G711编解码

http://blog.csdn.net/rightorwrong/article/details/4209467

搞语音对讲几天了，播放时声音干扰太大了。拖得时间久有两个原因：

1.每次采样的位数这个值设置的问题。本来是用的采样位数为16，但是服务端那边说用8。导致编解码时8位的始终有杂音。

1.G711编解码的问题：用了一个错误的编解码程序。目前用的编解码代码对于8为始终有杂音

下面把编解码的代码放在这里，16为采集效果很好

//编码
int CG711Decoder::G711_EnCode(unsigned char* pCodecBits, const char* pBuffer, int nBufferSize)
{
    short* buffer = (short*)pBuffer;
    for(int i=0; i<nBufferSize/2; i++)
    {
        pCodecBits[i] = encode(buffer[i]);
    }

    return nBufferSize/2;
}

//解码
int CG711Decoder::G711_Decode(char* pRawData, const unsigned char* pBuffer, int nBufferSize)
{
    short *out_data = (short*)pRawData;
    for(int i=0; i<nBufferSize; i++)
    {
        out_data[i] = decode(pBuffer[i]);
    }

    return nBufferSize*2;
}

#define MAX (32635)
unsigned char CG711Decoder::encode(short pcm)
{
    int sign = (pcm & 0x8000) >> 8;
    if (sign != 0)
        pcm = -pcm;
    if (pcm > MAX) pcm = MAX;
    int exponent = 7;
    int expMask;
    for (expMask = 0x4000; (pcm & expMask) == 0
        && exponent>0; exponent--, expMask >>= 1) { }
    int mantissa = (pcm >> ((exponent == 0) ? 4 : (exponent + 3))) & 0x0f;
    unsigned char alaw = (unsigned char)(sign | exponent << 4 | mantissa);
    return (unsigned char)(alaw^0xD5);
}
short CG711Decoder::decode(unsigned char alaw)
{
    alaw ^= 0xD5;
    int sign = alaw & 0x80;
    int exponent = (alaw & 0x70) >> 4;
    int data = alaw & 0x0f;
    data <<= 4;
    data += 8;
    if (exponent != 0)
        data += 0x100;
    if (exponent > 1)
        data <<= (exponent - 1);

    return (short)(sign == 0 ? data : -data);
}