g711.c

RTP协议

/*
 * This source code is a product of Sun Microsystems, Inc. and is provided
 * for unrestricted use.  Users may copy or modify this source code without
 * charge.
 *
 * SUN SOURCE CODE IS PROVIDED AS IS WITH NO WARRANTIES OF ANY KIND INCLUDING
 * THE WARRANTIES OF DESIGN, MERCHANTIBILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE, OR ARISING FROM A COURSE OF DEALING, USAGE OR TRADE PRACTICE.
 *
 * Sun source code is provided with no support and without any obligation on
 * the part of Sun Microsystems, Inc. to assist in its use, correction,
 * modification or enhancement.
 *
 * SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE
 * INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS SOFTWARE
 * OR ANY PART THEREOF.
 *
 * In no event will Sun Microsystems, Inc. be liable for any lost revenue
 * or profits or other special, indirect and consequential damages, even if
 * Sun has been advised of the possibility of such damages.
 *
 * Sun Microsystems, Inc.
 * 2550 Garcia Avenue
 * Mountain View, California  94043
 */

/*
 * g711.c
 *
 * u-law, A-law and linear PCM conversions.
 */
#define	SIGN_BIT	(0x80)		/* Sign bit for a A-law byte. */
#define	QUANT_MASK	(0xf)		/* Quantization field mask. */
#define	NSEGS		(8)		/* Number of A-law segments. */
#define	SEG_SHIFT	(4)		/* Left shift for segment number. */
#define	SEG_MASK	(0x70)		/* Segment field mask. */

static short seg_end[8] = {0xFF, 0x1FF, 0x3FF, 0x7FF,
                           0xFFF, 0x1FFF, 0x3FFF, 0x7FFF};

/* copy from CCITT G.711 specifications */
unsigned char _u2a[128] = {			/* u- to A-law conversions */
    1, 1, 2, 2, 3, 3, 4, 4,
    5, 5, 6, 6, 7, 7, 8, 8,
    9, 10, 11, 12, 13, 14, 15, 16,
    17, 18, 19, 20, 21, 22, 23, 24,
    25, 27, 29, 31, 33, 34, 35, 36,
    37, 38, 39, 40, 41, 42, 43, 44,
    46, 48, 49, 50, 51, 52, 53, 54,
    55, 56, 57, 58, 59, 60, 61, 62,
    64, 65, 66, 67, 68, 69, 70, 71,
    72, 73, 74, 75, 76, 77, 78, 79,
    81, 82, 83, 84, 85, 86, 87, 88,
    89, 90, 91, 92, 93, 94, 95, 96,
    97, 98, 99, 100, 101, 102, 103, 104,
    105, 106, 107, 108, 109, 110, 111, 112,
    113, 114, 115, 116, 117, 118, 119, 120,
    121, 122, 123, 124, 125, 126, 127, 128};

unsigned char _a2u[128] = {			/* A- to u-law conversions */
    1, 3, 5, 7, 9, 11, 13, 15,
    16, 17, 18, 19, 20, 21, 22, 23,
    24, 25, 26, 27, 28, 29, 30, 31,
    32, 32, 33, 33, 34, 34, 35, 35,
    36, 37, 38, 39, 40, 41, 42, 43,
    44, 45, 46, 47, 48, 48, 49, 49,
    50, 51, 52, 53, 54, 55, 56, 57,
    58, 59, 60, 61, 62, 63, 64, 64,
    65, 66, 67, 68, 69, 70, 71, 72,
    73, 74, 75, 76, 77, 78, 79, 79,
    80, 81, 82, 83, 84, 85, 86, 87,
    88, 89, 90, 91, 92, 93, 94, 95,
    96, 97, 98, 99, 100, 101, 102, 103,
    104, 105, 106, 107, 108, 109, 110, 111,
    112, 113, 114, 115, 116, 117, 118, 119,
    120, 121, 122, 123, 124, 125, 126, 127};

static int
search(
    int	val,
    short	*table,
    int	size)
{
    int	i;

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

/*
 * linear2alaw() - Convert a 16-bit linear PCM value to 8-bit A-law
 *
 * linear2alaw() accepts an 16-bit integer and encodes it as A-law data.
 *
 *		Linear Input Code	Compressed Code
 *	------------------------	---------------
 *	0000000wxyza			000wxyz
 *	0000001wxyza			001wxyz
 *	000001wxyzab			010wxyz
 *	00001wxyzabc			011wxyz
 *	0001wxyzabcd			100wxyz
 *	001wxyzabcde			101wxyz
 *	01wxyzabcdef			110wxyz
 *	1wxyzabcdefg			111wxyz
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 */
unsigned char
linear2alaw(
    int	pcm_val)	/* 2's complement (16-bit range) */
{
    int	mask;
    int	seg;
    unsigned char	aval;

    if (pcm_val >= 0)
    {
        mask = 0xD5; 		/* sign (7th) bit = 1 */
    }
    else
    {
        mask = 0x55; 		/* sign bit = 0 */
        pcm_val = -pcm_val - 8;
    }

