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/* Copyright 2017 The Chromium OS Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include <stdlib.h>
#include "dcblock.h"
#define RAMP_TIME_MS 20
struct dcblock {
float R;
float x_prev;
float y_prev;
float ramp_factor;
float ramp_increment;
int initialized;
};
struct dcblock *dcblock_new(float R, unsigned long sample_rate)
{
struct dcblock *dcblock = (struct dcblock *)calloc(1, sizeof(*dcblock));
dcblock->R = R;
dcblock->ramp_increment = 1000. / (float)(RAMP_TIME_MS * sample_rate);
return dcblock;
}
void dcblock_free(struct dcblock *dcblock)
{
free(dcblock);
}
/* This is the prototype of the processing loop. */
void dcblock_process(struct dcblock *dcblock, float *data, int count)
{
int n;
float x_prev = dcblock->x_prev;
float y_prev = dcblock->y_prev;
float R = dcblock->R;
if (!dcblock->initialized) {
x_prev = data[0];
dcblock->initialized = 1;
}
for (n = 0; n < count; n++) {
float x = data[n];
float d = x - x_prev + R * y_prev;
y_prev = d;
x_prev = x;
/*
* It takes a while for this DC-block filter to completely
* filter out a large DC-offset, so apply a mix-in ramp to
* avoid any residual jump discontinuities that can lead to
* "pop" during capture.
*/
if (dcblock->ramp_factor < 1.0) {
d *= dcblock->ramp_factor;
dcblock->ramp_factor += dcblock->ramp_increment;
}
data[n] = d;
}
dcblock->x_prev = x_prev;
dcblock->y_prev = y_prev;
}
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