alawdecoderutil.java

FMJ（freedom media for java)是java视频开发的新选择

package net.sf.fmj.media.codec.audio.alaw;


/**
 *  Turns 8-bit A-law bytes back into 16-bit PCM values.
 *  
 * Adapted from code by Marc Sweetgall at http://www.codeproject.com/csharp/g711audio.asp
 */
public class ALawDecoderUtil
{
    /**
     *  An array where the index is the a-law input, and the value is
     *  the 16-bit PCM result.
     *  
     */
    private static short[] aLawToPcmMap;

    static
    {
        aLawToPcmMap = new short[256];
        for (int i = 0; i < aLawToPcmMap.length; i++)
            aLawToPcmMap[i] = decode((byte) i);
    }

    /**
     *  Decode one a-law byte. For internal use only.
     *  
     *  @param alaw The encoded a-law byte
     *  @return A short containing the 16-bit result
     */
    private static short decode(byte alaw)
    {
        //Invert every other bit, and the sign bit (0xD5 = 1101 0101)
        alaw ^= 0xD5;

        //Pull out the value of the sign bit
        int sign = alaw & 0x80;
        //Pull out and shift over the value of the exponent
        int exponent = (alaw & 0x70) >> 4;
        //Pull out the four bits of data
        int data = alaw & 0x0f;

        //Shift the data four bits to the left
        data <<= 4;
        //Add 8 to put the result in the middle of the range (like adding a half)
        data += 8;
        
        //If the exponent is not 0, then we know the four bits followed a 1,
        //and can thus add this implicit 1 with 0x100.
        if (exponent != 0)
            data += 0x100;
        /* Shift the bits to where they need to be: left (exponent - 1) places
         * Why (exponent - 1) ?
         * 1 2 3 4 5 6 7 8 9 A B C D E F G
         * . 7 6 5 4 3 2 1 . . . . . . . . <-- starting bit (based on exponent)
         * . . . . . . . Z x x x x 1 0 0 0 <-- our data (Z is 0 only when exponent is 0)
         * We need to move the one under the value of the exponent,
         * which means it must move (exponent - 1) times
         * It also means shifting is unnecessary if exponent is 0 or 1.
         */
        if (exponent > 1)
            data <<= (exponent - 1);

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

    /**
     *  Decode one a-law byte
     *  
     *  @param alaw The encoded a-law byte
     *  @return A short containing the 16-bit result
     */
    public static short aLawDecode(byte alaw)
    {
        return aLawToPcmMap[alaw & 0xff];
    }

//    /**
//     *  Decode an array of a-law encoded bytes
//     *  
//     *  @param data An array of a-law encoded bytes
//     *  @return An array of shorts containing the results
//     */
//    public static short[] aLawDecode(byte[] data)
//    {
//        int size = data.length;
//        short[] decoded = new short[size];
//        for (int i = 0; i < size; i++)
//            decoded[i] = aLawToPcmMap[data[i]];
//        return decoded;
//    }
//
//    /**
//     *  Decode an array of a-law encoded bytes
//     *  
//     *  @param data An array of a-law encoded bytes
//     *  @param decoded An array of shorts containing the results
//     *  Same as the other method that returns an array of shorts
//     */
//    public static void aLawDecode(byte[] data, short[] decoded)
//    {
//        int size = data.length;
//       for (int i = 0; i < size; i++)
//            decoded[i] = aLawToPcmMap[data[i] & 0xff];
//    }

    public static void aLawDecode(boolean bigEndian, byte[] data, int offset, int length, byte[] decoded)
    {
    	if (bigEndian)
    		aLawDecodeBigEndian(data, offset, length, decoded);
    	else
    		aLawDecodeLittleEndian(data, offset, length, decoded);
    }
    
    /**
     *  Decode an array of a-law encoded bytes
     *  
     *  @param data An array of a-law encoded bytes
     *  @param decoded An array of bytes in Little-Endian format containing the results.  Should be twice as big as data.
     */
    public static void aLawDecodeLittleEndian(byte[] data, int offset, int length, byte[] decoded)
    {
        int size = length;
        for (int i = 0; i < size; i++)
        {
            //First byte is the less significant byte
            decoded[2 * i] = (byte)(aLawToPcmMap[data[offset + i] & 0xff] & 0xff);
            //Second byte is the more significant byte
            decoded[2 * i + 1] = (byte)(aLawToPcmMap[data[offset + i] & 0xff] >> 8);
        }
    }
    
    public static void aLawDecodeBigEndian(byte[] data, int offset, int length, byte[] decoded)
    {
        int size = length;
        for (int i = 0; i < size; i++)
        {
            decoded[2 * i + 1] = (byte)(aLawToPcmMap[data[offset + i] & 0xff] & 0xff);
            decoded[2 * i] = (byte)(aLawToPcmMap[data[offset + i] & 0xff] >> 8);
        }
    }
}