compressor.c

/*
 * compressor.h - Fast and lightweight soft limiter
 * This algorithm produces no distortion below a threshold, and increasingly reduces gain above the threshold
 *
 * Concept for algoritm from https://www.kvraudio.com/forum/viewtopic.php?t=195315
 * Modified and optimized for int32_t by Dan Green
 *
 * Author: Dan Green (danngreen1@gmail.com)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 * See http://creativecommons.org/licenses/MIT/ for more information.
 *
 * -----------------------------------------------------------------------------
 */
/*
 * To use, the compressor should first be initialized, which pre-calculates some values
 * to save processor cycles.
 *
 * run compress() on each sample to apply the compression.
 *
 *
 *
 *
 */

#include "globals.h"
#include "compressor.h"

float MAX_SAMPLEVAL;
float THRESHOLD_COMPILED;
int32_t THRESHOLD_VALUE;

//
//  max_sample_val is usually going to be 1<<15, or 1<<23, or 1<<31
//  threshold_percent is the threshold at which limiting should begin (0.75 is a good place to start)
//
void init_compressor(uint32_t max_sample_val, float threshold_percent)
{
	MAX_SAMPLEVAL=max_sample_val;

	float m = (float)max_sample_val;
	THRESHOLD_COMPILED = m * m * threshold_percent * (1.0 - threshold_percent);

	THRESHOLD_VALUE = threshold_percent*max_sample_val;
}

int32_t compress(int32_t val)
{
	float tv = THRESHOLD_COMPILED / ((float)val);
	if (val > THRESHOLD_VALUE) return (MAX_SAMPLEVAL - tv);
	else if (val < -THRESHOLD_VALUE) return (-MAX_SAMPLEVAL - tv);
	else return val;

}