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libavcodec/mpegaudiodecheader.c

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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 #include "mpegaudiodecheader.h"
00032 
00033 
00034 int ff_mpegaudio_decode_header(MPADecodeHeader *s, uint32_t header)
00035 {
00036     int sample_rate, frame_size, mpeg25, padding;
00037     int sample_rate_index, bitrate_index;
00038     if (header & (1<<20)) {
00039         s->lsf = (header & (1<<19)) ? 0 : 1;
00040         mpeg25 = 0;
00041     } else {
00042         s->lsf = 1;
00043         mpeg25 = 1;
00044     }
00045 
00046     s->layer = 4 - ((header >> 17) & 3);
00047     /* extract frequency */
00048     sample_rate_index = (header >> 10) & 3;
00049     sample_rate = ff_mpa_freq_tab[sample_rate_index] >> (s->lsf + mpeg25);
00050     sample_rate_index += 3 * (s->lsf + mpeg25);
00051     s->sample_rate_index = sample_rate_index;
00052     s->error_protection = ((header >> 16) & 1) ^ 1;
00053     s->sample_rate = sample_rate;
00054 
00055     bitrate_index = (header >> 12) & 0xf;
00056     padding = (header >> 9) & 1;
00057     //extension = (header >> 8) & 1;
00058     s->mode = (header >> 6) & 3;
00059     s->mode_ext = (header >> 4) & 3;
00060     //copyright = (header >> 3) & 1;
00061     //original = (header >> 2) & 1;
00062     //emphasis = header & 3;
00063 
00064     if (s->mode == MPA_MONO)
00065         s->nb_channels = 1;
00066     else
00067         s->nb_channels = 2;
00068 
00069     if (bitrate_index != 0) {
00070         frame_size = ff_mpa_bitrate_tab[s->lsf][s->layer - 1][bitrate_index];
00071         s->bit_rate = frame_size * 1000;
00072         switch(s->layer) {
00073         case 1:
00074             frame_size = (frame_size * 12000) / sample_rate;
00075             frame_size = (frame_size + padding) * 4;
00076             break;
00077         case 2:
00078             frame_size = (frame_size * 144000) / sample_rate;
00079             frame_size += padding;
00080             break;
00081         default:
00082         case 3:
00083             frame_size = (frame_size * 144000) / (sample_rate << s->lsf);
00084             frame_size += padding;
00085             break;
00086         }
00087         s->frame_size = frame_size;
00088     } else {
00089         /* if no frame size computed, signal it */
00090         return 1;
00091     }
00092 
00093 #if defined(DEBUG)
00094     dprintf(NULL, "layer%d, %d Hz, %d kbits/s, ",
00095            s->layer, s->sample_rate, s->bit_rate);
00096     if (s->nb_channels == 2) {
00097         if (s->layer == 3) {
00098             if (s->mode_ext & MODE_EXT_MS_STEREO)
00099                 dprintf(NULL, "ms-");
00100             if (s->mode_ext & MODE_EXT_I_STEREO)
00101                 dprintf(NULL, "i-");
00102         }
00103         dprintf(NULL, "stereo");
00104     } else {
00105         dprintf(NULL, "mono");
00106     }
00107     dprintf(NULL, "\n");
00108 #endif
00109     return 0;
00110 }

Generated on Fri Sep 16 2011 17:17:40 for FFmpeg by  doxygen 1.7.1