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/* Compander to use various networks like C-Netz / NMT / AMPS
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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 <stdint.h> |
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#include <string.h> |
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#include <math.h> |
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#include "compander.h" |
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/* this is the 0 DB level that stays 0 DB after compression / espansion */ |
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#define ZERO_DB_LEVEL 16384.0 |
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/* what factor shall the gain raise and fall after given attack/recovery time */ |
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#define ATTACK_FACTOR 1.5 |
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#define RECOVERY_FACTOR 0.75 |
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/* Minimum level value to keep state */ |
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#define ENVELOP_MIN 0.001 |
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static double sqrt_tab[10000]; |
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/*
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* Generate companding tables according to NMT specification |
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* |
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* Hopefully this is correct |
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* |
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*/ |
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void init_compander(compander_t *state, int samplerate, double attack_ms, double recovery_ms) |
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{ |
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int i; |
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memset(state, 0, sizeof(*state)); |
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state->envelop_e = 1.0; |
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state->envelop_c = 1.0; |
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/* ITU-T G.162: 1.5 times the steady state after attack_ms */ |
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state->step_up = pow(ATTACK_FACTOR, 1000.0 / attack_ms / (double)samplerate); |
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/* ITU-T G.162: 0.75 times the steady state after recovery_ms */ |
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state->step_down = pow(RECOVERY_FACTOR, 1000.0 / recovery_ms / (double)samplerate); |
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// FIXME: make global, not at instance
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for (i = 0; i < 10000; i++) |
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sqrt_tab[i] = sqrt(i * 0.001); |
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} |
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void compress_audio(compander_t *state, int16_t *samples, int num) |
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{ |
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int32_t sample; |
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double value, envelop, step_up, step_down; |
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int i; |
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step_up = state->step_up; |
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step_down = state->step_down; |
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envelop = state->envelop_c; |
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// printf("envelop=%.4f\n", envelop);
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for (i = 0; i < num; i++) { |
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/* normalize sample value to 0 DB level */ |
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value = (double)(*samples) / ZERO_DB_LEVEL; |
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if (fabs(value) > envelop) |
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envelop *= step_up; |
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else |
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envelop *= step_down; |
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if (envelop < ENVELOP_MIN) |
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envelop = ENVELOP_MIN; |
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value = value / sqrt_tab[(int)(envelop / 0.001)]; |
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/* convert back from 0 DB level to sample value */ |
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sample = (int)(value * ZERO_DB_LEVEL); |
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if (sample > 32767) |
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sample = 32767; |
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else if (sample < -32768) |
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sample = -32768; |
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*samples++ = sample; |
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} |
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state->envelop_c = envelop; |
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} |
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void expand_audio(compander_t *state, int16_t *samples, int num) |
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{ |
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int32_t sample; |
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double value, envelop, step_up, step_down; |
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int i; |
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step_up = state->step_up; |
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step_down = state->step_down; |
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envelop = state->envelop_e; |
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for (i = 0; i < num; i++) { |
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/* normalize sample value to 0 DB level */ |
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value = (double)(*samples) / ZERO_DB_LEVEL; |
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if (fabs(value) > envelop) |
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envelop *= step_up; |
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else |
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envelop *= step_down; |
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if (envelop < ENVELOP_MIN) |
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envelop = ENVELOP_MIN; |
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value = value * envelop; |
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/* convert back from 0 DB level to sample value */ |
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sample = (int)(value * ZERO_DB_LEVEL); |
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if (sample > 32767) |
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sample = 32767; |
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else if (sample < -32768) |
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sample = -32768; |
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*samples++ = sample; |
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} |
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state->envelop_e = envelop; |
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} |
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typedef struct compander { |
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double step_up; |
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double step_down; |
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double envelop_c; |
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double envelop_e; |
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} compander_t; |
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void init_compander(compander_t *state, int samplerate, double attack_ms, double recovery_ms); |
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void compress_audio(compander_t *state, int16_t *samples, int num); |
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void expand_audio(compander_t *state, int16_t *samples, int num); |
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