AudioFFT.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreMinimal.h"
#include "Misc/CoreMiscDefines.h"
#include "DSP/BufferVectorOperations.h"
#include "DSP/Dsp.h"
#include "DSP/FFTAlgorithm.h"
 
namespace Audio
{
    // Types of spectrums which can be directly derived from FFTFreqDomainData
    
    enum class ESpectrumType : uint8
    {
        MagnitudeSpectrum,
        PowerSpectrum
    };
 
 
    enum class EWindowType : uint8
    {
        None, // No window is applied. Technically a boxcar window.
        Hamming, // Mainlobe width of -3 dB and sidelove attenuation of ~-40 dB. Good for COLA.
        Hann, // Mainlobe width of -3 dB and sidelobe attenuation of ~-30dB. Good for COLA.
        Blackman // Mainlobe width of -3 dB and sidelobe attenuation of ~-60db. Tricky for COLA.
    };
 
    // Utility functions for generating different types of windows. Called in FWindow::Generate.
    SIGNALPROCESSING_API void GenerateRectangularWindow(float* WindowBuffer, int32 NumFrames, int32 NumChannels);
    SIGNALPROCESSING_API void GenerateHammingWindow(float* WindowBuffer, int32 NumFrames, int32 NumChannels, bool bIsPeriodic);
    SIGNALPROCESSING_API void GenerateHannWindow(float* WindowBuffer, int32 NumFrames, int32 NumChannels, bool bIsPeriodic);
    SIGNALPROCESSING_API void GenerateBlackmanWindow(float* WindowBuffer, int32 NumFrames, int32 NumChannels, bool bIsPeriodic);
    SIGNALPROCESSING_API void GenerateWindow(float* InOutWindowBuffer, EWindowType InType, int32 InNumFrames, int32 InNumChannels, bool bIsPeriodic);
 
    // Returns the hop size in samples necessary to maintain constant overlap add.
    // For more information on COLA, see the following page:
    // https://ccrma.stanford.edu/~jos/sasp/Overlap_Add_OLA_STFT_Processing.html
    SIGNALPROCESSING_API uint32 GetCOLAHopSizeForWindow(EWindowType InType, uint32 WindowLength);
 
    class FWindow
    {
    public:
        FWindow() = delete;
 
        SIGNALPROCESSING_API FWindow(EWindowType InType, int32 InNumFrames, int32 InNumChannels, bool bIsPeriodic);
 
        // Apply this window to InBuffer, which is expected to be an interleaved buffer with the same amount of frames
        // and channels this window was constructed with.
        SIGNALPROCESSING_API void ApplyToBuffer(float* InBuffer);
 
        SIGNALPROCESSING_API EWindowType GetWindowType() const;
 
    private:
        EWindowType WindowType;
        FAlignedFloatBuffer WindowBuffer;
        int32 NumSamples;
 
        // Generate the window. Called on constructor.
        SIGNALPROCESSING_API void Generate(int32 NumFrames, int32 NumChannels, bool bIsPeriodic);
    };
 
    // Return the ceiling of the log2 of InNum
    SIGNALPROCESSING_API int32 CeilLog2(int32 InNum);
 
    // Return the scaling factor needed to apply to a power spectrum given a current
    // and target FFT scaling. 
    SIGNALPROCESSING_API float GetPowerSpectrumScaling(int32 FFTSize, EFFTScaling InCurrentScaling, EFFTScaling InTargetScaling);
 
    // Scale the power spectrum to remove any scaling introduced by the FFT algorithm
    // implementation.
    SIGNALPROCESSING_API void ScalePowerSpectrumInPlace(int32 FFTSize, EFFTScaling InCurrentScaling, EFFTScaling InTargetScaling, TArrayView<float> InPowerSpectrum);
}