ConstantQ.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "DSP/AlignedBuffer.h"
#include "DSP/FloatArrayMath.h"
 
namespace Audio
{
    enum class EPseudoConstantQNormalization : uint8
    {
        EqualAmplitude,     //< No energy scaling. All bands weighted so max is 1.
        EqualEuclideanNorm, //< Scale energy by euclidean norm. Good when using magnitude spectrum.
        EqualEnergy         //< Scale energy by total energy. Good when using power spectrum.
    };
 
    struct FPseudoConstantQKernelSettings
    {
        int32 NumBands;                                 //< Total number of resulting constant Q bands.
        float NumBandsPerOctave;                            //< Number of bands to space within an octave.
        float KernelLowestCenterFreq;                   //< The starting frequency of the first band.
        float BandWidthStretch;                         //< Stretching factor to control overlap of adjacent bands.
        EPseudoConstantQNormalization Normalization;    //< Normalization scheme for bands.
 
        FPseudoConstantQKernelSettings()
        :   NumBands(96)
        ,   NumBandsPerOctave(12)
        ,   KernelLowestCenterFreq(40.f)
        ,   BandWidthStretch(1.0) 
        ,   Normalization(EPseudoConstantQNormalization::EqualEnergy)
        {}
    };
 
    SIGNALPROCESSING_API TUniquePtr<FContiguousSparse2DKernelTransform> NewPseudoConstantQKernelTransform(const FPseudoConstantQKernelSettings& InSettings, const int32 InFFTSize, const float InSampleRate);
 
    struct FPseudoConstantQBandSettings
    {
        float CenterFreq;
 
        float BandWidth; 
 
        int32 FFTSize; 
 
        float SampleRate; 
 
        EPseudoConstantQNormalization Normalization;
    };
 
    struct FPseudoConstantQ
    {
        // Returns the center frequency of a given CQT band.
        static SIGNALPROCESSING_API float GetConstantQCenterFrequency(const int32 InBandIndex, const float InBaseFrequency, const float InBandsPerOctave);
 
        // Returns the bandwidth for a given CQT band.
        static SIGNALPROCESSING_API float GetConstantQBandWidth(const float InBandCenter, const float InBandsPerOctave, const float InBandWidthStretch);
 
        // Fills an array with the spectrum weights to apply to an FFT magnitude or power spectrum output.
        static SIGNALPROCESSING_API void FillArrayWithConstantQBand(const FPseudoConstantQBandSettings& InSettings, FAlignedFloatBuffer& OutOffsetArray, int32& OutOffsetIndex);
    };
}