[linux-audio-user] Note tuning and quantizer in audio files

[Alejandro Lopez]

Hi again,

Just to briefly explain these two nice techniques as a couple of you have 
asked about them. They are both based on the same math, a kind of modified 
fast fourier transform which enables the software to stretch or shrink time 
without affecting pitch or sound timbre (the characteristic that enables us 
to recognize an instrument).

Note tuning, only applicable to samples of monophonic instruments: the 
software has a freq detector that will find the nearest note to what is 
being played. It will find a starting time, end time and deviation for that. 
Then it will apply this FFT to generate a sample equal in duration but in 
tune. This is done by stretching or shrinking as appropriate first (this 
does not change pitch), then re-sampling to length which changes the pitch 
as required.

Quantizer: only used for samples of percussion AFAIK, but should be 
applicable to other instruments with high attack like bass and others: first 
step is for the software to identify the location of the notes (that's the 
equivalent for MIDI events but this time in audio), by using a kind of peak 
detector this time instead of the freq detector for the other technique. 
Second step is to identify where the notes should exactly be time-wise. For 
this, it's neccessary that the software knows about the timing of the song / 
sample! (basically BPM and desired quantize resolution). Knowing this, it's 
pretty straightforward to "shrink and stretch" before and after the note so 
it falls exactly on time.

These two give best results when the notes are not too off. This is due to 
this special FFT not being a perfect method to extract exactly everything 
that's there, especially high frequencies. Different implementations of 
these use different algorithms, all are based on a FFT, but some use 
different techniques to mask certain side effects that appear on the 
modified sample. Some implementations are very impressive I have to say, and 
they can stretch or shrink to say two times or half with brilliant results.

All pretty impressive technology which has been around for a while now..

Cheers,

Alex

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