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
1Syncword12all bits must be 1
2MPEG version10 for MPEG-4, 1 for MPEG-2
3Layer2always 0
4Protection Absent1et to 1 if there is no CRC and 0 if there is CRC
5Profile2the MPEG-4 Audio Object Type minus 1
6MPEG-4 Sampling Frequency Index4MPEG-4 Sampling Frequency Index (15 is forbidden)
7Private Stream1set to 0 when encoding, ignore when decoding
8MPEG-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 decoding
10Home1set to 0 when encoding, ignore when decoding
11Copyrighted Stream1set to 0 when encoding, ignore when decoding
12Copyrighted Start1set to 0 when encoding, ignore when decoding
13Frame Length13this value must include 7 or 9 bytes of header length: FrameLength = (ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame)
14Buffer Fullness11buffer fullness
15Number of AAC Frames2number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame per ADTS frame
16CRC16CRC 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 <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;
    }
}

1.ADTS是个啥

ADTS全称是(Audio Data Transport Stream),是AAC的一种十分常见的传输格式。

记得第一次做demux的时候,把AAC音频的ES流从FLV封装格式中抽出来送给硬件解码器时,不能播;保存到本地用pc的播放器播时,我靠也不能播。当时崩溃了,后来通过查找资料才知道。一般的AAC解码器都需要把AAC的ES流打包成ADTS的格式,一般是在AAC ES流前添加7个字节的ADTS header。也就是说你可以吧ADTS这个头看作是AAC的frameheader。

ADTS AAC
ADTS_headerAAC ESADTS_headerAAC ES
...
ADTS_headerAAC ES

2.ADTS内容及结构

ADTS 头中相对有用的信息 采样率、声道数、帧长度。想想也是,我要是解码器的话,你给我一堆得AAC音频ES流我也解不出来。每一个带ADTS头信息的AAC流会清晰的告送解码器他需要的这些信息。

一般情况下ADTS的头信息都是7个字节,分为2部分:

adts_fixed_header();

adts_variable_header();

AAC音频格式详解-LMLPHP

 

syncword :同步头 总是0xFFF, all bits must be 1,代表着一个ADTS帧的开始

ID:MPEG Version: 0 for MPEG-4, 1 for MPEG-2

Layer:always: '00'

profile:表示使用哪个级别的AAC,有些芯片只支持AAC LC 。在MPEG-2 AAC中定义了3种:

AAC音频格式详解-LMLPHP

sampling_frequency_index:表示使用的采样率下标,通过这个下标在 Sampling Frequencies[ ]数组中查找得知采样率的值。

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

channel_configuration: 表示声道数

  • 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

AAC音频格式详解-LMLPHP

frame_length : 一个ADTS帧的长度包括ADTS头和AAC原始流.

adts_buffer_fullness:0x7FF 说明是码率可变的码流

3.将AAC打包成ADTS格式

如果是通过嵌入式高清解码芯片做产品的话,一般情况的解码工作都是由硬件来完成的。所以大部分的工作是把AAC原始流打包成ADTS的格式,然后丢给硬件就行了。

通过对ADTS格式的了解,很容易就能把AAC打包成ADTS。我们只需得到封装格式里面关于音频采样率、声道数、元数据长度、aac格式类型等信息。然后在每个AAC原始流前面加上个ADTS头就OK了。

贴上ffmpeg中添加ADTS头的代码,就可以很清晰的了解ADTS头的结构:

  1. int ff_adts_write_frame_header(ADTSContext *ctx,
  2. uint8_t *buf, int size, int pce_size)
  3. {
  4. PutBitContext pb;
  5. init_put_bits(&pb, buf, ADTS_HEADER_SIZE);
  6. /* adts_fixed_header */
  7. put_bits(&pb, 12, 0xfff);   /* syncword */
  8. put_bits(&pb, 1, 0);        /* ID */
  9. put_bits(&pb, 2, 0);        /* layer */
  10. put_bits(&pb, 1, 1);        /* protection_absent */
  11. put_bits(&pb, 2, ctx->objecttype); /* profile_objecttype */
  12. put_bits(&pb, 4, ctx->sample_rate_index);
  13. put_bits(&pb, 1, 0);        /* private_bit */
  14. put_bits(&pb, 3, ctx->channel_conf); /* channel_configuration */
  15. put_bits(&pb, 1, 0);        /* original_copy */
  16. put_bits(&pb, 1, 0);        /* home */
  17. /* adts_variable_header */
  18. put_bits(&pb, 1, 0);        /* copyright_identification_bit */
  19. put_bits(&pb, 1, 0);        /* copyright_identification_start */
  20. put_bits(&pb, 13, ADTS_HEADER_SIZE + size + pce_size); /* aac_frame_length */
  21. put_bits(&pb, 11, 0x7ff);   /* adts_buffer_fullness */
  22. put_bits(&pb, 2, 0);        /* number_of_raw_data_blocks_in_frame */
  23. flush_put_bits(&pb);
  24. return 0;
  25. }
 
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