Class FFT

FFT Class

Description

A class used for performing FFT calculations with real-number inputs and outputs.

Syntax

class FFT

Members

Public Constructors
FFT::FFT	Constructs a FFT class object.
Public Methods
FFT::setWindow	Set window function used for FFT calculations
FFT::calculate	Calculate FFT for an array of input values.
FFT::getFrequencyBins	Get the FFT output frequency bins.
FFT::getFFTSize	Get the size of FFT output for a given SampleCount.

FFT::FFT

Syntax

void FFT(Void);

Example Code

Example: FFT

Note

“FFT.h” must be included to use the class function.

FFT::setWindow

Description

Set window function used for FFT calculations.

Syntax

void setWindow(FFTWindow_t window, uint16_t sampleCount);

window: The window function to be used in FFT calculations. Valid values: - None (Default rectangular window) - Hann - Hamming sampleCount: Number of sample datapoints in the input array. Valid values: 16, 32, 64, 128, 256, 512, 1024, 2048.

Returns

NA

Example Code

Example: FFT

Note

The window function is used to reduce the effects of discontinuities that occur when the input signal has frequencies that do not fit an integer number of periods in the sample datapoints. More information on FFTs and window functions can be seen at: https://download.ni.com/evaluation/pxi/Understanding%20FFTs%20and%20Windowing.pdf https://en.wikipedia.org/wiki/Window_function “FFT.h” must be included to use the class function.

FFT::Calculate

Description

Calculate FFT for an array of input values.

Syntax

void calculate(float* inputBuf, float* outputBuf, uint16_t sampleCount);
void calculate(int16_t* inputBuf, float* outputBuf, uint16_t sampleCount);

Parameters

inputBuf: pointer to an array of sampleCount size, containing input sample datapoints, in float or uint16_t format. outputBuf: pointer to a float array of (sampleCount / 2) size, for containing calculated FFT result. sampleCount: number of sample datapoints in the input array. Valid values: 16, 32, 64, 128, 256, 512, 1024, 2048.

Returns

NA

Example Code

Example: FFT

Note

Larger sample counts will require more time for FFT calculations, but will provide a higher frequency resolution output. “FFT.h” must be included to use the class function.

FFT::getFrequencyBins

Description

Get a corresponding output FFT frequency bin for a given sample rate and sample count.

Syntax

void getFrequencyBins(uint16_t* outputBuf, uint16_t sampleCount, uint32_t sampleRate);

Parameters

outputBuf: pointer to a uint16_t array of (sampleCount / 2) size, for containing the calculated center frequency of each FFT output element. sampleCount: number of sample datapoints in the input array. Valid values: 16, 32, 64, 128, 256, 512, 1024, 2048. sampleRate: Defines how many times an audio signal is sampled in a second. In the example “FFT.ino”, sample rate is set at 16000, which means that each audio signal contains 16000 individual samples.

Returns

NA

Example Code

Example: FFT

Note

“FFT.h” must be included to use the class function.

uint16_t getFFTSize(uint16_t sampleCount);

“FFT.h” must be included to use the class function.