{"id":6588,"date":"2025-02-21T12:12:31","date_gmt":"2025-02-21T11:12:31","guid":{"rendered":"https:\/\/www.pschatzmann.ch\/home\/?p=6588"},"modified":"2025-03-04T17:16:10","modified_gmt":"2025-03-04T16:16:10","slug":"reverse-fft","status":"publish","type":"post","link":"https:\/\/www.pschatzmann.ch\/home\/2025\/02\/21\/reverse-fft\/","title":{"rendered":"Using Inverse FFT to generate Audio"},"content":{"rendered":"

So far I was using the Fast Fourier Transform (FFT)<\/strong> functionality of my “Arduino Audio Tools Library”<\/a> for analyzing the audio. This week I spent some time to make sure that we also support the inverse FFT properly.<\/p>\n

The inverse FFT<\/strong> is generating audio samples<\/strong> in the time domain from the spektrum information<\/strong>. This functionality is executed automatically when we read the data from the fft source: The spektrum information can be set in the callback.<\/p>\n

My implementation also supports a stride and a window function: In this case the samples are combined using the overlap add method.<\/p>\n

The output is automatically scaled, therefore the max input values are not really important. In the example we just use 1.0.<\/p>\n

Here is an example that generates a tone across the full spektrum:<\/p>\n

#include \"AudioTools.h\"\n#include \"AudioTools\/AudioLibs\/AudioRealFFT.h\" \/\/ using RealFFT\n\nAudioInfo info(44100, 2, 16);\nAudioRealFFT afft; \/\/ or AudioKissFFT\n\/\/CsvOutput<int16_t> out(Serial);\nI2SStream out;\nStreamCopy copier(out, afft);\nint bin_idx = 0;\n\n\/\/ provide fft frequency\nvoid fftSelectFrequencies(AudioFFTBase &fft) {\n  fft.clearBins();\n  FFTBin bin{1.0f,1.0f};\n  fft.setBin(bin_idx, bin);\n\n  \/\/ increase bin: restart from first bin\n  if (++bin_idx>=fft.size()) bin_idx = 0;\n}\n\nvoid setup() {\n  AudioToolsLogger.begin(Serial, AudioToolsLogLevel::Warning);\n\n  \/\/ Setup FFT\n  auto tcfg = afft.defaultConfig(RX_MODE);\n  tcfg.copyFrom(info);\n  tcfg.length = 1024;\n  tcfg.callback = fftSelectFrequencies;\n  afft.begin(tcfg);\n\n  \/\/ setup output\n  auto ocfg = out.defaultConfig(TX_MODE);\n  ocfg.copyFrom(info);\n  out.begin(ocfg);\n}\n\nvoid loop() { copier.copy(); }\n\n<\/code><\/pre>\n
Dependencies<\/h2>\n
    \n
  • https:\/\/github.com\/pschatzmann\/arduino-audio-tools<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"
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