BiQuadIIRによるLPF

BiQuadIIR.cpp

#define _USE_MATH_DEFINES
#include <math.h>
class BiQuadIIR
{
	double b0, b1, b2, a1, a2;
	double x1, x2, y1, y2;

public:
	BiQuadIIR(double b0, double b1, double b2, double a0, double a1, double a2)
		: b0(b0 / a0), b1(b1 / a0), b2(b2 / a0), a1(a1 / a0), a2(a2 / a0), x1(0), x2(0), y1(0), y2(0)
	{
	}

	double proc(double x)
	{
		double y = b0 * x + b1 * x1 + b2 * x2 - a1 * y1 - a2 * y2;
		y2 = y1; x2 = x1; y1 = y; x1 = x;
		return y;
	}

	// LPFを作って返す
	static BiQuadIIR LPF(float sampleFreq, int cutOffFreq, double q = 0.707106781186547524)
	{
		double w0 = 2 * M_PI * cutOffFreq / sampleFreq;
		double a = sin(w0) / (2 * q);
		double b = cos(w0);
		return BiQuadIIR((1 - b) / 2, 1 - b, (1 - b) / 2, 1 + a, -2 * b, 1 - a);
	}
};

main.cpp

#define WIN32_LEAN_AND_MEAN
#include <Windows.h>
#include <stdio.h>
#include <mmsystem.h>
#pragma comment(lib, "winmm.lib")

int main(int argc, char *argv[])
{
	short *pData;
	int count;

	// ヘッダ読み飛ばしてWAVEファイル読む (RIFFで生PCMで余計なタグがなければ、データは44バイト目から)
	{
		FILE *fp = fopen("D:\\test.wav", "rb");
		fseek(fp, 0, SEEK_END);
		count = (ftell(fp) - 44) / 2;
		fseek(fp, 44, SEEK_SET);
		pData = (short*)malloc(count * 2);
		fread(pData, 2, count, fp);
		fclose(fp);
	}

	// 左右それぞれLPFをかける。200Hz以下通す
	BiQuadIIR iirL = BiQuadIIR::LPF(44100, 200);
	BiQuadIIR iirR = BiQuadIIR::LPF(44100, 200);
	for (int i = 0; i < count / 2; i += 2)
	{
		// 要 clipping な気がする
		pData[i] = iirL.proc(pData[i]);
		pData[i + 1] = iirR.proc(pData[i + 1]);
	}

	// 音出す
	WAVEFORMATEX wex = {};
	wex.wFormatTag = WAVE_FORMAT_PCM;
	wex.wBitsPerSample = 16;
	wex.nSamplesPerSec = 44100;
	wex.nChannels = 2;
	wex.nBlockAlign = wex.wBitsPerSample * wex.nChannels / 8;
	wex.nAvgBytesPerSec = wex.nSamplesPerSec * wex.nBlockAlign;
	wex.cbSize = 0;

	HWAVEOUT hwo;
	WAVEHDR hdr = {};
	hdr.lpData = (LPSTR)pData;
	hdr.dwBufferLength = count;

	MMRESULT ret = waveOutOpen(&hwo, WAVE_MAPPER, &wex, NULL, NULL, CALLBACK_NULL);
	ret = waveOutPrepareHeader(hwo, &hdr, sizeof(hdr));
	ret = waveOutWrite(hwo, &hdr, sizeof(hdr));

	getchar();
	waveOutClose(hwo);
	delete[] pData;

	return 0;
}