G711编解码，alaw、ulaw与PCB相互转换

//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;     //最高位，符号位  

    if (pcm > MAX) pcm = MAX;  //max=32635  
    int exponent = 7;    
    int expMask;    
    for (expMask = 0x4000; (pcm & expMask) == 0  ///bit14->14-7，找到高字节中第一个不为0的位  
        && 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);  //11010101  
}    
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);    
}