I am testing out the Example 5 from the ArduinoFFT library by kosme, shown below.

```
/*
Example of use of the FFT libray
Copyright (C) 2014 Enrique Condes
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
/*
In this example, the Arduino simulates the sampling of a sinusoidal 1000 Hz
signal with an amplitude of 100, sampled at 5000 Hz. Samples are stored
inside the vReal array. The samples are windowed according to Hamming
function. The FFT is computed using the windowed samples. Then the magnitudes
of each of the frequencies that compose the signal are calculated. Finally,
the frequency with the highest peak is obtained, being that the main frequency
present in the signal. This frequency is printed, along with the magnitude of
the peak.
*/
#include "arduinoFFT.h"
arduinoFFT FFT = arduinoFFT(); /* Create FFT object */
/*
These values can be changed in order to evaluate the functions
*/
const uint16_t samples = 64; //This value MUST ALWAYS be a power of 2
const double signalFrequency = 1000;
const double samplingFrequency = 5000;
const uint8_t amplitude = 100;
/*
These are the input and output vectors
Input vectors receive computed results from FFT
*/
double vReal[samples];
double vImag[samples];
#define SCL_INDEX 0x00
#define SCL_TIME 0x01
#define SCL_FREQUENCY 0x02
#define SCL_PLOT 0x03
void setup()
{
Serial.begin(115200);
Serial.println("Ready");
}
void loop()
{
/* Build raw data */
double cycles = (((samples-1) * signalFrequency) / samplingFrequency); //Number of signal cycles that the sampling will read
for (uint16_t i = 0; i < samples; i++)
{
vReal[i] = int8_t((amplitude * (sin((i * (twoPi * cycles)) / samples))) / 2.0);/* Build data with positive and negative values*/
//vReal[i] = uint8_t((amplitude * (sin((i * (twoPi * cycles)) / samples) + 1.0)) / 2.0);/* Build data displaced on the Y axis to include only positive values*/
vImag[i] = 0.0; //Imaginary part must be zeroed in case of looping to avoid wrong calculations and overflows
}
/* Print the results of the simulated sampling according to time */
Serial.println("Data:");
PrintVector(vReal, samples, SCL_TIME);
FFT.Windowing(vReal, samples, FFT_WIN_TYP_HAMMING, FFT_FORWARD); /* Weigh data */
Serial.println("Weighed data:");
PrintVector(vReal, samples, SCL_TIME);
FFT.Compute(vReal, vImag, samples, FFT_FORWARD); /* Compute FFT */
Serial.println("Computed Real values:");
PrintVector(vReal, samples, SCL_INDEX);
Serial.println("Computed Imaginary values:");
PrintVector(vImag, samples, SCL_INDEX);
FFT.ComplexToMagnitude(vReal, vImag, samples); /* Compute magnitudes */
Serial.println("Computed magnitudes:");
PrintVector(vReal, (samples >> 1), SCL_FREQUENCY);
double x;
double v;
FFT.MajorPeak(vReal, samples, samplingFrequency, &x, &v);
Serial.print(x, 6);
Serial.print(", ");
Serial.println(v, 6);
while(1); /* Run Once */
// delay(2000); /* Repeat after delay */
}
void PrintVector(double *vData, uint16_t bufferSize, uint8_t scaleType)
{
for (uint16_t i = 0; i < bufferSize; i++)
{
double abscissa;
/* Print abscissa value */
switch (scaleType)
{
case SCL_INDEX:
abscissa = (i * 1.0);
break;
case SCL_TIME:
abscissa = ((i * 1.0) / samplingFrequency);
break;
case SCL_FREQUENCY:
abscissa = ((i * 1.0 * samplingFrequency) / samples);
break;
}
Serial.print(abscissa, 6);
if(scaleType==SCL_FREQUENCY)
Serial.print("Hz");
Serial.print(" ");
Serial.println(vData[i], 4);
}
Serial.println();
}
```

I connected the Arduino Uno board to my computer and ran the exact example from the library, which yielded the attached graph.

The beginning looks pretty normal, but then everything seems to go wrong.

- I’m assuming the blue line is the FFT, but what does the red line represent?
- Why is there an extra red peak at the beginning of the graph?
- Shouldn’t there be only one peak at 1000 on the x-axis if it is simulating a sinusoidal signal?
- Why is the amplitude so high, if I set it to 100?