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mpegaudiodecheader.c
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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
24  * MPEG Audio header decoder.
25  */
26 
27 #include "avcodec.h"
28 #include "mpegaudio.h"
29 #include "mpegaudiodata.h"
30 #include "mpegaudiodecheader.h"
31 
32 
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 = avpriv_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 = avpriv_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  av_dlog(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  av_dlog(NULL, "ms-");
99  if (s->mode_ext & MODE_EXT_I_STEREO)
100  av_dlog(NULL, "i-");
101  }
102  av_dlog(NULL, "stereo");
103  } else {
104  av_dlog(NULL, "mono");
105  }
106  av_dlog(NULL, "\n");
107 #endif
108  return 0;
109 }
110 
111 int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate)
112 {
113  MPADecodeHeader s1, *s = &s1;
114 
115  if (ff_mpa_check_header(head) != 0)
116  return -1;
117 
118  if (avpriv_mpegaudio_decode_header(s, head) != 0) {
119  return -1;
120  }
121 
122  switch(s->layer) {
123  case 1:
124  avctx->codec_id = AV_CODEC_ID_MP1;
125  *frame_size = 384;
126  break;
127  case 2:
128  avctx->codec_id = AV_CODEC_ID_MP2;
129  *frame_size = 1152;
130  break;
131  default:
132  case 3:
133  avctx->codec_id = AV_CODEC_ID_MP3;
134  if (s->lsf)
135  *frame_size = 576;
136  else
137  *frame_size = 1152;
138  break;
139  }
140 
141  *sample_rate = s->sample_rate;
142  *channels = s->nb_channels;
143  *bit_rate = s->bit_rate;
144  return s->frame_size;
145 }