From f7953a166b871c47d0c3a602142b8e498f9ce278 Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Thu, 2 May 2013 20:46:37 +0200
Subject: [PATCH] Add calibration support.

---
 decode.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/decode.c b/decode.c
index e20c8c9..923913d 100644
--- a/decode.c
+++ b/decode.c
@@ -8,6 +8,9 @@
 #define LANCZOS_RADIUS 30
 #define BUFSIZE 4096
 #define HYSTERESIS_LIMIT 1000
+#define SAMPLE_RATE 44100
+#define C64_FREQUENCY 985248
+#define TAP_RESOLUTION 8
 
 double sinc(double x)
 {
@@ -98,6 +101,10 @@ int main(int argc, char **argv)
 		printf("%d\n", in[i]);
 	}
 #endif
+
+	std::vector<float> pulse_lengths;  // in seconds
+
+	// Find the flanks.
 	int last_bit = -1;
 	double last_upflank = -1;
 	int last_max_level = 0;
@@ -105,11 +112,9 @@ int main(int argc, char **argv)
 		int bit = (pcm[i] > 0) ? 1 : 0;
 		if (bit == 1 && last_bit == 0 && last_max_level > HYSTERESIS_LIMIT) {
 			// up-flank!
-			double t = find_zerocrossing(pcm, i - 1) * (123156.0/44100.0);
+			double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
 			if (last_upflank > 0) {
-//				fprintf(stderr, "length: %f (0x%x)\n", t - last_upflank, lrintf(t - last_upflank));
-				int len = lrintf(t - last_upflank);
-				printf("0x%x\n", len);
+				pulse_lengths.push_back(t - last_upflank);
 			}
 			last_upflank = t;
 			last_max_level = 0;
@@ -117,4 +122,25 @@ int main(int argc, char **argv)
 		last_max_level = std::max(last_max_level, abs(pcm[i]));
 		last_bit = bit;
 	}
+
+	// Calibrate on the first ~15k pulses (skip a few, just to be sure).
+	float calibration_factor = 1.0f;
+	if (pulse_lengths.size() < 20000) {
+		fprintf(stderr, "Too few pulses, not calibrating!\n");
+	} else {
+		double sum = 0.0;
+		for (int i = 1000; i < 16000; ++i) {
+			sum += pulse_lengths[i];
+		}
+		double mean_length = C64_FREQUENCY * sum / 15000.0f;
+		calibration_factor = 380.0 / mean_length;
+		fprintf(stderr, "Cycle length: %.2f -> 380.0 (change %+.2f%%)\n",
+			mean_length, 100.0 * (calibration_factor - 1.0));
+	}
+
+	for (int i = 0; i < pulse_lengths.size(); ++i) {
+		int len = lrintf(pulse_lengths[i] * calibration_factor * C64_FREQUENCY / TAP_RESOLUTION);
+		//fprintf(stderr, "length: %f (0x%x)\n", t - last_upflank, len);
+		printf("0x%x\n", len);
+	}
 }
-- 
2.39.2