I coded up the spectral freeze core of the Audio Damage "Spectre" module in a similar way:

http://www.audiodamage.com/hardware/product.php?pid=ADM15

It's a basic phase vocoder with forward and inverse FFTs but we added some fun little tweaks to shift, stretch and randomize the spectrum. It runs nicely on an STM32F405 ARM Cortex M4F processor.

Eric

On 09/16/2016 10:59 AM, Giulio Moro wrote:
Hi,
I actually implemented this a few years back using an FFT algorithm, I
can dig out the code if you need it (it was a VST written using Juce and
fftw, but there was no threading on the FFT if I remember correctly, so
it is flawed as it is and it requires running with large blocksizes.
I doubt simple time-domain algorithm would work without obvious
artefacts, as the periodicity is not guaranteed and the combined period
could be very long anyhow.

The FFT implementation I made was the text-book phase vocoder but I was
doing the forward FFT only once at the beginning of a freeze, to
"sample" the signal, then I would keep the vector of amplitudes constant
while updating the phase.

Best,
Giulio


    ------------------------------------------------------------------------
    *From:* Spencer Jackson <ssjackso...@gmail.com>
    *To:* music-dsp@music.columbia.edu
    *Sent:* Friday, 16 September 2016, 18:30
    *Subject:* Re: [music-dsp] Help with "Sound Retainer"/Sostenuto Effect

    On Fri, Sep 16, 2016 at 11:24 AM, gm <g...@voxangelica.net
    <mailto:g...@voxangelica.net>> wrote:
     > Did you consider a reverb or an FFT time stretch algorithm?
     >

    I haven't looked into an FFT algorithm. I'll have to read up on that,
    but what do you mean with reverb? Would you feed the loop into a
    reverb or apply some reverberant filter before looping?


    Thanks,
    _Spencer
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