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/*
* Copyright (C) 2026 Mark Hills <mark@xwax.org>
*
* This file is part of "xwax".
*
* "xwax" is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License, version 3 as
* published by the Free Software Foundation.
*
* "xwax" is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <https://www.gnu.org/licenses/>.
*
*/
/*
* Experimental program to generate a timecode signal for use
* with xwax.
*/
#define _GNU_SOURCE /* sincos() */
#include <assert.h>
#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#define BANNER "xwax timecode generator " \
"(C) Copyright 2026 Mark Hills <mark@xwax.org>"
#define RATE 44100
#define RESOLUTION 4000
#define SEED 0x00001
#define TAPS 0x00002
#define BITS 22
#define MAX(x,y) ((x)>(y)?(x):(y))
typedef unsigned int bits_t;
/*
* Calculate the next bit in the LFSR sequence
*/
static inline bits_t lfsr(bits_t code, bits_t taps)
{
bits_t taken;
int xrs;
taken = code & taps;
xrs = 0;
while (taken != 0x0) {
xrs += taken & 0x1;
taken >>= 1;
}
return xrs & 0x1;
}
/*
* LFSR in the forward direction
*/
static inline bits_t fwd(bits_t current, bits_t taps, unsigned int nbits)
{
bits_t l;
/* New bits are added at the MSB; shift right by one */
l = lfsr(current, taps | 0x1);
return (current >> 1) | (l << (nbits - 1));
}
static inline double dither(void)
{
return (double)(rand() % 32768) / 32768.0 - 0.5;
}
int main(int argc, char *argv[])
{
unsigned int s;
bits_t b;
int length;
fputs(BANNER, stderr);
fputc('\n', stderr);
fprintf(stderr, "Generating %d-bit %dHz timecode sampled at %dKhz\n",
BITS, RESOLUTION, RATE);
b = SEED;
length = 0;
for (s = 0; ; s++) {
double time, cycle, angle, modulate, x, y;
signed short c[2];
time = (double)s / RATE;
cycle = time * RESOLUTION;
angle = cycle * M_PI * 2;
sincos(angle, &x, &y);
/* Modulate the waveform according to the bitstream */
modulate = 1.0 - (-cos(angle) + 1.0) * 0.25 * ((b & 0x1) == 0);
x *= modulate;
y *= modulate;
/* Write out 16-bit PCM data */
c[0] = -y * SHRT_MAX * 0.5 + dither();
c[1] = x * SHRT_MAX * 0.5 + dither();
fwrite(c, sizeof(signed short), 2, stdout);
/* Advance the bitstream if required */
if ((int)cycle > length) {
assert((int)cycle - length == 1);
b = fwd(b, TAPS, BITS);
if (b == SEED) /* LFSR period reached */
break;
length = cycle;
}
}
fprintf(stderr, "Generated %0.1f seconds of timecode\n",
(double)length / RESOLUTION);
fprintf(stderr, "\n");
fprintf(stderr, " {\n");
fprintf(stderr, " .resolution = %d,\n", RESOLUTION);
fprintf(stderr, " .bits = %d,\n", BITS);
fprintf(stderr, " .seed = 0x%08x,\n", SEED);
fprintf(stderr, " .taps = 0x%08x,\n", TAPS);
fprintf(stderr, " .length = %d,\n", length);
fprintf(stderr, " .safe = %d,\n", MAX(0, length - 4 * RESOLUTION));
fprintf(stderr, " }\n");
return 0;
}
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