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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 //#define DEBUG
28 #include "avcodec.h"
29 #include "mpegaudio.h"
30 #include "mpegaudiodata.h"
31 #include "mpegaudiodecheader.h"
32 
33 
35 {
36  int sample_rate, frame_size, mpeg25, padding;
37  int sample_rate_index, bitrate_index;
38  if (header & (1<<20)) {
39  s->lsf = (header & (1<<19)) ? 0 : 1;
40  mpeg25 = 0;
41  } else {
42  s->lsf = 1;
43  mpeg25 = 1;
44  }
45 
46  s->layer = 4 - ((header >> 17) & 3);
47  /* extract frequency */
48  sample_rate_index = (header >> 10) & 3;
49  sample_rate = avpriv_mpa_freq_tab[sample_rate_index] >> (s->lsf + mpeg25);
50  sample_rate_index += 3 * (s->lsf + mpeg25);
51  s->sample_rate_index = sample_rate_index;
52  s->error_protection = ((header >> 16) & 1) ^ 1;
53  s->sample_rate = sample_rate;
54 
55  bitrate_index = (header >> 12) & 0xf;
56  padding = (header >> 9) & 1;
57  //extension = (header >> 8) & 1;
58  s->mode = (header >> 6) & 3;
59  s->mode_ext = (header >> 4) & 3;
60  //copyright = (header >> 3) & 1;
61  //original = (header >> 2) & 1;
62  //emphasis = header & 3;
63 
64  if (s->mode == MPA_MONO)
65  s->nb_channels = 1;
66  else
67  s->nb_channels = 2;
68 
69  if (bitrate_index != 0) {
70  frame_size = avpriv_mpa_bitrate_tab[s->lsf][s->layer - 1][bitrate_index];
71  s->bit_rate = frame_size * 1000;
72  switch(s->layer) {
73  case 1:
74  frame_size = (frame_size * 12000) / sample_rate;
75  frame_size = (frame_size + padding) * 4;
76  break;
77  case 2:
78  frame_size = (frame_size * 144000) / sample_rate;
79  frame_size += padding;
80  break;
81  default:
82  case 3:
83  frame_size = (frame_size * 144000) / (sample_rate << s->lsf);
84  frame_size += padding;
85  break;
86  }
87  s->frame_size = frame_size;
88  } else {
89  /* if no frame size computed, signal it */
90  return 1;
91  }
92 
93 #if defined(DEBUG)
94  av_dlog(NULL, "layer%d, %d Hz, %d kbits/s, ",
95  s->layer, s->sample_rate, s->bit_rate);
96  if (s->nb_channels == 2) {
97  if (s->layer == 3) {
98  if (s->mode_ext & MODE_EXT_MS_STEREO)
99  av_dlog(NULL, "ms-");
100  if (s->mode_ext & MODE_EXT_I_STEREO)
101  av_dlog(NULL, "i-");
102  }
103  av_dlog(NULL, "stereo");
104  } else {
105  av_dlog(NULL, "mono");
106  }
107  av_dlog(NULL, "\n");
108 #endif
109  return 0;
110 }
111 
112 int avpriv_mpa_decode_header(AVCodecContext *avctx, uint32_t head, int *sample_rate, int *channels, int *frame_size, int *bit_rate)
113 {
114  MPADecodeHeader s1, *s = &s1;
115 
116  if (ff_mpa_check_header(head) != 0)
117  return -1;
118 
119  if (avpriv_mpegaudio_decode_header(s, head) != 0) {
120  return -1;
121  }
122 
123  switch(s->layer) {
124  case 1:
125  avctx->codec_id = AV_CODEC_ID_MP1;
126  *frame_size = 384;
127  break;
128  case 2:
129  avctx->codec_id = AV_CODEC_ID_MP2;
130  *frame_size = 1152;
131  break;
132  default:
133  case 3:
134  avctx->codec_id = AV_CODEC_ID_MP3;
135  if (s->lsf)
136  *frame_size = 576;
137  else
138  *frame_size = 1152;
139  break;
140  }
141 
142  *sample_rate = s->sample_rate;
143  *channels = s->nb_channels;
144  *bit_rate = s->bit_rate;
145  return s->frame_size;
146 }