158 lines
4.3 KiB
C
158 lines
4.3 KiB
C
/* Sample rate conversion
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*
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* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include <errno.h>
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#include <string.h>
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#include <stdlib.h>
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#include "../common/sample.h"
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#include "samplerate.h"
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int init_samplerate(samplerate_t *state, double low_samplerate, double high_samplerate, double filter_cutoff)
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{
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memset(state, 0, sizeof(*state));
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state->factor = high_samplerate / low_samplerate;
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if (state->factor < 1.0) {
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fprintf(stderr, "Software error: Low sample rate must be lower than high sample rate, aborting!\n");
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abort();
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}
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iir_lowpass_init(&state->up.lp, filter_cutoff, high_samplerate, 2);
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iir_lowpass_init(&state->down.lp, filter_cutoff, high_samplerate, 2);
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return 0;
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}
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/* convert high sample rate to low sample rate */
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int samplerate_downsample(samplerate_t *state, sample_t *samples, int input_num)
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{
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int output_num = 0, i, idx;
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double factor = state->factor, in_index, diff;
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sample_t output[(int)((double)input_num / factor + 0.5) + 10]; /* add some safety */
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sample_t last_sample;
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/* filter down */
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iir_process(&state->down.lp, samples, input_num);
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/* get last sample for interpolation */
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last_sample = state->down.last_sample;
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/* resample filtered result */
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in_index = state->down.in_index;
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for (i = 0; ; i++) {
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/* convert index to int */
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idx = (int)in_index;
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/* if index is outside input sample range, we are done */
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if (idx >= input_num)
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break;
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/* linear interpolation */
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diff = in_index - (double)idx;
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if (idx)
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output[i] = samples[idx - 1] * (1.0 - diff) + samples[idx] * diff;
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else
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output[i] = last_sample * (1.0 - diff) + samples[idx] * diff;
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/* count output number */
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output_num++;
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/* increment input index */
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in_index += factor;
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}
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/* store last sample for interpolation */
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if (input_num)
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state->down.last_sample = samples[input_num - 1];
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/* remove number of input samples from index */
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in_index -= (double)input_num;
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/* in_index cannot be negative, excpet due to rounding error, so... */
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if ((int)in_index < 0)
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in_index = 0.0;
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state->down.in_index = in_index;
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/* copy samples */
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for (i = 0; i < output_num; i++)
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*samples++ = output[i];
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return output_num;
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}
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/* convert low sample rate to high sample rate */
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int samplerate_upsample(samplerate_t *state, sample_t *input, int input_num, sample_t *output)
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{
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int output_num = 0, i, idx;
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double factor = 1.0 / state->factor, in_index, diff;
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sample_t buff[(int)((double)input_num / factor + 0.5) + 10]; /* add some safety */
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sample_t *samples, last_sample;
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/* get last sample for interpolation */
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last_sample = state->up.last_sample;
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if (input == output)
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samples = buff;
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else
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samples = output;
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/* resample input */
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in_index = state->up.in_index;
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for (i = 0; ; i++) {
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/* convert index to int */
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idx = (int)in_index;
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/* if index is outside input sample range, we are done */
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if (idx >= input_num)
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break;
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/* linear interpolation */
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diff = in_index - (double)idx;
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if (idx)
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samples[i] = input[idx - 1] * (1.0 - diff) + input[idx] * diff;
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else
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samples[i] = last_sample * (1.0 - diff) + input[idx] * diff;
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/* count output number */
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output_num++;
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/* increment input index */
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in_index += factor;
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}
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/* store last sample for interpolation */
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if (input_num)
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state->up.last_sample = input[input_num - 1];
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/* remove number of input samples from index */
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in_index -= (double)input_num;
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/* in_index cannot be negative, excpet due to rounding error, so... */
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if ((int)in_index < 0)
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in_index = 0.0;
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state->up.in_index = in_index;
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/* filter up */
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iir_process(&state->up.lp, samples, output_num);
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if (input == output) {
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/* copy samples */
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for (i = 0; i < output_num; i++)
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*output++ = samples[i];
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}
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return output_num;
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}
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