CODEWITHSUNDEEP
signal-processingfftweb-audio-api
Kevin
Kevin

Reputation: 61

Fast-fourier transform for low-frequency signals in the WebAudio API / javascript

I have a single-channel wave coming in at an 8000 Hz sampling rate.

I need to analyze frequencies that are between 5 Hz and 300 Hz in real-time, with emphasis on signals from 10 to 60 Hz.

My thought initially is to run the 8000 Hz sample into a buffer, collecting about 32000 samples. Then, run a 32000 window-sized fourier transform on it.

The reasoning here is that for lower-frequency signals, you need a larger window size (right?)

However, if I'm trying to display this signal in real-time, it seems like the AudioAnalyserNode might not be a good choice here. I know the WebAudio API would allow me to get the raw data, but ideally the AudioAnalyserNode would be able to run a new fft based on the previous 32000 samples, even if a smaller amount of samples have become newly available. At this point, it seems like the fft data is only updating once every four seconds.

Do I have to create a special "running bin" so that the display updates more frequently than once every 4 seconds? Or, what's the smallest window size I can use to still get reasonable values in this range? Is 32000 a large enough window size?

I am using the WebAudio API analyser node in javascript, but if I have to get the raw data, I'm also willing to change libraries to another one in javascript.

Upvotes: 2

Views: 776

Answers (1)

Raymond Toy
Raymond Toy

Reputation: 6048

Using an AnalyserNode, you can call getFloatFrequencyData as often as you like. This will return the FFT of the last fftSize samples. These get smoothed together. For full details, see AnalyserNode Interface

Also, the WebAudio spec allows you to construct an AudioContext with a user-selectable sample rate. You could set your sample rate to 8000 Hz. Then your FFTs can have finer resolution with less complexity.

However, I don't think any browser has implemented this capability yet.

An alternative would be to get a supported audio card that allows a sample rate of 8000 Hz and set up your system to use that as the default audio output device, Then the audio context will have a sample rate of 8000 Hz.

Upvotes: 2

Related Questions