Add an RC highpass filter, which seemingly does a good job of emulating the one in...

[c64tapwav] / filter.cpp

diff --git a/filter.cpp b/filter.cpp
index d8f97010f2670dd2f4566fa5153f01223640672f..ecd45025cc2441476243feb89b58c88d94fa617f 100644 (file)
--- a/filter.cpp
+++ b/filter.cpp
@@ -23,3 +23,20 @@ Filter Filter::lpf(float cutoff_radians)
 
        return Filter(a0, a1, a2, b0, b1, b2);
 }
 
        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);
+}