libavcodec/mpegaudiodecheader.c

   1 /*
   2  * MPEG Audio header decoder
   3  * Copyright (c) 2001, 2002 Fabrice Bellard
   4  *
   5  * This file is part of FFmpeg.
   6  *
   7  * FFmpeg is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * FFmpeg is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with FFmpeg; if not, write to the Free Software
  19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  20  */
  21
  22 /**
  23  * @file libavcodec/mpegaudiodecheader.c
  24  * MPEG Audio header decoder.
  25  */
  26
  27 //#define DEBUG
  28 #include "avcodec.h"
  29 #include "mpegaudio.h"
  30 #include "mpegaudiodata.h"
  31
  32
  33 int ff_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
  34 {
  35     int sample_rate, frame_size, mpeg25, padding;
  36     int sample_rate_index, bitrate_index;
  37     if (header & (1<<20)) {
  38         s->lsf = (header & (1<<19)) ? 0 : 1;
  39         mpeg25 = 0;
  40     } else {
  41         s->lsf = 1;
  42         mpeg25 = 1;
  43     }
  44
  45     s->layer = 4 - ((header >> 17) & 3);
  46     /* extract frequency */
  47     sample_rate_index = (header >> 10) & 3;
  48     sample_rate = ff_mpa_freq_tab[sample_rate_index] >> (s->lsf + mpeg25);
  49     sample_rate_index += 3 * (s->lsf + mpeg25);
  50     s->sample_rate_index = sample_rate_index;
  51     s->error_protection = ((header >> 16) & 1) ^ 1;
  52     s->sample_rate = sample_rate;
  53
  54     bitrate_index = (header >> 12) & 0xf;
  55     padding = (header >> 9) & 1;
  56     //extension = (header >> 8) & 1;
  57     s->mode = (header >> 6) & 3;
  58     s->mode_ext = (header >> 4) & 3;
  59     //copyright = (header >> 3) & 1;
  60     //original = (header >> 2) & 1;
  61     //emphasis = header & 3;
  62
  63     if (s->mode == MPA_MONO)
  64         s->nb_channels = 1;
  65     else
  66         s->nb_channels = 2;
  67
  68     if (bitrate_index != 0) {
  69         frame_size = ff_mpa_bitrate_tab[s->lsf][s->layer - 1][bitrate_index];
  70         s->bit_rate = frame_size * 1000;
  71         switch(s->layer) {
  72         case 1:
  73             frame_size = (frame_size * 12000) / sample_rate;
  74             frame_size = (frame_size + padding) * 4;
  75             break;
  76         case 2:
  77             frame_size = (frame_size * 144000) / sample_rate;
  78             frame_size += padding;
  79             break;
  80         default:
  81         case 3:
  82             frame_size = (frame_size * 144000) / (sample_rate << s->lsf);
  83             frame_size += padding;
  84             break;
  85         }
  86         s->frame_size = frame_size;
  87     } else {
  88         /* if no frame size computed, signal it */
  89         return 1;
  90     }
  91
  92 #if defined(DEBUG)
  93     dprintf(NULL, "layer%d, %d Hz, %d kbits/s, ",
  94            s->layer, s->sample_rate, s->bit_rate);
  95     if (s->nb_channels == 2) {
  96         if (s->layer == 3) {
  97             if (s->mode_ext & MODE_EXT_MS_STEREO)
  98                 dprintf(NULL, "ms-");
  99             if (s->mode_ext & MODE_EXT_I_STEREO)
 100                 dprintf(NULL, "i-");
 101         }
 102         dprintf(NULL, "stereo");
 103     } else {
 104         dprintf(NULL, "mono");
 105     }
 106     dprintf(NULL, "\n");
 107 #endif
 108     return 0;
 109 }