X-Git-Url: https://git.sesse.net/?p=c64tapwav;a=blobdiff_plain;f=filter.cpp;h=ecd45025cc2441476243feb89b58c88d94fa617f;hp=d8f97010f2670dd2f4566fa5153f01223640672f;hb=0f23da84ab2a790a5a9450c1b05f6300362fc91c;hpb=cec99d15ad920457eea61bc442b5ba610c13430e

diff --git a/filter.cpp b/filter.cpp
index d8f9701..ecd4502 100644
--- a/filter.cpp
+++ b/filter.cpp
@@ -23,3 +23,20 @@ Filter Filter::lpf(float cutoff_radians)
 
 	return Filter(a0, a1, a2, b0, b1, b2);
 }
+
+Filter Filter::hpf(float cutoff_radians)
+{
+	float resonance = 1.0f / sqrt(2.0f);
+	float sn = sin(cutoff_radians), cs = cos(cutoff_radians);
+	float alpha = float(sn / (2 * resonance));
+
+	// coefficients for highpass filter
+	float a0 = 1 + alpha;
+	float b0 = (1 + cs) * 0.5f;
+	float b1 = -(1 + cs);
+	float b2 = b0;
+	float a1 = -2 * cs;
+	float a2 = 1 - alpha;
+
+	return Filter(a0, a1, a2, b0, b1, b2);
+}