MPEG-4 Audio 关于esds 记录

当解析Mp4的时候，会碰到这个esds 的box，这个box 有很多信息。

当时是为了解决在MP4A中如何判断是acc还是 MP3 然后找到下面答案，现在梳理下:

在Audio Trak 的esds box中

首先是 1、version 和 flag 占4个字节 不使用就跳过

get_be32(m_it); /* version + flags */1
2、MP4ESDescrTag 读取 MP4ESDescrTag tag 和 len 和附属内容
tag = get_byte(m_it); len = getTaglen(); if (tag == MP4ESDescrTag) { get_be16(m_it); /* ID */ get_byte(m_it); /* priority */ } else get_be16(m_it); /* ID */1234567
3、MP4DecConfigDescrTag 读取 MP4DecConfigDescrTag tag 和 len 和附属内容
tag = get_byte(m_it);len = getTaglen(); if (tag == MP4DecConfigDescrTag) { int object_type_id = get_byte(m_it); get_byte(m_it); /* stream type */ get_byte(m_it); get_byte(m_it); get_byte(m_it); get_be32(m_it); /* max bitrate */ get_be32(m_it); /* avg bitrate */ st->codec->codec_id= codec_get_id(ff_mp4_obj_type, object_type_id);12345678910

这边的附属内容我要说下 重要的是 这边对codec 解码进行了调整，id 对应表附件1，这边决定了MP3 或者 acc等

4、 MP4DecSpecificDescrTag 读取MP4DecSpecificDescrTag 和len 和附属内容

tag = get_byte(m_it);len=getTagLen();if (tag == MP4DecSpecificDescrTag) { st->codec->extradata = (uint8_t *)malloc(len); if (st->codec->extradata) { ReadData(st->codec->extradata, len,m_it); st->codec->extradata_size = len; int ad = *st->codec->extradata; int aa =*st->codec->extradata >> 3; if ((*st->codec->extradata >> 3) == 29) { st->codec->codec_id = CODEC_ID_MP3ON4; } if (st->codec->codec_id == CODEC_ID_AAC) { MPEG4AudioConfig cfg = {0}; avpriv_mpeg4audio_get_config(&cfg, st->codec->extradata, st->codec->extradata_size, 1); st->codec->channels = cfg.channels;// if (cfg.object_type == 29 && cfg.sampling_index < 3) // old mp3on4//st->codec->sample_rate = avpriv_mpa_freq_tab[cfg.sampling_index];// else if (cfg.ext_sample_rate)// //st->codec->sample_rate = cfg.ext_sample_rate;// else// //st->codec->sample_rate = cfg.sample_rate; // if (!(st->codec->codec_id = ff_codec_get_id(mp4_audio_types,// cfg.object_type)))// st->codec->codec_id = AV_CODEC_ID_AAC; }}1234567891011121314151617181920212223242526272829

这边就严重了，这边如果是老的版本 只需如此

int aa =*st->codec->extradata >> 3;if ((*st->codec->extradata >> 3) == 29) { st->codec->codec_id = CODEC_ID_MP3ON4;}1234

但是新的 版本 调整增加了 很多内容

The Audio Specific Config is the global header for MPEG-4 Audio:5 bits: object typeif (object type == 31) 6 bits + 32: object type4 bits: frequency indexif (frequency index == 15) 24 bits: frequency4 bits: channel configurationvar bits: AOT Specific Config123456789

• 主要如上 object type 读取5个bit object type列表见附件2（对codec解码器进行调整，MP3 还是ACC还是 MP3on4）
• 然后 frequency index 读取4个bit frequency index 列表见 附件3（对采样率进行调整）
• 最后channel 读取4个位 列表见附件4 （对声道数进行调整）

