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：

ADTS帧首部结构：

A

AC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：

// 创建解码环境并返回一个句柄
NeAACDecHandle NEAACAPI NeAACDecOpen(void);

// 关闭解码环境
void NEAACAPI NeAACDecClose(NeAACDecHandle hDecoder);

// 获取当前解码器库的配置
NeAACDecConfigurationPtr NEAACAPI NeAACDecGetCurrentConfiguration(NeAACDecHandle hDecoder);

// 为解码器库设置一个配置结构
unsigned char NEAACAPI NeAACDecSetConfiguration(NeAACDecHandle hDecoder, NeAACDecConfigurationPtr config);

// 初始化解码器库
long NEAACAPI NeAACDecInit(NeAACDecHandle hDecoder, unsigned char *buffer, unsigned longbuffer_size, unsigned long *samplerate, unsigned char *channels);

// 解码AAC数据
void* NEAACAPI NeAACDecDecode(NeAACDecHandle hDecoder, NeAACDecFrameInfo *hInfo, unsigned char*buffer, unsigned long buffer_size);

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__*/

#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;
	}
}