros to be able to convolve something that lasts longer than a single frame of an fft?

i believe increasing the overlap factor farther than 2x requires some kind of trick. ezfft seems to do it by cutting off the top half mirrored part of the fft and then alternating the output of two ffts, so the useful half of two ffts fits are output in the time that a single one with its redundant upper half normally would. not sure if zero padding is needed for that or not. from what i've read zero padding an fft is equivelent to interpolating it with a sinc filter

@tr97 it seems stuff i've seen of your that used greater than 2x overlap used either that iterator event blast trick or distributing the overlaps across voices. guess you can't increase the overlap more without sacrificing the '2 continuous audio streams' attribute?

Can't remember if it was overlap save or overlap add in the convolution.

Overlap add in the case of this vocoder thing is just having two (or more) channels where they are windowed and offset so that when one is at a frame transition, it's volume is zero, and the other is max, and vice versa. That way, you remove the artefacts at the frame transitions from the audible output.

I found so far that for this vocoder, the best sound quality was using four channels, noticeably better then two for some things, but going higher didn't make a difference to my ears. Maybe best compromise is just two though - better cpu.

The 512 and 1024 fft sizes seemed the best, and different enough to both be useful. I guess 2048 and 256 would also be useful for some source audio. going down to 128 didn't work so well for me (also found a bug in the Davies 128 iFFT module - the phase output isn't connected internally)

Next is to try and understand how to use this for pitch and speed manipulation :)