附件1 const MP4CodecTag ff_mp4_obj_type[] = { { CODEC_ID_MPEG4 , 32 }, { CODEC_ID_H264 , 33 }, { CODEC_ID_AAC , 64 }, { CODEC_ID_MPEG2VIDEO, 96 }, /* MPEG2 Simple */ { CODEC_ID_MPEG2VIDEO, 97 }, /* MPEG2 Main */ { CODEC_ID_MPEG2VIDEO, 98 }, /* MPEG2 SNR */ { CODEC_ID_MPEG2VIDEO, 99 }, /* MPEG2 Spatial */ { CODEC_ID_MPEG2VIDEO, 100 }, /* MPEG2 High */ { CODEC_ID_MPEG2VIDEO, 101 }, /* MPEG2 422 */ { CODEC_ID_AAC , 102 }, /* MPEG2 AAC Main */ { CODEC_ID_AAC , 103 }, /* MPEG2 AAC Low */ { CODEC_ID_AAC , 104 }, /* MPEG2 AAC SSR */ { CODEC_ID_MP3 , 105 }, /* 13818-3 */ { CODEC_ID_MPEG1VIDEO, 106 }, /* 11172-2 */ { CODEC_ID_MP3 , 107 }, /* 11172-3 */ { CODEC_ID_MJPEG , 108 }, /* 10918-1 */ { CODEC_ID_PNG , 109 }, { CODEC_ID_JPEG2000 , 110 }, /* 15444-1 */ { CODEC_ID_VC1 , 163 }, { CODEC_ID_VORBIS , 221 }, { CODEC_ID_PCM_S16LE , 224 }, { CODEC_ID_QCELP , 225 }, { CODEC_ID_AC3 , 226 }, { CODEC_ID_PCM_ALAW , 227 }, { CODEC_ID_PCM_MULAW , 228 }, { CODEC_ID_PCM_S16BE , 230 }, { CODEC_ID_H263 , 242 }, { CODEC_ID_H261 , 243 }, { CODEC_ID_NONE, 0 },

};1

附件2 MPEG-4 Audio Object Types: ● 0: Null ● 1: AAC Main ● 2: AAC LC (Low Complexity) ● 3: AAC SSR (Scalable Sample Rate) ● 4: AAC LTP (Long Term Prediction) ● 5: SBR (Spectral Band Replication) ● 6: AAC Scalable ● 7: TwinVQ ● 8: CELP (Code Excited Linear Prediction) ● 9: HXVC (Harmonic Vector eXcitation Coding) ● 10: Reserved ● 11: Reserved ● 12: TTSI (Text-To-Speech Interface) ● 13: Main Synthesis ● 14: Wavetable Synthesis ● 15: General MIDI ● 16: Algorithmic Synthesis and Audio Effects ● 17: ER (Error Resilient) AAC LC ● 18: Reserved ● 19: ER AAC LTP ● 20: ER AAC Scalable ● 21: ER TwinVQ ● 22: ER BSAC (Bit-Sliced Arithmetic Coding) ● 23: ER AAC LD (Low Delay) ● 24: ER CELP ● 25: ER HVXC ● 26: ER HILN (Harmonic and Individual Lines plus Noise) ● 27: ER Parametric ● 28: SSC (SinuSoidal Coding) ● 29: PS (Parametric Stereo) ● 30: MPEG Surround ● 31: (Escape value) ● 32: Layer-1 ● 33: Layer-2 ● 34: Layer-3 ● 35: DST (Direct Stream Transfer) ● 36: ALS (Audio Lossless) ● 37: SLS (Scalable LosslesS) ● 38: SLS non-core ● 39: ER AAC ELD (Enhanced Low Delay) ● 40: SMR (Symbolic Music Representation) Simple ● 41: SMR Main ● 42: USAC (Unified Speech and Audio Coding) (no SBR) ● 43: SAOC (Spatial Audio Object Coding) ● 44: LD MPEG Surround ● 45: USAC 附件3 Sampling Frequencies There are 13 supported frequencies: ● 0: 96000 Hz ● 1: 88200 Hz ● 2: 64000 Hz ● 3: 48000 Hz ● 4: 44100 Hz ● 5: 32000 Hz ● 6: 24000 Hz ● 7: 22050 Hz ● 8: 16000 Hz ● 9: 12000 Hz ● 10: 11025 Hz ● 11: 8000 Hz ● 12: 7350 Hz ● 13: Reserved ● 14: Reserved ● 15: frequency is written explictly 附件3

Channel Configurations These are the channel configurations: ● 0: Defined in AOT Specifc Config ● 1: 1 channel: front-center ● 2: 2 channels: front-left, front-right ● 3: 3 channels: front-center, front-left, front-right ● 4: 4 channels: front-center, front-left, front-right, back-center ● 5: 5 channels: front-center, front-left, front-right, back-left, back-right ● 6: 6 channels: front-center, front-left, front-right, back-left, back-right, LFE-channel ● 7: 8 channels: front-center, front-left, front-right, side-left, side-right, back-left, back-right, LFE-channel ● 8-15: Reserved