    /* Convert the scaled magnitude to segment number. */
    seg = search(pcm_val, seg_end, 8);

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

    if (seg >= 8)		/* out of range, return maximum value. */
        return (0x7F ^ mask);
    else
    {
        aval = seg << SEG_SHIFT;
        if (seg < 2)
            aval |= (pcm_val >> 4) & QUANT_MASK;
        else
            aval |= (pcm_val >> (seg + 3)) & QUANT_MASK;
        return (aval ^ mask);
    }
}

/*
 * alaw2linear() - Convert an A-law value to 16-bit linear PCM
 *
 */
int
alaw2linear(
    unsigned char	a_val)
{
    int	t;
    int	seg;

    a_val ^= 0x55;

    t = (a_val & QUANT_MASK) << 4;
    seg = ((unsigned)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) ? t : -t);
}

#define	BIAS		(0x84)		/* Bias for linear code. */

/*
 * linear2ulaw() - Convert a linear PCM value to u-law
 *
 * In order to simplify the encoding process, the original linear magnitude
 * is biased by adding 33 which shifts the encoding range from (0 - 8158) to
 * (33 - 8191). The result can be seen in the following encoding table:
 *
 *	Biased Linear Input Code	Compressed Code
 *	------------------------	---------------
 *	00000001wxyza			000wxyz
 *	0000001wxyzab			001wxyz
 *	000001wxyzabc			010wxyz
 *	00001wxyzabcd			011wxyz
 *	0001wxyzabcde			100wxyz
 *	001wxyzabcdef			101wxyz
 *	01wxyzabcdefg			110wxyz
 *	1wxyzabcdefgh			111wxyz
 *
 * Each biased linear code has a leading 1 which identifies the segment
 * number. The value of the segment number is equal to 7 minus the number
 * of leading 0's. The quantization interval is directly available as the
 * four bits wxyz.  * The trailing bits (a - h) are ignored.
 *
 * Ordinarily the complement of the resulting code word is used for
 * transmission, and so the code word is complemented before it is returned.
 *
 * For further information see John C. Bellamy's Digital Telephony, 1982,
 * John Wiley & Sons, pps 98-111 and 472-476.
 */
unsigned char
linear2ulaw(
    int	pcm_val)	/* 2's complement (16-bit range) */
{
    int	mask;
    int	seg;
    unsigned char	uval;

    /* if someone has passed in a bum short (e.g. they manipulated an
     * int rather than a short, so the value looks positive, not
     * negative) fix that */
    if(pcm_val & 0x8000)
    {
	pcm_val = - (~pcm_val & 0x7fff) - 1;
    }

    /* Get the sign and the magnitude of the value. */
    if (pcm_val < 0)
    {
        pcm_val = BIAS - pcm_val;
        mask = 0x7F;
    }
    else
    {
        pcm_val += BIAS;
        mask = 0xFF;
    }

    /* Convert the scaled magnitude to segment number. */
    seg = search(pcm_val, seg_end, 8);

    /*
     * Combine the sign, segment, quantization bits;
     * and complement the code word.
     */
    if (seg >= 8)		/* out of range, return maximum value. */
        return (0x7F ^ mask);
    else
    {
        uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0xF);
        return (uval ^ mask);
    }

}

/*
 * ulaw2linear() - Convert a u-law value to 16-bit linear PCM
 *
 * First, a biased linear code is derived from the code word. An unbiased
 * output can then be obtained by subtracting 33 from the biased code.
 *
 * Note that this function expects to be passed the complement of the
 * original code word. This is in keeping with ISDN conventions.
 */
int
ulaw2linear(
    unsigned char	u_val)
{
    int	t;

    /* Complement to obtain normal u-law value. */
    u_val = ~u_val;

    /*
     * Extract and bias the quantization bits. Then
     * shift up by the segment number and subtract out the bias.
     */
    t = ((u_val & QUANT_MASK) << 3) + BIAS;
    t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;

    return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}

/* A-law to u-law conversion */
unsigned char
alaw2ulaw(
    unsigned char	aval)
{
    aval &= 0xff;
    return ((aval & 0x80) ? (0xFF ^ _a2u[aval ^ 0xD5]) :
            (0x7F ^ _a2u[aval ^ 0x55]));
}

/* u-law to A-law conversion */
unsigned char
ulaw2alaw(
    unsigned char	uval)
{
    uval &= 0xff;
    return ((uval & 0x80) ? (0xD5 ^ (_u2a[0xFF ^ uval] - 1)) :
            (0x55 ^ (_u2a[0x7F ^ uval] - 1)));
}