FFT Class
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
Description
Constructs a FFT class object.
Syntax
void FFT(void);
Parameters
NA
Returns
NA
Example Code
Example:FFT
(https://github.com/ambiot/ambd_arduino/blob/dev/Arduino_package/hardware/libraries/AudioCodec/examples/FFT/FFT.ino)
Notes and Warnings
“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);
Parameters
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
(https://github.com/ambiot/ambd_arduino/blob/dev/Arduino_package/hardware/libraries/AudioCodec/examples/FFT/FFT.ino)
Notes and Warnings
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
(https://github.com/ambiot/ambd_arduino/blob/dev/Arduino_package/hardware/libraries/AudioCodec/examples/FFT/FFT.ino)
Notes and Warnings
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
(https://github.com/ambiot/ambd_arduino/blob/dev/Arduino_package/hardware/libraries/AudioCodec/examples/FFT/FFT.ino)
Notes and Warnings
“FFT.h” must be included to use the class function.
FFT::getFFTSize
Description
Get the size of FFT output for a given sampleCount.
Syntax
uint16_t getFFTSize(uint16_t sampleCount);
Parameters
sampleCount: number of sample datapoints in the input array. Valid values: 16, 32, 64, 128, 256, 512, 1024, 2048.
Returns
This function returns the FFT output size for the given sampleCount, which is (sampleCount / 2).
Example Code
NA
Notes and Warnings
“FFT.h” must be included to use the class function.
