[vox-tech] Marking Audio file based on Freq.

Alex Mandel tech_dev at wildintellect.com
Tue Mar 11 10:04:29 PDT 2008

Jeff Newmiller wrote:
> Alex Mandel wrote:
>> Fun challenge if anyone is up for it.
>> How do I locate all instances of a particular frequency in an audio file?
>> Don't worry about format.
>> Preferred solution will use Audacity, Octave, R or some other FOSS 
>> software. More points the easier it is to use (really a Audacity plugin 
>> at least in concept would be great)
>> Bonus if it flags all the points in say the marker track of audacity, 
>> which would make it easy for me to shuffle through and listen to the 
>> identified segments.
>> Really any ideas on how to approach this would be welcome.
> Some ideas:
> Multiply the waveform both by a sinewave of the desired frequency,
> and by a cosinewave of the same frequency, obtaining two multiplied
> waveforms.
> Compute moving averages of each of these multiplied waves with a
> window of time long enough to detect the signal. This interval should
> be "many" cycles long.  At each timestep, compute the squares of the
> averages, add them, and take the square root.  The result should be
> a time profile showing roughly how well the sample data matches the
> desired frequency.  The longer the averaging interval, the better
> the selectivity of the frequency match will be, but the poorer the
> identification of "when" the frequency matched will be.
> Also, audacity has an "FFT filter" function, which could be configured
> as a bandpass filter, and the amplitude of the filtered signal could
> represent the frequency match with the bandpass filter frequency.

I think this puts me little closer to what I'm after. All my boss wants 
is to flag likely places where there might be a match and then use a 
human to decide if it's the bird species we want. So I think one pass 
with a fixed window size might work well, especially since I have an 
example to work with. The push to make it a plugin for Audacity is so 
that we have an easy to use interface for shuttling around the audio 
file and listening to pieces, although I question it's ability to work 
with 6 hour long files, I can also auto cut those with other apps before 


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