UnrealEngineUWP/Engine/Source/Runtime/SignalProcessing/Private/BufferOnePoleLPF.cpp

// Copyright 1998-2019 Epic Games, Inc. All Rights Reserved.

#include "DSP/BufferOnePoleLPF.h"
#include "DSP/BufferVectorOperations.h"

using namespace Audio;

// Constructor 
FBufferOnePoleLPF::FBufferOnePoleLPF(float InG)
	: CutoffFrequency(0.0f)
	, B1(0.0f)
	, A0(1.0f)
	, Z1(0.0f)
{
	SetG(InG);
}

// Set the LPF gain coefficient
void FBufferOnePoleLPF::SetG(float InG)
{ 
	B1 = InG;
	A0 = 1.0f - B1;
}

// Resets the sample delay to 0
void FBufferOnePoleLPF::Reset()
{
	SetG(0.0f);
	Z1 = 0.0f;
}

/** Sets the filter frequency using normalized frequency (between 0.0 and 1.0f or 0.0 hz and Nyquist Frequency in Hz) */
void FBufferOnePoleLPF::SetFrequency(const float InFrequency)
{
	if (!FMath::IsNearlyEqual(InFrequency, CutoffFrequency))
	{
		CutoffFrequency = InFrequency;
		SetG(FMath::Exp(-PI * CutoffFrequency));
	}
}

void FBufferOnePoleLPF::ProcessAudio(const Audio::AlignedFloatBuffer& InSamples, Audio::AlignedFloatBuffer& OutSamples)
{
	int32 Index;
	const int32 InNum = InSamples.Num();
	const float* InSampleData = InSamples.GetData();
	
	// Prepare output samples
	OutSamples.Reset(InNum);
	OutSamples.AddUninitialized(InNum);
	float* OutSampleData = OutSamples.GetData();

	// Filter audio
	int32 DelayIndex;
	OutSampleData[0] = InSampleData[0] * A0 + B1 * Z1;
	for(Index = 1, DelayIndex = 0; Index < InNum; Index++, DelayIndex++)
	{
		//OutSampleData[Index] = UnderflowClamp(InSampleData[Index] * A0 + B1 * OutSampleData[DelayIndex]);
		OutSampleData[Index] = InSampleData[Index] * A0 + B1 * OutSampleData[DelayIndex];
	}
	BufferUnderflowClampFast(OutSampleData, InNum);
	// Store delay value
	Z1 = OutSampleData[InNum - 1];
}