FeedbackDelayNetwork.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreMinimal.h"
#include "DSP/BufferVectorOperations.h"
#include "DSP/IntegerDelay.h"
#include "DSP/LongDelayAPF.h"
#include "DSP/BufferOnePoleLPF.h"
 
namespace Audio
{
    // Delay settings of 4 channel feedback delay network
    struct FFDNDelaySettings
    {
        // Number of samples for delay line 0
        int32 APF0DelayNumSamples;
        // Number of samples for delay line 1
        int32 APF1DelayNumSamples;
        // Number of samples for delay line 2
        int32 APF2DelayNumSamples;
        // Number of samples for delay line 3
        int32 APF3DelayNumSamples;
        
        // Default delay line settings for left channel.
        static SIGNALPROCESSING_API FFDNDelaySettings DefaultLeftDelays(float InSampleRate);
        // Default delay line settings for right channel. 
        static SIGNALPROCESSING_API FFDNDelaySettings DefaultRightDelays(float InSampleRate);
    };
 
    // Filter coefficients of 4 channel feedback delay network.
    struct FFDNCoefficients
    {
        // Sample multiplier of input samples before they enter the delay line.
        float InputScale;
        // All pass filter coefficients. One coefficient for each filter.
        float APFG[4];
        // Low pass filter feed back coefficients. One coefficient for each filter.
        float LPFA[4];
        // Low pass filter feed forward coefficients. One coefficient for each filter.
        float LPFB[4];
        // Feedback multiplier of mixing matrix output to network input.
        float Feedback;
    };
 
    // 4 channel feedback delay network (FDN) for artificial reverberation.
    class FFeedbackDelayNetwork
    {
    public:
        // InMaxNumInternalBufferSamples controls the internal buffer size used for vector operations. 
        // InSettings controls the delay line lengths.
        SIGNALPROCESSING_API FFeedbackDelayNetwork(int32 InMaxNumInternalBufferSamples, const FFDNDelaySettings& InSettings);
 
        SIGNALPROCESSING_API ~FFeedbackDelayNetwork();
 
        // Sets the coefficient values of the all pass filters, low pass filters, input scalers and feedback scalers.
        SIGNALPROCESSING_API void SetCoefficients(const FFDNCoefficients& InCoefficients);
 
        // Generates artificial reverberation for InSamples and places results in OutSamples.
        SIGNALPROCESSING_API void ProcessAudio(const FAlignedFloatBuffer& InSamples, FAlignedFloatBuffer& OutSamples);
 
        // Sets internal audio samples to silence. 
        SIGNALPROCESSING_API void FlushAudio();
 
        // Returns the actual number of samples in an internal buffer.
        SIGNALPROCESSING_API int32 GetNumInternalBufferSamples() const;
 
 
    private:
 
        // Process single internal buffer of audio.
        void ProcessAudioBuffer(const float* InSampleData, const int32 InNum, float* OutSampleData);
 
        FFDNDelaySettings Settings;
 
        // Internal buffer size.
        int32 NumInternalBufferSamples;
 
        // Coefficients structure for APF, LPF, input scaling and feedback.
        FFDNCoefficients Coefficients;
 
        // Previous output of low pass filters.
        float LPFZ[4];
 
        // Previous output of mixing matrix.
        float FMO[4];
 
        TUniquePtr<FAlignedBlockBuffer> DelayLine0;
        TUniquePtr<FAlignedBlockBuffer> DelayLine1;
        TUniquePtr<FAlignedBlockBuffer> DelayLine2;
        TUniquePtr<FAlignedBlockBuffer> DelayLine3;
 
        FAlignedFloatBuffer WorkBuffer0;
        FAlignedFloatBuffer WorkBuffer1;
        FAlignedFloatBuffer WorkBuffer2;
        FAlignedFloatBuffer WorkBuffer3;
    };
}