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Beat-Counting

Posted on 1998-06-30
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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).
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Question by:Odie
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jhance earned 100 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.
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by:overworked
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?

-Rob
rob@gosupersonic.com
http://www.gosupersonic.com

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by:Odie
ID: 1166975
Overworked,

No problem
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by:Odie
ID: 1166976
JHance,

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?
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by:jhance
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.
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by:ozo
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.
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by:Odie
ID: 1166979
JHance,

I'm nearly ready with implementing your method.


Ozo,

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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