From 2acfa0424af341c8c2aa7446dfb720aa5e74db31 Mon Sep 17 00:00:00 2001
From: "sgunderson@bigfoot.com" <>
Date: Tue, 20 Jul 2010 00:25:08 +0200
Subject: [PATCH] Lots of small but important changes to undertone detection:

 - When comparing peak amplitudes, compare interpolated peaks, not raw bins.
 - Find the best candidate bin _before_ sliding up or down one bin.
 - Verify that a candidate bin is indeed a local peak before testing it.
 - Remove some pointless out-of-bounds checks.
 - Formatting changes to undertone debug printouts.
 - Change the undertone limit from 10dB to 20dB.

Seems to work quite a lot better, especially for the low E string.
It's still not perfect, of course, and I'm sure there are regression
situations, but I think it's a significant improvement.
---
 pitchdetector.cpp | 56 ++++++++++++++++++++++++++++-------------------
 1 file changed, 34 insertions(+), 22 deletions(-)

diff --git a/pitchdetector.cpp b/pitchdetector.cpp
index a73fb86..8805840 100644
--- a/pitchdetector.cpp
+++ b/pitchdetector.cpp
@@ -93,9 +93,13 @@ std::pair<double, double> PitchDetector::find_peak(double *in, unsigned num_samp
 	if (best_bin == 0 || best_bin == num_samples / 2) {
 		return std::make_pair(-1.0, 0.0);
 	}
+	std::pair<double, double> peak = 
+		interpolate_peak(in[best_bin - 1],
+				 in[best_bin],
+				 in[best_bin + 1]);
 
 #if 0
-	printf("undertone strength: %+4.2f %+4.2f %+4.2f [%+4.2f] %+4.2f %+4.2f %+4.2f\n",
+	printf("undertone strength: %+6.2f %+6.2f %+6.2f [%+6.2f] %+6.2f %+6.2f %+6.2f\n",
 		20.0 * log10(in[best_bin*4] / fft_length),
 		20.0 * log10(in[best_bin*3] / fft_length),
 		20.0 * log10(in[best_bin*2] / fft_length),
@@ -105,35 +109,43 @@ std::pair<double, double> PitchDetector::find_peak(double *in, unsigned num_samp
 		20.0 * log10(in[best_bin/4] / fft_length));
 #endif
 
-	// see if we might have hit an overtone (set a limit of 10dB)
+	// see if we might have hit an overtone (set a limit of 20dB)
 	for (unsigned i = 6; i >= 1; --i) {
-		if (best_bin != best_bin / i &&
-		    20.0 * log10(in[best_bin] / in[best_bin / i]) < 10.0f &&
-		    best_bin / i >= 5) {
+		unsigned candidate_bin = best_bin / i;
+		if (best_bin == candidate_bin ||
+		    candidate_bin < 5 ||
+		    candidate_bin >= num_samples / 2 - 1) {
+			continue;
+		}
+
+		// Check that we indeed have a peak in this area.
+		if (candidate_bin - 1 != best_bin &&
+		    in[candidate_bin - 1] > in[candidate_bin] &&
+		    in[candidate_bin - 1] > in[candidate_bin - 2]) {
+			--candidate_bin;
+		} else if (candidate_bin + 1 != best_bin &&
+		           in[candidate_bin + 1] > in[candidate_bin] &&
+			   in[candidate_bin + 1] > in[candidate_bin + 2]) {
+			++candidate_bin;
+		} else if (in[candidate_bin] < in[candidate_bin - 1] ||
+		           in[candidate_bin] < in[candidate_bin + 1]) {
+			continue;
+		}
+
+		std::pair<double, double> candidate_peak = 
+			interpolate_peak(in[candidate_bin - 1],
+					 in[candidate_bin],
+					 in[candidate_bin + 1]);
+		if (candidate_peak.second + 20.0f > peak.second) {
 #if 0
 			printf("Overtone of degree %u!\n", i);
 #endif
-			best_bin /= i;
-
-			// consider sliding one bin up or down
-			if (best_bin > 1 && in[best_bin - 1] > in[best_bin] && in[best_bin - 1] > in[best_bin - 2]) {
-				--best_bin;
-			} else if (best_bin < num_samples / 2 - 1 && in[best_bin + 1] > in[best_bin] && in[best_bin + 1] > in[best_bin + 2]) {
-				++best_bin;
-			}
-			   
+			best_bin = candidate_bin;
+			peak = candidate_peak;
 			break;
 		}
 	}
 
-	if (best_bin == 0 || best_bin == num_samples / 2) {
-		return std::make_pair(-1.0, 0.0);
-	}
-	std::pair<double, double> peak = 
-		interpolate_peak(in[best_bin - 1],
-				 in[best_bin],
-				 in[best_bin + 1]);
-		
 	return std::make_pair(bin_to_freq(double(best_bin) + peak.first, num_samples), peak.second);
 }
 
-- 
2.39.2