• AAC音频格式分析与解码


    一直在做一个语音项目,到了测试阶段,近来不是很忙,想把之前做的内容整理一下。

    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 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;
        }
    }
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  • 原文地址:https://www.cnblogs.com/caosiyang/p/2594029.html
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