aac文件解析

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AAC音频格式分析

AAC音频格式有ADIF和ADTS：

ADIF：Audio Data Interchange Format 音频数据交换格式。这种格式的特征是可以确定的找到这个音频数据的开始，不需进行在音频数据流中间开始的解码，即它的解码必须在明确定义的开始处进行。故这种格式常用在磁盘文件中。

ADTS：Audio Data Transport Stream 音频数据传输流。这种格式的特征是它是一个有同步字的比特流，解码可以在这个流中任何位置开始。它的特征类似于mp3数据流格式。

简单说，ADTS可以在任意帧解码，也就是说它每一帧都有头信息。ADIF只有一个统一的头，所以必须得到所有的数据后解码。且这两种的header的格式也是不同的，目前一般编码后的和抽取出的都是ADTS格式的音频流。

语音系统对实时性要求较高，基本是这样一个流程，采集音频数据，本地编码，数据上传，服务器处理，数据下发，本地解码

ADTS是帧序列，本身具备流特征，在音频流的传输与处理方面更加合适。

ADTS帧首部结构：

序号域长度（bits）说明1Syncword12all bits must be 12MPEG version10 for MPEG-4, 1 for MPEG-23Layer2always 04Protection Absent1et to 1 if there is no CRC and 0 if there is CRC5Profile2the MPEG-4 Audio Object Type minus 16MPEG-4 Sampling Frequency Index4MPEG-4 Sampling Frequency Index (15 is forbidden)7Private Stream1set to 0 when encoding, ignore when decoding8MPEG-4 Channel Configuration3MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an inband PCE)9Originality1set to 0 when encoding, ignore when decoding10Home1set to 0 when encoding, ignore when decoding11Copyrighted Stream1set to 0 when encoding, ignore when decoding12Copyrighted Start1set to 0 when encoding, ignore when decoding13Frame Length13this value must include 7 or 9 bytes of header length: FrameLength = (ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame)14Buffer Fullness11buffer fullness15Number of AAC Frames2number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame per ADTS frame16CRC16CRC if protection absent is 0

int get_audio_samplerate(int index){int rate = 0;switch(index){case 0x0: rate=96000; break;case 0x1: rate=88200; break;case 0x2: rate=64000; break;case 0x3: rate=48000; break;case 0x4: rate=44100; break;case 0x5: rate=32000; break;case 0x6: rate=24000; break;case 0x7: rate=22050; break;case 0x8: rate=16000; break;case 0x9: rate=2000; break;case 0xa: rate=11025; break;case 0xb: rate=8000; break;default: break;}return rate;}

if ((Adts_Headr_Buf[0] == 0xFF)&&((Adts_Headr_Buf[1] & 0xF0) == 0xF0))    //syncword 12个1{adtsheader->syncword = (Adts_Headr_Buf[0] << 4 )  | (Adts_Headr_Buf[1]  >> 4);adtsheader->id = ((unsigned int) Adts_Headr_Buf[1] & 0x08) >> 3;adtsheader->layer = ((unsigned int) Adts_Headr_Buf[1] & 0x06) >> 1;adtsheader->protection_absent = (unsigned int) Adts_Headr_Buf[1] & 0x01;adtsheader->profile = ((unsigned int) Adts_Headr_Buf[2] & 0xc0) >> 6;adtsheader->sf_index = ((unsigned int) Adts_Headr_Buf[2] & 0x3c) >> 2;adtsheader->private_bit = ((unsigned int) Adts_Headr_Buf[2] & 0x02) >> 1;adtsheader->channel_configuration = ((((unsigned int) Adts_Headr_Buf[2] & 0x01) << 2) | (((unsigned int) Adts_Headr_Buf[3] & 0xc0) >> 6));adtsheader->original = ((unsigned int) Adts_Headr_Buf[3] & 0x20) >> 5;adtsheader->home = ((unsigned int) Adts_Headr_Buf[3] & 0x10) >> 4;adtsheader->copyright_identification_bit = ((unsigned int) Adts_Headr_Buf[3] & 0x08) >> 3;adtsheader->copyright_identification_start = (unsigned int) Adts_Headr_Buf[3] & 0x04 >> 2;adtsheader->aac_frame_length = (((((unsigned int) Adts_Headr_Buf[3]) & 0x03) << 11) | (((unsigned int) Adts_Headr_Buf[4] & 0xFF) << 3)| ((unsigned int) Adts_Headr_Buf[5] & 0xE0) >> 5) ;adtsheader->adts_buffer_fullness = (((unsigned int) Adts_Headr_Buf[5] & 0x1f) << 6 | ((unsigned int) Adts_Headr_Buf[6] & 0xfc) >> 2);adtsheader->no_raw_data_blocks_in_frame = ((unsigned int) Adts_Headr_Buf[6] & 0x03);}

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