From 7f4c3a651fdeb6037f2ee2f34e34cc70f6d7ded4 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Fri, 22 Feb 2019 09:05:07 +0100
Subject: [PATCH] added biquad highpass

---
 README.md |  4 ++--
 biquad.hh | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index e317dfb..2bbc0f2 100644
--- a/README.md
+++ b/README.md
@@ -37,8 +37,8 @@ There is also support for cascading, to improve [roll-off](https://en.wikipedia.
 
 The following [infinite impulse response](https://en.wikipedia.org/wiki/Infinite_impulse_response) [digital biquad filter](https://en.wikipedia.org/wiki/Digital_biquad_filter) implementations are available:
 
-* [second-order Butterworth low pass filter](https://en.wikipedia.org/wiki/Butterworth_filter)
-* [2n-order Butterworth cascade of second-order low pass filters](https://en.wikipedia.org/wiki/Butterworth_filter)
+* [second-order Butterworth low or high pass filter](https://en.wikipedia.org/wiki/Butterworth_filter)
+* [2n-order Butterworth cascade of second-order low or high pass filters](https://en.wikipedia.org/wiki/Butterworth_filter)
 
 ### [phasor.hh](phasor.hh)
 
diff --git a/biquad.hh b/biquad.hh
index e2db34a..c928b1e 100644
--- a/biquad.hh
+++ b/biquad.hh
@@ -37,6 +37,22 @@ public:
 		a1a0 = a1 / a0;
 		a2a0 = a2 / a0;
 	}
+	void highpass(int n, int N, VALUE Q = Const<VALUE>::InvSqrtTwo())
+	{
+		VALUE alpha = UnitCircle<VALUE>::sin(n, N) / (VALUE(2) * Q),
+			cn = UnitCircle<VALUE>::cos(n, N),
+			b0 = (VALUE(1) + cn) / VALUE(2),
+			b1 = -(VALUE(1) + cn),
+			b2 = (VALUE(1) + cn) / VALUE(2),
+			a0 = VALUE(1) + alpha,
+			a1 = -VALUE(2) * cn,
+			a2 = VALUE(1) - alpha;
+		b0a0 = b0 / a0;
+		b1a0 = b1 / a0;
+		b2a0 = b2 / a0;
+		a1a0 = a1 / a0;
+		a2a0 = a2 / a0;
+	}
 	TYPE operator()(TYPE x0)
 	{
 		TYPE y0 = b0a0*x0 + b1a0*x1 + b2a0*x2
@@ -59,6 +75,12 @@ public:
 			cascade[i].lowpass(n, N, VALUE(1) / (VALUE(2) *
 				UnitCircle<VALUE>::cos(2*i+1, 4*ORDER)));
 	}
+	void highpass(int n, int N)
+	{
+		for (int i = 0; i < NUM; ++i)
+			cascade[i].highpass(n, N, VALUE(1) / (VALUE(2) *
+				UnitCircle<VALUE>::cos(2*i+1, 4*ORDER)));
+	}
 	TYPE operator()(TYPE input)
 	{
 		for (int i = 0; i < NUM; ++i)