Program to generate timecode

mktimecode.c

+/*
+ * Copyright (C) 2011 Mark Hills <mark@pogo.org.uk>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program 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 version 2 for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 2 along with this program; if not, write to the Free
+ * Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+ * MA 02110-1301, USA.
+ *
+ */
+
+/*
+ * 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>
+
+#define BANNER "xwax timecode generator " \
+    "(C) Copyright 2011 Mark Hills <mark@pogo.org.uk>"
+
+#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, "\n");
+    fprintf(stderr, ".resolution = %d,\n", RESOLUTION);
+    fprintf(stderr, ".flags = 0,\n");
+    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));
+
+    return 0;
+}