moth/packages/p2/src/modem.c

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2012-05-29 22:45:20 -06:00
#include <unistd.h>
#include <stdlib.h>
#define NOISE_PROB 300
#define NOISE_BITS 16
int badbits = 0;
char
line_noise(char c)
{
int i = 7;
while (badbits && (i >= 0)) {
c = c ^ ((rand() % 2) << i);
badbits -= 1;
i -= 1;
}
if (rand() % NOISE_PROB == 0) {
badbits = rand() % NOISE_BITS;
}
return c;
}
int
main(int argc, char *argv[])
{
char c;
ssize_t ret;
int baud = 0;
useconds_t usec;
if (argv[1]) {
baud = atoi(argv[1]);
}
if (! baud) {
baud = 1200;
}
srandom(getpid());
/*
N81 uses 1 stop bit, and 1 parity bit. That works out to
exactly 10 bits per byte.
*/
usec = 10000000 / baud;
while (1) {
ret = read(0, &c, 1);
if (ret != 1) {
break;
}
c = line_noise(c);
write(1, &c, 1);
usleep(usec);
}
return 0;
}