Teensy 3.1 FFT Adafruit AGC Electret Microphone - first two bins not zeroing out

ksumrall · Jul 4, 2016

Hello,

I'm working on a light to music project and stuck getting a reliable spectrum.

I'm using a Teensy 3.1 and an Adafruit AGC Electrit Microphone (https://www.adafruit.com/product/1713).
I'm running the teensy through USB for now so that I can use the serial monitor ro view results.
My code is as follows:

Code:

// FFT Test
//
// Compute a 1024 point Fast Fourier Transform (spectrum analysis)
// on audio connected to an adafruit AGC Electret Microphone MAX9814
// https://www.adafruit.com/product/1713
//
// The 512 bins are calculated into 32 bins.
//

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

#define debug

const unsigned int matrix_width = 32;
const unsigned int matrix_height = 256;

// These parameters adjust the vertical thresholds
const float maxLevel = 0.4;      // 1.0 = max, lower is more "sensitive"
const float dynamicRange = 40.0; // total range to display, in decibels
const float linearBlend = 0.3;   // useful range is 0 to 0.7

// Create the Audio components.  These should be created in the
// order data flows, inputs/sources -> processing -> outputs
AudioInputAnalog adc1(A2); 
AudioAnalyzeFFT1024 myFFT;
AudioConnection patchCord1(adc1, myFFT);

// This array specifies how many of the FFT frequency bins to use for each horizontal pixel.
// Because humans hear in octaves, and FFT bins are linear, the low frequencies
// use a small number of bins, higher frequencies use more.
const int lowerFFTBins[] = {0,1,2,3,4,5,6,8,10,12,15,18,22,27,32,38,45,53,63,74,87,102,119,138,160,186,216,250,289,334,385,444};
const int upperFFTBins[] = {0,1,2,3,4,5,7,9,11,14,17,21,26,31,37,44,52,62,73,86,101,118,137,159,185,215,249,288,333,384,443,511};
float thresholdVertical[matrix_height];
float thresholdVert[matrix_height];

void setup() {
  analogReadAveraging(4);
  AudioMemory(12);

  // compute the vertical thresholds before starting
  computeVerticalLevels();

  for(unsigned int j = 0; j < matrix_height; j++) {
    thresholdVert[j] = thresholdVertical[matrix_height-j-1];
  }

  myFFT.windowFunction(AudioWindowHanning1024);
}

void loop() {
  float level;
  int maxY;

  if (myFFT.available()) {
#ifdef debug    
    Serial.print("FFT: ");
#else
    Serial.write(255);
#endif
    float bar[60];
    int index = 2;
    for (int x = 0; x < matrix_width; x++) {
      level = myFFT.read(lowerFFTBins[x], upperFFTBins[x]);
      maxY = 0;
      for(int y = 0; y < matrix_height; y++){
        if(level >= thresholdVert[y]){
          maxY = y;
        }
      }
#ifdef debug
      Serial.print(maxY);
      Serial.print(" ");
#else
      Serial.write(maxY);
#endif
    }
#ifdef debug
    Serial.println();
#endif

    delay(20);
  }
}

// Run once from setup, the compute the vertical levels
void computeVerticalLevels() {
  unsigned int y;
  float n, logLevel, linearLevel;

  for (y=0; y < matrix_height; y++) {
    n = (float)y / (float)(matrix_height - 1);
    logLevel = pow10f(n * -1.0 * (dynamicRange / 20.0));
    linearLevel = 1.0 - n;
    linearLevel = linearLevel * linearBlend;
    logLevel = logLevel * (1.0 - linearBlend);
    thresholdVertical[y] = (logLevel + linearLevel) * maxLevel;
  }
}

The results look good for all but the first two bands. The first two bands seem to latch on to the maximum value experienced...

Code:

FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
FFT: 24 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Those first two bands keep latching onto the max value experienced. What's interesting is that I do see them show lower values as there is sound being calculated, but when it goes quiet, they appear in their 'latched' state.
I remember seeing some example code of Paul's where he skipped the first two bins. That makes me wonder if I should do the same and move on with just 30 bins.

*EDIT*
The scale of the values is 0 - 255
I have seen through the course of one song a the first two bins reading 248 and 196 respectively during silence. While the song is playing, the first bins seem to show properly fluctuating levels.

defragster · Jul 4, 2016

I was seeing something similar intermittently on my Sparkfun mic breakout and thought the mic was giving an odd value - Then I got distracted doing other stuff first and never got back to it.

One ran fine for days on my downstairs desktop I could see when I breathed or clicked my mouse - then it started acting oddly . . .

DerekR · Jul 5, 2016

By any chance is there a small dc offset on your mic input that accounts for bins 0 & 1? Bin 0 is the dc level, and that will be "smeared" into 1 by the Hanning window function.

ksumrall · Jul 5, 2016

There is a 1.25 dc offset with a 1Vpp swing. That will leave at least a 0.25V value. I'm sure that has some influence on the values, but I don't see how that would cause the values in those two bins only to continually rise and latch during silence.

B.Jacquot · Jul 5, 2016

ksumrall said:
...continually rise ... during silence.

(Ellepsis mine)

That sounds like it could be a side effect of the AGC.

Does the time it takes change if you pull the A/D pin to ground or VCC?

ksumrall · Jul 9, 2016

B.Jacquot -
I have just created a video to demonstrate my observations - https://youtu.be/TQP7IqIM3zI.
I tried a microphone without AGC which produced better results. Here's a link to the mic on ebay.
I think it does have something to do with AGC, but not sure what. Electronics is hard.
Until I have a better understanding of what is happening, I'll either use the mic without AGC or use the mic with AGC and ignore the first two bins.
Any thoughts?

Teensy 3.1 FFT Adafruit AGC Electret Microphone - first two bins not zeroing out

ksumrall

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defragster

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ksumrall

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