rate.c

IP网络语音通讯软件源代码. 不可多得的语音源代码

/*
 * July 5, 1991
 * Copyright 1991 Lance Norskog And Sundry Contributors
 * This source code is freely redistributable and may be used for
 * any purpose.  This copyright notice must be maintained. 
 * Lance Norskog And Sundry Contributors are not responsible for 
 * the consequences of using this software.
 */

/*

 * Modified for use in Speak Freely by John Walker

 * Sound Tools rate change effect file.
 */

#include "netfone.h"

/*
 * Least Common Multiple Linear Interpolation 
 *
 * Find least common multiple of the two sample rates.
 * Construct the signal at the LCM by interpolating successive
 * input samples as straight lines.  Pull output samples from
 * this line at output rate.
 *
 * Of course, actually calculate only the output samples.
 *
 * LCM must be 32 bits or less.  Two prime number sample rates
 * between 32768 and 65535 will yield a 32-bit LCM, so this is 
 * stretching it.
 */

/*
 * Algorithm:
 *	
 *	Generate a master sample clock from the LCM of the two rates.
 *	Interpolate linearly along it.	Count up input and output skips.
 *
 *	Input:	 |inskip |		 |		 |		 |		 |
 *																		
 *																		
 *																		
 *	LCM:	 |	 |	 |	 |	 |	 |	 |	 |	 |	 |	 |
 *																		
 *																		
 *																		
 *	Output:  |	outskip  |			 |			 | 
 *
 *																		
 */

/* here for linear interp.	might be useful for other things */

static long gcd(a, b) 
  long a, b;
{
		if (b == 0)
				return a;
		else
				return gcd(b, a % b);
}

static long lcm(a, b) 
  long a, b;
{
		return (a * b) / gcd(a, b);
}

/*
 * Prepare processing.
 */
void rate_start(rate_t *rate, int inrate, int outrate)
{
		rate->lcmrate = lcm((long) inrate, (long) outrate);
		/* Cursory check for LCM overflow.	
		 * If both rate are below 65k, there should be no problem.
		 * 16 bits x 16 bits = 32 bits, which we can handle.
		 */
		rate->inskip = rate->lcmrate / inrate;
		rate->outskip = rate->lcmrate / outrate; 
		rate->total = rate->intot = rate->outtot = 0;
		rate->lastsamp = 0;
}

/*
 * Processed signed long samples from ibuf to obuf.
 * Return number of samples processed.
 */

void rate_flow(rate_t *rate,
			   unsigned char *ibuf, unsigned char *obuf,
			   int *isamp, int *osamp)
{
		int len, done;
		unsigned char *istart = ibuf;
		long last;
		

		done = 0;

		if (rate->total == 0) {
				/* Emit first sample.  We know the fence posts meet. */
				*obuf = *ibuf++;
				/* Count up until have right input samples */
				rate->lastsamp = audio_u2s(*obuf++);
				done = 1;
				rate->total = 1;
				/* advance to second output */
				rate->outtot += rate->outskip;
				/* advance input range to span next output */
				while ((rate->intot + rate->inskip) <= rate->outtot){
						rate->lastsamp = audio_u2s(*ibuf++);
//						ibuf++;
						rate->intot += rate->inskip;
				}
		}

		/* number of output samples the input can feed */

		len = (*isamp * rate->inskip) / rate->outskip;
		if (len > *osamp)
				len = *osamp;
		last = rate->lastsamp;
		for(; done < len; done++) {
				int osl;

/*				osl = last;*/
				osl = last + ((long) (audio_u2s(*ibuf) - last) *
						((long) rate->outtot - rate->intot)) / rate->inskip;
				*obuf++ = audio_s2u(osl);
				/* advance to next output */
				rate->outtot += rate->outskip;
				/* advance input range to span next output */
				while ((rate->intot + rate->inskip) <= rate->outtot){
						last = audio_u2s(*ibuf++);
						rate->intot += rate->inskip;
						if (ibuf - istart == *isamp)
								goto out;
				}
                /* long samples with high LCM's overrun counters! */
				if (rate->outtot == rate->intot)
						rate->outtot = rate->intot = 0;
		}
out:
		*isamp = ibuf - istart;
		*osamp = len;
		rate->lastsamp = last;
}