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Posted on 1998-06-30
Medium Priority
Last Modified: 2010-04-10
Does anyone how I can implement Beat-Counting (let my program 'know' wether or not there is a beat at a certain moment and determin the amount of this per minute) if I have acces to fft and pcm data (real-time).
Question by:Odie
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LVL 32

Accepted Solution

jhance earned 200 total points
ID: 1166973
I've not done this in software but have done it in hardware.  I once built a light system for a dance floor and the requirement was to index a series of lights once for every beat of the music.  I built a low pass filter that rolled off sharply above 100Hz.  This then went to an integrator that needed a signal for about 0.125 seconds before triggering it's output.  This output went to a flip-flop circuit that performed a divide-by-2 function (it looked better to do every other beat that every one).  

Since you are doing this in software and you have the fft, it should be a easy task to run this filter and make it work.  You can even add adaptive features so that as the music changes it can "search" different frequencies and windows for a beat if the default one is lost.

Expert Comment

ID: 1166974
I'm very interested in doing the same thing in my app SuperSonic. If you come up with a successful beat counting routine, would you be willing to put it in a dll and license it to me?



Author Comment

ID: 1166975

No problem

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Author Comment

ID: 1166976

Could you explain that a little more detailed?

I know I have to search for "pulses" in the lower fft-values, but could you be a little more specific in your explanation?  How do I do this?
LVL 32

Expert Comment

ID: 1166977
You pick you range of frequencies and then watch for the presence of a signal for a given time period.  For example, let's say you have broken down your input signal using a FFT into 3 bands, 20-200Hz, 200-2000Hz, and 2000-20000Hz and the values are normalized to 8-bits.  Now your software can "watch" the 20-200Hz band for a signal above a threshold that is present for "X" mS.  After that time, you assume that there is a beat.  Then you wait for it to go away for "X" mS.  Now you assume that the beat is over.
LVL 84

Expert Comment

ID: 1166978
I might have been inclined to look for peaks in the residuals from the Linear Prediction Coefficients
But windowed pulses in low FFT bands sounds lika promising idea too.

Author Comment

ID: 1166979

I'm nearly ready with implementing your method.


Could you explain that Linear Prediction Coefficient stuff a little more.  I might want to implement different methods to make sure I'm counting beats (and not noise or something).


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