AAC音频格式分析与解码

#AAC音频格式分析与解码| 来源: 网络整理| 查看: 265

关于AAC音频格式基本情况，可参考维基百科http://en.wikipedia.org/wiki/Advanced_Audio_Coding

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帧结构：

header

body

ADTS帧首部结构：

序号域长度（bits）说明 1 Syncword 12 all bits must be 1 2 MPEG version 1 0 for MPEG-4, 1 for MPEG-2 3 Layer 2 always 0 4 Protection Absent 1 et to 1 if there is no CRC and 0 if there is CRC 5 Profile 2 the MPEG-4 Audio Object Type minus 1 6 MPEG-4 Sampling Frequency Index 4 MPEG-4 Sampling Frequency Index (15 is forbidden) 7 Private Stream 1 set to 0 when encoding, ignore when decoding 8 MPEG-4 Channel Configuration 3 MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an inband PCE) 9 Originality 1 set to 0 when encoding, ignore when decoding 10 Home 1 set to 0 when encoding, ignore when decoding 11 Copyrighted Stream 1 set to 0 when encoding, ignore when decoding 12 Copyrighted Start 1 set to 0 when encoding, ignore when decoding 13 Frame Length 13 this value must include 7 or 9 bytes of header length: FrameLength = (ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame) 14 Buffer Fullness 11 buffer fullness 15 Number of AAC Frames 2 number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame per ADTS frame 16 CRC 16 CRC if protection absent is 0

AAC解码

在解码方面，使用了开源的FAAD，http://www.audiocoding.com/faad2.html

sdk解压缩后，docs目录有详细的api说明文档，主要用到的有以下几个：

NeAACDecHandle NEAACAPI NeAACDecOpen(void); 创建解码环境并返回一个句柄 void NEAACAPI NeAACDecClose(NeAACDecHandle hDecoder); 关闭解码环境 NeAACDecConfigurationPtr NEAACAPI NeAACDecGetCurrentConfiguration(NeAACDecHandle hDecoder); 获取当前解码器库的配置 unsigned char NEAACAPI NeAACDecSetConfiguration(NeAACDecHandle hDecoder, NeAACDecConfigurationPtr config); 为解码器库设置一个配置结构 long NEAACAPI NeAACDecInit(NeAACDecHandle hDecoder, unsigned char *buffer, unsigned long buffer_size, unsigned long *samplerate, unsigned char *channels); 初始化解码器库 void* NEAACAPI NeAACDecDecode(NeAACDecHandle hDecoder, NeAACDecFrameInfo *hInfo, unsigned char *buffer, unsigned long buffer_size); 解码AAC数据

对以上api做了简单封装，写了一个解码类，涵盖了FAAD库的基本用法，感兴趣的朋友可以看看

MyAACDecoder.h：

/** * * filename: MyAACDecoder.h * summary: convert aac to wave * author: caosiyang * email: [email protected] * */ #ifndef __MYAACDECODER_H__ #define __MYAACDECODER_H__ #include "Buffer.h" #include "mytools.h" #include "WaveFormat.h" #include "faad.h" #include using namespace std; class MyAACDecoder { public: MyAACDecoder(); ~MyAACDecoder(); int32_t Decode(char *aacbuf, uint32_t aacbuflen); const char* WavBodyData() const { return _mybuffer.Data(); } uint32_t WavBodyLength() const { return _mybuffer.Length(); } const char* WavHeaderData() const { return _wave_format.getHeaderData(); } uint32_t WavHeaderLength() const { return _wave_format.getHeaderLength(); } private: MyAACDecoder(const MyAACDecoder &dec); MyAACDecoder& operator=(const MyAACDecoder &rhs); //init AAC decoder int32_t _init_aac_decoder(char *aacbuf, int32_t aacbuflen); //destroy aac decoder void _destroy_aac_decoder(); //parse AAC ADTS header, get frame length uint32_t _get_frame_length(const char *aac_header) const; //AAC decoder properties NeAACDecHandle _handle; unsigned long _samplerate; unsigned char _channel; Buffer _mybuffer; WaveFormat _wave_format; }; #endif /*__MYAACDECODER_H__*/

