I'm working on a telemetry board and wish to include software FSK1200
(MX-614's are getting harder to find). The official Bell 202 spec is
out of print and hard to find as well, so I pieced this together from
what I can find. Multimon won't decode the output, so I'm likely doing
something wrong. This is supposed to be (once it works) my reference
implementation before I port it to my telemetry board, so I'm trying
to keep it as simple as possible.

The program outputs 22050 hz 8-bit signed data to stdout;
gcc fsk.c -o fsk -lm; ./fsk test; sox -s -b -r 22050 test -w
test.wav

Phase is coherent, and there are peaks at 1200 and 2200 in the
spectrum analysis.

Any help or suggestions would be appreciated.

Robert Cicconetti
KG4MVB

-- begin fsk.c --
#include math.h
#include stdio.h

// $#@$#@
//#define BAUDRATE 1200
// octet rate is 150 octet / sec. (1200 / 8). BIT period is
// .000666, or 1/1500 (10 bits per symbol, including start and stop).
// BAUDRATE is therefor used improperly here; fixme.
#define BAUDRATE 1500
#define MARK 1200
#define SPACE 2200

// For convenience when working with multimon
#define SAMPLERATE 22050

double sendbyte(char a, double angle);

int main (void)
{
// 8 bit output; encoder DAC is likely to be only 3 or 4 bit
short outbyte;
int i;
double angle = 0;
char test[]= "fm KG4MVB-0 to FFFFFF-0 SABM+";

for (i = 0 ; i 5000; i++) putchar(0);
//PREAMBLE
for (i = 0 ; i 24; i++) sendbyte(0, angle);

for (i = 0 ; i 29 ; i++)
{
angle = sendbyte(test[i], angle);
}

for (i = 0 ; i 2; i++) sendbyte(0, angle);
for (i = 0 ; i 50000; i++) putchar(0);
return 0;
}

double sendbyte(char a, double angle)
{
// LSB first. Start bit MARK end bit SPACE
// MARK is 1, SPACE is 0
//
double phase_offset = 0;
int freq, j = 0, i;
char bit;

for (i = 0; i 10; i++)
{
if (i == 0)
bit = 1;
else if (i == 9)
bit = 0;
else
bit =( a (i-1)) & 0x1;

if (bit == 1)
freq = MARK;
else
freq = SPACE;

phase_offset = angle -
(double)2*M_PI*freq*((double)(j-1)/SAMPLERATE);
while (j ((i+1)*SAMPLERATE)/BAUDRATE )
{
angle =
(double)2*M_PI*freq*((double)j/SAMPLERATE) + phase_offset;
putchar((short)(0.5*sin(angle)*128));
j++;
}
}
return angle;
}