CODEWITHSUNDEEP
iosobjective-cfftaudio-processing
Snick
Snick

Reputation: 115

Performing FFT on PCM file to generate a spectrogram

Im writing an app that visualizes music. So far I have an audio file from the ipod library converted to PCM and placed inside the APPs directory. Now I am trying to perform a FFT on that PCM file to give me frequency and db over time. Here is code I found which uses Apple Accelerate framework to perform the FFT:

int bufferFrames = 1024;
int bufferlog2 = round(log2(bufferFrames));
FFTSetup fftSetup = vDSP_create_fftsetup(bufferlog2, kFFTRadix2);
float outReal[bufferFrames / 2];
float outImaginary[bufferFrames / 2];
COMPLEX_SPLIT out = { .realp = outReal, .imagp = outImaginary };
vDSP_ctoz((COMPLEX *)data, 2, &out, 1, bufferFrames / 2);
vDSP_fft_zrip(fftSetup, &out, 1, bufferlog2, FFT_FORWARD);

Now I dont understand how to feed this the PCM file. 'data' I believe is an array of COMPLEX objects which hold the portion of the audio that the FFT will be applied to. How do I build such a data structure from the PCM file?

I found some Java code that might be useful but not sure how to convert this to C. Also was is audioData and how do I fill that from the PCM file:

Complex[] complexData = new Complex[audioData.length];
for (int i = 0; i < complexData.length; i++) {
    complextData[i] = new Complex(audioData[i], 0);
}

Upvotes: 1

Views: 1050

Answers (1)

zaph
zaph

Reputation: 112857

The "Apple Accelerate framework" is fine, I have used it on a project. Just read the docs carefully. Basically you will need to read the PCM data into memory and perhaps massage the format, then call the FFT on it.

To massage the data loop through your PCM data, sample by sample and copy as appropriate to another array.

The docs are here, you want the vDFT_fft* functions.

Mike Ash has a good writeup Friday Q&A site.

Audio data is the result of voltage samples taken periodically of the audio. The forma has many different forms. If you are going to work with this you are going to have to spend some time learning the basics. Also if you are going to run a FFT on audio data you need to learn about sampling frequency and "aliasing" sometimes called "fold back".

Upvotes: 1

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