How to generate the audio spectrum using fft in C++?

c++ audio fft spectrum
MRashid picture MRashid · Jan 13, 2011 · Viewed 36.6k times · Source

I want to generate an audio spectrum (as seen in this video) of a mp3 audio file. Basically this problem requires calculating the fft of the audio signal. How do I program this in C/C++?

I've looked at a couple of open source libraries such as FFTW and I really don't know how to use these for my problem. Any help would be greatly appreciated. Thanks in advance!

Answer

Paul R picture Paul R · Jan 13, 2011

There are quite a few similar/related questions on SO already which are well worth reading as the answers contain a lot of useful information and advice, but in essence you need to do this:

  • convert audio data to format required by FFT (e.g. int -> float, separate L/R channels)
  • apply suitable window function (e.g. Hann aka Hanning window)
  • apply FFT (NB: if using typical complex-to-complex FFT then set imaginary parts of input array to zero)
  • calculate magnitude of first N/2 FFT output bins (sqrt(re*re + im*im))
  • optionally convert magnitude to dB (log) scale (20 * log10(magnitude))
  • plot N/2 (log) magnitude values

Note that while FFTW is a very good and very fast FFT it may be a little overwhelming for a beginner - it's also very expensive if you want to include it as part of a commercial product - I recommend starting with KissFFT instead.

Related questions

Real-time pitch detection using FFT

I'm trying to do real-time pitch detection using C++. I'm testing some code from performous (http://performous.org/), because everything else hasn't worked for me. I know for sure that this works, but i just cant get it to work. …

Read More
c++ FFT Beat detection library?

I am currently looking around for a good allround beat detection library / source code in C++ since I found it really hard to achieve satisfying results with the beat detection code I wrote myself using this tutorial: http://www.gamedev.…

Read More
How are VST Plugins made?

I would like to make (or learn how to make) VST plugins. Is there a special SDK for this? how does one yield a .vst instead of a .exe? Also, if one is looking to make Audio Units for Logic …

Read More