http://www.musicdsp.org/archive.php?classid=3#38

#define FRAC 29

FixedPoint<int, FRAC> a1 = FixedPoint<int, FRAC>(3.221897E-05f);
FixedPoint<int, FRAC> a2 = FixedPoint<int, FRAC>(6.443795E-05f);
FixedPoint<int, FRAC> a3 = FixedPoint<int, FRAC>(3.221897E-05f);
FixedPoint<int, FRAC> b1 = FixedPoint<int, FRAC>(-1.983881f);
FixedPoint<int, FRAC> b2 = FixedPoint<int, FRAC>(0.9840099f);


FixedPoint<int, 16> f1 = FixedPoint<int, 16>(0.1f);
FixedPoint<int, 16> f2 = FixedPoint<int, 16>(0.9f);

short iirTest(short in, bool Channel)
{
	FixedPoint<int, 16> Out;
	if (Channel)
	  {
	  	static FixedPoint<int, 16> x0 = 0, x1 = 0, x2 = 0;
		static FixedPoint<int, 16> y0 = 0, y1 = 0, y2 = 0;
		
		x2 = x1; x1 = x0; x0 = in;
		y2 = y1; y1 = y0;
		
		Out = y0 = x0*a1 + x1*a2 + x2*a3 - y1*b1 - y2*b2;
	  }
	else 
	  {
	  	static FixedPoint<int, 16> x0 = 0, x1 = 0, x2 = 0;
		static FixedPoint<int, 16> y0 = 0, y1 = 0, y2 = 0;
		
		x2 = x1; x1 = x0; x0 = in;
		y2 = y1; y1 = y0;
		
		Out = y0 = x0*a1 + x1*a2 + x2*a3 - y1*b1 - y2*b2;
	  }
	Out *= f2;
	Out += f1*in;
	return Out.Data>>16;		
}