/* the Music Player Daemon (MPD) * Copyright (C) 2008 Max Kellermann <max@duempel.org> * This project's homepage is: http://www.musicpd.org * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include "pcm_dither.h" static unsigned long prng(unsigned long state) { return (state * 0x0019660dL + 0x3c6ef35fL) & 0xffffffffL; } static int16_t pcm_dither_sample_24_to_16(int32_t sample, struct pcm_dither_24 *dither) { int32_t output, rnd; enum { from_bits = 24, to_bits = 16, scale_bits = from_bits - to_bits, round = 1 << (scale_bits - 1), mask = (1 << scale_bits) - 1, ONE = 1 << (from_bits - 1), MIN = -ONE, MAX = ONE - 1 }; sample += dither->error[0] - dither->error[1] + dither->error[2]; dither->error[2] = dither->error[1]; dither->error[1] = dither->error[0] / 2; /* round */ output = sample + round; rnd = prng(dither->random); output += (rnd & mask) - (dither->random & mask); dither->random = rnd; /* clip */ if (output > MAX) { output = MAX; if (sample > MAX) sample = MAX; } else if (output < MIN) { output = MIN; if (sample < MIN) sample = MIN; } output &= ~mask; dither->error[0] = sample - output; return (int16_t)(output >> scale_bits); } void pcm_dither_24_to_16(struct pcm_dither_24 *dither, int16_t *dest, const int32_t *src, unsigned num_samples) { while (num_samples-- > 0) *dest++ = pcm_dither_sample_24_to_16(*src++, dither); }