[sdiy] Digital accumulator VCO core?

Brian Willoughby brianw at audiobanshee.com
Mon Feb 15 22:22:29 CET 2021

I think you're conflating autotune and spectral gate. They do both have FFT in common, but so many effects have FFT in common and that doesn't mean they're necessarily related.

My point is that the spectral gate does not need to make any attempt to work out the prime element in each band, and that's a very expensive algorithm. If you're not dealing with the most expensive calculation in one effect (autotune) then there's no real need to consider it the same as or even related to another effect (spectral gate).

Spectral Gate can ignore the phase element of the real FFT results, and focus only on the amplitude. Gate (of a single FFT bin) is basically a pure amplitude operation.

I worked on porting a Spectral Gate effect from VST to AudioUnit in the early days of Mac OS X. The project never got finished, and I worried about the artifacts from altering sound in the frequency domain. This effect had the usual Gate threshold, but it was applied individually to each band. I recall that there was a way to measure the "noise" during "silent" excerpts of the audio, and this frequency domain curve was used to apply a different threshold to each bin of the FFT.

If I'm missing something about Spectral Gate that makes it more like Autotune than I've described, please point it out.


On Feb 15, 2021, at 12:37, Mike Bryant wrote:
> It was never designed as an effect.  Autotune splits a signal up into hundreds of frequency bands, then works out if the prime element in that band is 'on-tune' to whatever key you specify, and if not moves the frequency slightly so that it is.  It then adds all the reconstructed elements back together so that the singer is in-tune.  It's thus quite easy to add a noise gate in each band so that if there is no significant activity in that band the band is nulled.
> -----Original Message-----
> From: cheater cheater
> Sent: 15 February 2021 20:31
> OK, I'm still lost, what does cher's favourite vocal filter have to do with removing noise from a signal? I'm sure there is a smart reason, I'm just not privy to it.
> On Mon, Feb 15, 2021 at 9:26 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>> I'm not sure if the Antares original ever implemented that feature.  But some of the many other ones inspired by Antares did.
>> Long before that there were similar products used to pull information out of noise, notably on digitised old film soundtracks.  But that was usually proprietary software and wouldn't work in real time.
>> -----Original Message-----
>> From: cheater cheater
>> Sent: 15 February 2021 20:21
>> Sorry, autotune? Like the Antares Auto-Tune plugin?
>> On Mon, Feb 15, 2021 at 8:51 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>>>> On a separate note, has anyone come across something that's like a spectral "gate"? like a normal time-domain noise gate, but for each frequency separately: i.e. a frequency will pass through only if it's above a certain threshold.
>>> Yes that's possible with many wavelet based autotune implementations.

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