Audio Soft Limiter

limit.cpp

#include <math.h>
#include <float.h>

static inline float f_max(float v1,float v2)
{	
	return v1>v2 ? v1 : v2;
}

static const float limiter_max = (float)0.9999;

enum
{
	bufPow=8,
	bufLength=1<<bufPow,
	bufMax =	bufLength - 1,
};

class limiter
{
	int bufPos;

	float backbuffer[bufLength];

	float buffer[bufLength*2];
	
	float gain,minGainLP;

	bool active;

public:
	limiter()
	{
		bufPos = 0;
		gain		= 1.0f,
		minGainLP	= 1.0f;
		active = false;
	}

	float process_sample(float in)
	{
		float max,out;

		float val = (float)fabs(in);
		
		if (val > limiter_max)
		{
			if (!active) memset(buffer,0,sizeof(buffer));
			active = true;
		}

		{
			
			if (active)
			{
				int offset = bufPos, level = bufPow;
				do
				{
					float * ptr = buffer + bufLength*2 - (2<<level);
					ptr[offset]=val;
					val = f_max(val,ptr[offset^1]);
					offset>>=1;
					level--;
				} while(level>0);
				max = val;
				if (max<=limiter_max) active = false;
			}
			

			backbuffer[bufPos] = in;
			bufPos = (bufPos+1)&bufMax;
			out = backbuffer[bufPos];
		}

		// attenute when getting close to the point of clipping
		float minGain;

		if(active)
			minGain = limiter_max/max;
		else
			minGain = limiter_max;
			
		
		// Low-pass filter of the control signal
		minGainLP = 0.9f*minGainLP + 0.1f * minGain;

		// choose the lesser of these two control signals
		gain = 0.001f + 0.999f*gain;
		if (minGainLP<gain) gain = minGainLP;
		
		if (fabs(out*gain)>limiter_max) gain = (float)(limiter_max/fabs(out));
		
		return out * gain;
	}
};