diff options
Diffstat (limited to 'src/fft.hpp')
| -rw-r--r-- | src/fft.hpp | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/src/fft.hpp b/src/fft.hpp index 9008c05..1cf3e7e 100644 --- a/src/fft.hpp +++ b/src/fft.hpp @@ -12,6 +12,9 @@ #include <fftw3.h> +/** + * Computes the fast Fourier transform of an input window. + */ class FFT { private: /* FFT window width */ @@ -28,21 +31,95 @@ private: std::unique_ptr<WindowFunction> window_function_; public: + /* plan and buffers are not copiable */ FFT() = delete; FFT(const FFT &c) = delete; FFT(FFT &&) = delete; FFT & operator=(const FFT&) = delete; + /** + * @param win_width Width of window this object must support. + * NOTE: Only supports this window size! + */ explicit FFT(std::size_t win_width); + + /** + * @param win_width Width of window this object must support. + * @param win_func Window function to apply before FFT computations. + * NOTE: Only supports this window size! + */ FFT(std::size_t win_width, std::unique_ptr<WindowFunction>& win_func); + virtual ~FFT(); + /** + * Compute the fast Fourier transform. + * @param input Array of complex input values. + * @return Complex values corresponding to the FFT terms, equal in size to input. + * + * NOTE: This functions normalizes the output by 1/N. + * NOTE: This function provides the negative frequencies in the first half + * and the positive frequencies in the second half. + * NOTE: For even-sized inputs, the Nyquist frequency term is the last in + * the output vector. + */ ComplexWindow Compute(const ComplexWindow& input); + /** + * Retrieve the (real) magnitudes of a complex input vector (window). + * @param input Array of complex values. + * @param alias If true, will alias negative and positive frequencies. + * @return Array of real values, containing amplitudes. + * + * NOTE: When aliasing, each output value will be the sum of that term's + * magnitude as well as the corresponding negative or positive + * frequency term's magnitude. + */ static RealWindow GetMagnitude(const ComplexWindow& input, bool alias); + + /** + * Compute the frequency bounds of a specific FFT window. + * @param rate Sampling rate of the input signal. + * @param width Width of the FFT window. + * @return Lower and upper bounds, in Hz. + */ static std::tuple<double, double> GetFrequencyLimits(double rate, std::size_t width); + + /** + * Compute the index in the FFT window of a given frequency. + * @param rate Sampling rate of input. + * @param width Width of the FFT window. + * @param f Frequency, in Hz. + * @return Index (computed exactly) of the frequency. + */ static double GetFrequencyIndex(double rate, std::size_t width, double f); + + /** + * Resample a real, scaled output of the FFT. + * @param input Real window, values between [0..1]. + * @param rate Sampling rate of input. + * @param width Desired output width. + * @param fmin Frequency lower bound (from input). + * @param fmax Frequency upper bound (from input). + * @return Resampled values, clamped to [0..1]. + * + * NOTE: Uses Lanczos resampling algorithm. + * NOTE: Will resize the [fmin..fmax] band from input (computed as if + * input is a FFT output) to a width-sized output window. + */ static RealWindow Resample(const RealWindow& input, double rate, std::size_t width, double fmin, double fmax); + + /** + * Crop a real, scaled output of the FFT. + * @param input Real window, values between [0..1]. + * @param rate Sampling rate of input. + * @param fmin fmin Frequency lower bound (from input). + * @param fmax fmax Frequency upper bound (from input). + * @return Cropped window. + * + * NOTE: The [fmin..fmax] band from input is computed as if input is a FFT + * output, which under normal circumstances it should be. + */ static RealWindow Crop(const RealWindow& input, double rate, double fmin, double fmax); }; |