Custom FFT Frequency Range

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tcontrada

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I have a project where I have to measure the frequencies of a mechanical device at low audio frequencies, mostly below 1KHz. The Audio Library's FFT1024 and 256 are set for a 22KHz frequency range with about 43Hz between each freq bin. For my project, I need a resolution down to about 5Hz or so...

With the help of a couple other posts from JBeale and Frank B. I was able to code a generic FFT with variable frequency range.

I am using the Teensy 3.2 @ 72MHz

Here is the code I am using:

Code: 
#include <Audio.h>
#include <Wire.h>
#include <SPI.h>
#include <SD.h>
#include <SerialFlash.h>

AudioInputAnalog         adc1;       
AudioAnalyzeFFT1024      myFFT;
AudioConnection          patchCord1(adc1, myFFT);
AudioSynthWaveform       waveform1;      
AudioOutputAnalog        dac1;           
AudioConnection          patchCord2(waveform1, dac1);


void setDACFreq(int freq) {  // change sampling rate of internal ADC and DAC
const unsigned config = PDB_SC_TRGSEL(15) | PDB_SC_PDBEN | PDB_SC_CONT | PDB_SC_PDBIE | PDB_SC_DMAEN;
    PDB0_SC = 0; //<--add this line
    PDB0_IDLY = 1;
    PDB0_MOD = round((float)F_BUS / freq ) - 1;    
    PDB0_SC = config | PDB_SC_LDOK;
    PDB0_SC = config | PDB_SC_SWTRIG;
    PDB0_CH0C1 = 0x0101;    
}

const uint16_t dacOutputFreq = 500;//Hz - Test Frequency for FFT
const uint16_t dacFreqMultiplier = 22;//If using FFT256 set to 44. This give the correct Output Frequency of the DAC

void setup() {
  AudioMemory(12);
  AudioNoInterrupts();
  setDACFreq(2000);//For a 1KHz Frequency FFT Span
  myFFT.windowFunction(AudioWindowHanning1024);
  AudioInterrupts();
  
  Serial.begin(115200);

  //Using DAC Output to Test FFT input on A2
  waveform1.begin(1.0,(dacOutputFreq * dacFreqMultiplier),WAVEFORM_SINE);
  
  delay(1000);
}

void loop() {
  if (myFFT.available()) {
    for(int i = 0; i < 511; i++)
    {
      Serial.println((myFFT.read(i)*10),1);
    }
    Serial.println();
    delay(200);//Delay if using SerialPlot for Visual Output
  }
}

In order to test the program, I am using the Teensy's onboard DAC set to a specific frequency, but can run at any test frequency by setting it in code.
Basically, the DAC output on the Teensy is connected to the A2 pin.

Since I am changing the frequency of the update rate on the DAC/ADC there has to be a multiplier for the DAC output frequency. See code comment.

This test program works with either the FFT1024 or FFT256.

Here is a screenshot of the FFT output (1024) in SerialPlot which works well with the Serial output of the Teensy.

Teensy FFT Example.png

SerialPlot can be downloaded here:

http://https://bitbucket.org/hyOzd/serialplot

I hope this help with other FFT projects...
 
Last edited:
That's strange about my link not working, but as long as folks know about SerialPlot as it's great for working with the Teensy, especially in FFT mode.
 
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