G711转AAC

来源：互联网发布：气象数据网编辑：程序博客网时间：2024/05/19 13:27

在嵌入式系统中，因资源限制，一般音频采用G711压缩编码，但在上层应用中却很少直接支持G711，一般都市WMA,AAC,MP3等。

如视频系统存储文件采用MP4格式存储就不直接支持 G711，故需要转换成 AAC，普通播放器才能播放。

分为2步

1.将 G711转成 PCM ------解码成原是流

int G711::ALawDecode(uint8_t alaw)
{
alaw ^= 0x55; // A-law has alternate bits inverted for transmission

unsigned sign = alaw&0x80;

int linear = alaw&0x1f;
linear <<= 4;
linear += 8; // Add a 'half' bit (0x08) to place PCM value in middle of range

alaw &= 0x7f;
if(alaw>=0x20)
  {
  linear |= 0x100; // Put in MSB
  unsigned shift = (alaw>>4)-1;
  linear <<= shift;
  }

if(!sign)
return -linear;
else
return linear;
}

unsigned G711::ALawDecode(int16_t* dst, const uint8_t* src, size_t srcSize)
{
int16_t* end = dst+srcSize;
while(dst<end)
*dst++ = ALawDecode(*src++);
return srcSize<<1;
}

data --------G711 编码数据 datalen 对应G711数据长度

int sizePCM = G711::ALawDecode((short *)pbPCMBuffer,data,dataLen);

2.将PCM 转成 AAC--------编码成AAC

采用libfaac http://www.audiocoding.com/downloads.html

定义变量:

faacEncHandle m_hEncoder;
unsigned long maxBytesOutput =0;
unsigned long nInputSamples = 0;
UINT nPCMBitSize = 16;
int nPCMBufferSize;
BYTE* pbPCMBuffer = NULL;
BYTE* pbAACBuffer = NULL;

配置

m_hEncoder = faacEncOpen(8000,1,&nInputSamples,&maxBytesOutput);
pbAACBuffer = new unsigned char[maxBytesOutput];
memset(pbAACBuffer,0,maxBytesOutput);

nPCMBufferSize = nInputSamples * nPCMBitSize / 8;
pbPCMBuffer = new BYTE [nPCMBufferSize];

faacEncConfigurationPtr CurFormat=faacEncGetCurrentConfiguration(m_hEncoder);
CurFormat->inputFormat=FAAC_INPUT_16BIT;
faacEncSetConfiguration(m_hEncoder, CurFormat);

编码

nInputSamples = (sizePCM) / (nPCMBitSize / 8);
int nRet = faacEncEncode(m_hEncoder,
(int*) pbPCMBuffer, nInputSamples, pbAACBuffer, maxBytesOutput);

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