音频格式G711转PCM的代码
　　今天调试摄像头的音频格式，始终不对。原始音频为8000，16位，帧率50。每帧的数据应该是320字节。

接收到手上，RTP包为171字节，数据为160，只有一半。这是怎么回事？一开始总是认为RTP接收有问题，反复核查确认代码及流程正确。那么就是格式进行了转换。什么格式？开始认为是AAC，后来发现不对，AAC压缩率更高。

最后根据这个160、 320，搜索了一个，说G711会变成原来的一半。代码如下：

public static byte[] convertG711ToPcm(byte[] g711Buffer, int length, byte[] pcmBuffer)
	{
		if (pcmBuffer == null)
		{
			pcmBuffer = new byte[length*2];
		}
		for (int i=0; i<length; i++)
		{
			byte alaw = g711Buffer[i];
			alaw ^= 0xD5;
			
			int sign     =  alaw & 0x80;
			int exponent = (alaw & 0x70) >> 4;
			int value    = (alaw & 0x0F) >> 4 + 8;
			if (exponent != 0)
			{
				value += 0x0100;
			}
			if (exponent > 1)
			{
				value <<= (exponent - 1);
			}
			value = (char)((sign == 0 ? value : -value) & 0xFFFF);
			pcmBuffer[i*2+0] = (byte) (value      & 0xFF);
			pcmBuffer[i*2+1] = (byte) (value >> 8 & 0xFF);
		}
		return pcmBuffer;
	}

public class G711Code {
    private final static int SIGN_BIT = 0x80;
    private final static int QUANT_MASK = 0xf;
    private final static int SEG_SHIFT = 4;
    private final static int SEG_MASK = 0x70;

    static short[] seg_end = {0xFF, 0x1FF, 0x3FF, 0x7FF,0xFFF, 0x1FFF, 0x3FFF, 0x7FFF};

    static short search(short val,short[] table,short size){

        for (short i = 0 ; i < size; i++) {
            if(val <= table[i]){
                return i;
            }
        }
        return size;
    }

    static byte linear2alaw(short pcm_val){
        short mask;
        short seg;
        char aval;
        if(pcm_val >= 0){
            mask = 0xD5;   //* sign (7th) bit = 1 二进制的11010101
        }else{
            mask = 0x55; //* sign bit = 0  二进制的01010101
            pcm_val = (short) (-pcm_val - 1);//负数转换为正数计算
            if(pcm_val < 0){
                pcm_val = 32767;
            }
        }

        /* Convert the scaled magnitude to segment number. */
        seg = search(pcm_val, seg_end, (short) 8); //查找采样值对应哪一段折线

        /* Combine the sign, segment, and quantization bits. */

        if (seg >= 8)       /* out of range, return maximum value. */
            return (byte) (0x7F ^ mask);
        else {
            //以下按照表格第一二列进行处理，低4位是数据，5~7位是指数，最高位是符号
            aval = (char) (seg << SEG_SHIFT);
            if (seg < 2)
                aval |= (pcm_val >> 4) & QUANT_MASK;
            else
                aval |= (pcm_val >> (seg + 3)) & QUANT_MASK;
            return (byte) (aval ^ mask);
        }
    }


    static short alaw2linear(byte a_val){
        short       t;
        short       seg;

        a_val ^= 0x55;

        t = (short) ((a_val & QUANT_MASK) << 4);
        seg = (short) ((a_val & SEG_MASK) >> SEG_SHIFT);
        switch (seg) {
            case 0:
                t += 8;
                break;
            case 1:
                t += 0x108;
                break;
            default:
                t += 0x108;
                t <<= seg - 1;
        }
        return (a_val & SIGN_BIT) != 0 ? t : (short) -t;
    }

    /**
     * pcm 转 G711 a率
     * @param pcm
     * @param code
     * @param size
     */
    public static void G711aEncoder(short[] pcm,byte[] code,int size){
        for(int i=0;i<size;i++){
            code[i]=linear2alaw(pcm[i]);
        }
    }

    /**
     * G.711 转 PCM
     * @param pcm
     * @param code
     * @param size
     */
    public static void G711aDecoder(short[] pcm,byte[] code,int size)
    {
        for(int i=0;i<size;i++){
            pcm[i]=alaw2linear(code[i]);
        }
    }

}