Using a serial port as a sound card with a 2nd order delta-sigma DAC

sd-2nd-block.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <strings.h>

/* Written by Robert Jacobs sometime in 2008. Nothing new to me, I just
   learned about delta-sigma modulation around that time.

   Released under terms of CC0

   Credit is appreciated but unnecessary.
*/

// Scaled up because we lose two bits out of every ten, so we need to be a
// little quieter so that the remaining 8 can carry the other weight
#define FBN -32768*10/8
#define FBP 32767*10/8

#define BKSZ 128

int main (int argc, char **argv) {
	short in[BKSZ * 10];
	unsigned char * out;
	int i,t,p,d;
	int ratio,counter;
	int an,d1,i1,d2,i2,fb; /* Must be 32-bit to deal with 17-bit differenes */

	/* Can't test stdout: ttyS0 is a tty. */
	if (isatty(0)) {
		fprintf(stderr,
		        "use: sd-2nd [us] < audioin > serialdev\n"			\
		        " audioin should be-\n"					\
		        "  * the same sample rate as the serial dev's bitrate\n"	\
		        "  * us (default 1) is the upsample ratio\n"		\
		        "  * 16bit signed native-endian\n"				\
		        "  * headerless, single channel\n");
		return 1|(isatty(0)?2:0)|(isatty(1)?4:0);
	}

	if (argc==1)
		ratio=1;
	else
		ratio=atoi(argv[1]);

	if (ratio<1) {
		fprintf(stderr,"Can only upsample, not %d\n",ratio);
		return 8;
	}
 
	out = (unsigned char *)malloc(BKSZ*ratio);
	if (out==NULL) {
		fprintf(stderr,"Dynamic memory allocation of %d kiBytes failed",2*ratio);
		return 9;
	}

	an=d1=i1=d2=i2=fb=0;

	/* Xn1 Serial: StartLow 0 1 2 ... X-2 X-1 StopHigh */

	while (fread(in,20 * BKSZ,1,stdin)==1) { //BKSZ0 uint16s = 40960 bytes
		bzero(out,BKSZ*ratio);
		for (counter=ratio,d=p=0;p<BKSZ*ratio;p++) {
			{ /* Start bit always low */
				if (!counter) { counter=ratio; d++; } counter--;
				an=in[d];
				d1=an-fb;
				i1+=d1;
				d2=i1-fb;
				i2+=d2;
				fb=FBN;
			}
			for (i=0;i<8;i++) { /* Always assume 8N1 */
				if (!counter) { counter=ratio; d++; } counter--;
				an=in[d];
				d1=an-fb;
				i1+=d1;
				d2=i1-fb;
				i2+=d2;
				if (i2>0) {
					out[p] |= (1<<i);
					fb=FBP;
				} else
					fb=FBN;
			}
			{ /* Stop bit always high */
				if (!counter) { counter=ratio; d++; } counter--;
				an=in[d];
				d1=an-fb;
				i1+=d1;
				d2=i1-fb;
				i2+=d2;
				fb=FBP;
			}
		}
		fwrite(out,BKSZ*ratio,1,stdout);
	}
	return 0;
}