From 00501fa4333d8c71b317eb438f2bee744e547bc9 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Thu, 3 Jan 2019 21:13:56 +0100
Subject: [PATCH] added FIR LPF, HPF and BPF

---
 README.md |  7 +++++++
 filter.hh | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 filter.hh

diff --git a/README.md b/README.md
index 97e6cb7..7847b75 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,13 @@ Implemented are the follwing [Window functions](https://en.wikipedia.org/wiki/Wi
 * [Gaussian window](https://en.wikipedia.org/wiki/Window_function#Gaussian_window)
 * [Kaiser window](https://en.wikipedia.org/wiki/Window_function#Kaiser_window)
 
+### [filter.hh](filter.hh)
+
+Implemented are the following [finite impulse response](https://en.wikipedia.org/wiki/Finite_impulse_response) [filters](https://en.wikipedia.org/wiki/Filter_(signal_processing)):
+* [low-pass filter](https://en.wikipedia.org/wiki/Low-pass_filter)
+* [high-pass filter](https://en.wikipedia.org/wiki/High-pass_filter)
+* [band-pass filter](https://en.wikipedia.org/wiki/Band-pass_filter)
+
 ### [const.hh](const.hh)
 
 Some constants we need
diff --git a/filter.hh b/filter.hh
new file mode 100644
index 0000000..dc0fc0d
--- /dev/null
+++ b/filter.hh
@@ -0,0 +1,61 @@
+/*
+Some finite impulse response filter functions
+
+Copyright 2018 Ahmet Inan <inan@aicodix.de>
+*/
+
+#ifndef FILTER_HH
+#define FILTER_HH
+
+#include "const.hh"
+#include "utils.hh"
+#include "coeffs.hh"
+
+namespace DSP {
+
+template <typename TYPE>
+class LowPass : public CoeffsFunc<TYPE>
+{
+	TYPE f;
+public:
+	LowPass(TYPE cutoff) : f(TYPE(2) * cutoff) {}
+	TYPE operator () (int n, int N)
+	{
+		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
+		return f * sinc(f * x);
+	}
+};
+
+template <typename TYPE>
+class HighPass : public CoeffsFunc<TYPE>
+{
+	TYPE f;
+public:
+	HighPass(TYPE cutoff) : f(TYPE(2) * cutoff) {}
+	TYPE operator () (int n, int N)
+	{
+		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
+		// if (N%1) return delta(x) - f * sinc(f * x);
+		return sinc(x) - f * sinc(f * x);
+
+	}
+};
+
+template <typename TYPE>
+class BandPass : public CoeffsFunc<TYPE>
+{
+	TYPE f0, f1;
+public:
+	BandPass(TYPE cutoff0, TYPE cutoff1) :
+		f0(TYPE(2) * cutoff0), f1(TYPE(2) * cutoff1) {}
+	TYPE operator () (int n, int N)
+	{
+		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
+		return f1 * sinc(f1 * x) - f0 * sinc(f0 * x);
+	}
+};
+
+}
+
+#endif
+