alaw.cpp

DTMF检测程序

#include "alaw.h"

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

/*	Linear Code		Compressed Code
	---------------		---------------
	0000000wxyza		000wxyz
	0000001wxyza		001wxyz
	000001wxyzab		010wxyz
	00001wxyzabc		011wxyz
	0001wxyzabcd		100wxyz
	001wxyzabcde		101wxyz
	01wxyzabcdef		110wxyz
	1wxyzabcdefg		111wxyz
*/


inline int search(int val, const short *table, int size)
{
	int i;
	
	for (i = 0; i < size; i++) {
		if (val <= *table++)
			return (i);
	}
	return (size);
}

unsigned char linear2alaw(short pcm_val)     /* 2's complement (16-bit range) */
{
	int mask;
	int seg;
	unsigned char aval;

	pcm_val &= 0xfff8;

	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, alaw_seg_end, NSEGS);

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

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

short alaw2linear(unsigned char a_val)
{
	short 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);
}