Cycling 74 » Topic: Frequency detector

Frequency detector

periklis — Sat, 16 Apr 2011 14:18:17 +0000

Hi guys,

here is my question: i need a frequency detector which works without fft. Is there any autocorrelation patch or some idea of how i could detect frequency of a (relative…) periodic signal?

Re: Frequency detector

mudang — Sat, 16 Apr 2011 15:19:25 +0000

In the FTM library there’s an external called gbr.yin~. It implements the Yin algorithm which is a kind of improved autocorrelation and works quite well for monophonic sources.

If you have a very “clean” signal, zerox~ might also do the job…

Re: Frequency detector

Chris Muir — Sat, 16 Apr 2011 17:07:20 +0000

The old-school guitar synthesizer method is to clip the signal until you have a more or less square wave, then analyze the period of the square wave. It doesn’t work all that well, though. You probably want to low-pass filter the signal before you clip it.

Re: Frequency detector

periklis — Sat, 16 Apr 2011 17:39:14 +0000

>If you have a very “clean” signal, zerox~ might also do the job…

I tried zerox~ but there is this problem with the signal vector. To get a good response out of it, concerning the number of crossings-which by the way should EXACTLY do what i am looking for-i have to push mr. vector at 2048 samples which is (theoretically at least…) bringing some latency to the output. But i’ll try the yin~ thing right now!

Thanks a lot both of you guys!!!

Re: Frequency detector

mudang — Sat, 16 Apr 2011 20:50:53 +0000

Latency is always an issue with frequency detection – no matter if you use fft, autocorrelation or zero counting.
This of course depends on the lowest frequency you want to be able to detect.

For autocorrelation you’ll need twice the period length of the lowest frequency.

There seem to be techniques to do a frequency detection within a fraction of the period length (used in guitar to midi boxes, f.e.) – but I’ve never found any details on them…

Re: Frequency detector

Chris Muir — Sat, 16 Apr 2011 21:43:50 +0000

The best one I know of is the Axon / Blue Chip method, licensed to Yamaha, who appear to be sitting on it these days. It supposedly used a pre-trained neural network to look at the slope of the wave and make a decision based on the first quarter-cycle.

I have the Yamaha G-50, which incorporates this, and it is way faster than anything by Roland, which mostly uses period detection.

Re: Frequency detector

Jean-Francois Charles — Sun, 17 Apr 2011 16:20:15 +0000

Thanks Chris for the insight.
Do you know if it works as well for other instruments than guitar, or is this technique really optimized for guitar and less performant on other instruments?

About autocorrelation vs FFT analysis: when you use FFT, you need to use a window which size is a power-of-two number of samples. With autocorrelation, you can tune the window size more precisely to the lowest frequency you are looking for.

Anybody knows what kind of method the Eventide boxes use?

Re: Frequency detector

Chris Muir — Sun, 17 Apr 2011 17:37:51 +0000

I imagine that the Axon technique could work on many/most sounds, but it would need to be trained for different classes of sounds.

Re: Frequency detector

mudang — Mon, 18 Apr 2011 10:14:02 +0000

Here’s some info on axon’s technique:

http://www.axon-technologies.net/index.php?name=EZCMS&menu=17&page_id=28

The neural net is used to find the pluck impulse and the first (bridge) and second (fret) reflection on a string.

The second trick they use is to only count zero crossings with a maximum slope. As far as I understand it, this is only to avoid octave-errors due to strong harmonics.

Re: Frequency detector

periklis — Mon, 18 Apr 2011 14:14:32 +0000

This is very helpful. Thanks! very clever also :-))
I also used the yin~ object and it does an excellent job! And i must say, that pushing the vector at 2048 (for doing simple zerox detection) didn’t caused me any latency problems (even with other more demanding patches)

Re: Frequency detector

mzed — Mon, 18 Apr 2011 17:55:48 +0000

I’ve also done pitch detection using a wavelet transform. You still need a fair number of samples (say 512) before you get the result, but unlike fft’s the resolution is pretty good in the lower frequencies.

Re: Frequency detector

mudang — Mon, 18 Apr 2011 21:55:32 +0000

@mzed:
How do you do the wavelet transform in max ?

At the moment i’m writing a polyphonic pitch detection using fft. To get a good enough resolution to discern 12TET steps in the lower frequency i need to have a 8192 fft window :(

Re: Frequency detector

mzed — Thu, 12 May 2011 16:32:34 +0000

I’ve done the transform and detection as an external object written in C. IMHO, doing this kind of analysis is pretty inefficient, both on the programming side and when its running.