MyAACDecoder.cpp：

#include "MyAACDecoder.h" MyAACDecoder::MyAACDecoder(): _handle(NULL), _samplerate(44100), _channel(2), _mybuffer(4096, 4096) { } MyAACDecoder::~MyAACDecoder() { _destroy_aac_decoder(); } int32_t MyAACDecoder::Decode(char *aacbuf, uint32_t aacbuflen) { int32_t res = 0; if (!_handle) { if (_init_aac_decoder(aacbuf, aacbuflen) != 0) { ERR1(":::: init aac decoder failed ::::"); return -1; } } //clean _mybuffer _mybuffer.Clean(); uint32_t donelen = 0; uint32_t wav_data_len = 0; while (donelen < aacbuflen) { uint32_t framelen = _get_frame_length(aacbuf + donelen); if (donelen + framelen > aacbuflen) { break; } //decode NeAACDecFrameInfo info; void *buf = NeAACDecDecode(_handle, &info, (unsigned char*)aacbuf + donelen, framelen); if (buf && info.error == 0) { if (info.samplerate == 44100) { //44100Hz //src: 2048 samples, 4096 bytes //dst: 2048 samples, 4096 bytes uint32_t tmplen = info.samples * 16 / 8; _mybuffer.Fill((const char*)buf, tmplen); wav_data_len += tmplen; } else if (info.samplerate == 22050) { //22050Hz //src: 1024 samples, 2048 bytes //dst: 2048 samples, 4096 bytes short *ori = (short*)buf; short tmpbuf[info.samples * 2]; uint32_t tmplen = info.samples * 16 / 8 * 2; for (int32_t i = 0, j = 0; i < info.samples; i += 2) { tmpbuf[j++] = ori[i]; tmpbuf[j++] = ori[i + 1]; tmpbuf[j++] = ori[i]; tmpbuf[j++] = ori[i + 1]; } _mybuffer.Fill((const char*)tmpbuf, tmplen); wav_data_len += tmplen; } } else { ERR1("NeAACDecDecode() failed"); } donelen += framelen; } //generate Wave header _wave_format.setSampleRate(_samplerate); _wave_format.setChannel(_channel); _wave_format.setSampleBit(16); _wave_format.setBandWidth(_samplerate * 16 * _channel / 8); _wave_format.setDataLength(wav_data_len); _wave_format.setTotalLength(wav_data_len + 44); _wave_format.GenerateHeader(); return 0; } uint32_t MyAACDecoder::_get_frame_length(const char *aac_header) const { uint32_t len = *(uint32_t *)(aac_header + 3); len = ntohl(len); //Little Endian len = len > 19; return len; } int32_t MyAACDecoder::_init_aac_decoder(char* aacbuf, int32_t aacbuflen) { unsigned long cap = NeAACDecGetCapabilities(); _handle = NeAACDecOpen(); if (!_handle) { ERR1("NeAACDecOpen() failed"); _destroy_aac_decoder(); return -1; } NeAACDecConfigurationPtr conf = NeAACDecGetCurrentConfiguration(_handle); if (!conf) { ERR1("NeAACDecGetCurrentConfiguration() failed"); _destroy_aac_decoder(); return -1; } NeAACDecSetConfiguration(_handle, conf); long res = NeAACDecInit(_handle, (unsigned char *)aacbuf, aacbuflen, &_samplerate, &_channel); if (res < 0) { ERR1("NeAACDecInit() failed"); _destroy_aac_decoder(); return -1; } //fprintf(stdout, "SampleRate = %d\n", _samplerate); //fprintf(stdout, "Channel = %d\n", _channel); //fprintf(stdout, ":::: init aac decoder done ::::\n"); return 0; } void MyAACDecoder::_destroy_aac_decoder() { if (_handle) { NeAACDecClose(_handle); _handle = NULL; } }

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