• Main Page
  • Related Pages
  • Modules
  • Data Structures
  • Files
  • File List
  • Globals

libavcodec/mpegaudiodecheader.c

Go to the documentation of this file.
00001 /*
00002  * MPEG Audio header decoder
00003  * Copyright (c) 2001, 2002 Fabrice Bellard
00004  *
00005  * This file is part of FFmpeg.
00006  *
00007  * FFmpeg is free software; you can redistribute it and/or
00008  * modify it under the terms of the GNU Lesser General Public
00009  * License as published by the Free Software Foundation; either
00010  * version 2.1 of the License, or (at your option) any later version.
00011  *
00012  * FFmpeg is distributed in the hope that it will be useful,
00013  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00014  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00015  * Lesser General Public License for more details.
00016  *
00017  * You should have received a copy of the GNU Lesser General Public
00018  * License along with FFmpeg; if not, write to the Free Software
00019  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
00020  */
00021 
00027 //#define DEBUG
00028 #include "avcodec.h"
00029 #include "mpegaudio.h"
00030 #include "mpegaudiodata.h"
00031 
00032 
00033 int ff_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
00034 {
00035     int sample_rate, frame_size, mpeg25, padding;
00036     int sample_rate_index, bitrate_index;
00037     if (header & (1<<20)) {
00038         s->lsf = (header & (1<<19)) ? 0 : 1;
00039         mpeg25 = 0;
00040     } else {
00041         s->lsf = 1;
00042         mpeg25 = 1;
00043     }
00044 
00045     s->layer = 4 - ((header >> 17) & 3);
00046     /* extract frequency */
00047     sample_rate_index = (header >> 10) & 3;
00048     sample_rate = ff_mpa_freq_tab[sample_rate_index] >> (s->lsf + mpeg25);
00049     sample_rate_index += 3 * (s->lsf + mpeg25);
00050     s->sample_rate_index = sample_rate_index;
00051     s->error_protection = ((header >> 16) & 1) ^ 1;
00052     s->sample_rate = sample_rate;
00053 
00054     bitrate_index = (header >> 12) & 0xf;
00055     padding = (header >> 9) & 1;
00056     //extension = (header >> 8) & 1;
00057     s->mode = (header >> 6) & 3;
00058     s->mode_ext = (header >> 4) & 3;
00059     //copyright = (header >> 3) & 1;
00060     //original = (header >> 2) & 1;
00061     //emphasis = header & 3;
00062 
00063     if (s->mode == MPA_MONO)
00064         s->nb_channels = 1;
00065     else
00066         s->nb_channels = 2;
00067 
00068     if (bitrate_index != 0) {
00069         frame_size = ff_mpa_bitrate_tab[s->lsf][s->layer - 1][bitrate_index];
00070         s->bit_rate = frame_size * 1000;
00071         switch(s->layer) {
00072         case 1:
00073             frame_size = (frame_size * 12000) / sample_rate;
00074             frame_size = (frame_size + padding) * 4;
00075             break;
00076         case 2:
00077             frame_size = (frame_size * 144000) / sample_rate;
00078             frame_size += padding;
00079             break;
00080         default:
00081         case 3:
00082             frame_size = (frame_size * 144000) / (sample_rate << s->lsf);
00083             frame_size += padding;
00084             break;
00085         }
00086         s->frame_size = frame_size;
00087     } else {
00088         /* if no frame size computed, signal it */
00089         return 1;
00090     }
00091 
00092 #if defined(DEBUG)
00093     dprintf(s->avctx, "layer%d, %d Hz, %d kbits/s, ",
00094            s->layer, s->sample_rate, s->bit_rate);
00095     if (s->nb_channels == 2) {
00096         if (s->layer == 3) {
00097             if (s->mode_ext & MODE_EXT_MS_STEREO)
00098                 dprintf(s->avctx, "ms-");
00099             if (s->mode_ext & MODE_EXT_I_STEREO)
00100                 dprintf(s->avctx, "i-");
00101         }
00102         dprintf(s->avctx, "stereo");
00103     } else {
00104         dprintf(s->avctx, "mono");
00105     }
00106     dprintf(s->avctx, "\n");
00107 #endif
00108     return 0;
00109 }

Generated on Tue Nov 4 2014 12:59:22 for ffmpeg by  doxygen 1.7.1