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: csy3228@gmail.com
- *
- * //
- #ifndef __MYAACDECODER_H__
- #define __MYAACDECODER_H__
- #include "Buffer.h"
- #include "mytools.h"
- #include "WaveFormat.h"
- #include "faad.h"
- #include < iostream >
- 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 << 6;
- 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;
- }
- }
AAC音频格式分析
来源: http://www.bubuko.com/infodetail-2411595.html