From cd7d55a2c4a2ac1e65269bdbeff53dd393dc8a73 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Wed, 3 Apr 2019 20:53:17 +0200
Subject: [PATCH] added Fs/4 complex down conversion

---
 README.md |  4 ++++
 cdc.hh    | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 cdc.hh

diff --git a/README.md b/README.md
index 907af11..ac3cfe6 100644
--- a/README.md
+++ b/README.md
@@ -56,6 +56,10 @@ Normalizers for [periodic](https://en.wikipedia.org/wiki/Periodic_function) sign
 
 [Numerically controlled oscillator](https://en.wikipedia.org/wiki/Numerically_controlled_oscillator) implemented using a [phasor](https://en.wikipedia.org/wiki/Phasor) and [complex multiplication](https://en.wikipedia.org/wiki/Complex_number#Multiplication) instead of a [lookup table](https://en.wikipedia.org/wiki/Lookup_table).
 
+### [cdc.hh](cdc.hh)
+
+Fs/4 [Complex down conversion](https://en.wikipedia.org/wiki/Digital_down_converter)
+
 ### [fmd.hh](fmd.hh)
 
 [Frequency modulation](https://en.wikipedia.org/wiki/Frequency_modulation) [demodulation](https://en.wikipedia.org/wiki/Demodulation) with and without [atan2](https://en.wikipedia.org/wiki/Atan2).
diff --git a/cdc.hh b/cdc.hh
new file mode 100644
index 0000000..1d56553
--- /dev/null
+++ b/cdc.hh
@@ -0,0 +1,51 @@
+/*
+Fs/4 Complex down conversion
+
+Copyright 2019 Ahmet Inan <inan@aicodix.de>
+*/
+
+#pragma once
+
+#include "filter.hh"
+#include "window.hh"
+
+namespace DSP {
+
+template <typename TYPE, int TAPS>
+class CDC
+{
+	typedef TYPE complex_type;
+	typedef typename TYPE::value_type value_type;
+	value_type real[(TAPS-1)/4];
+	value_type imag[(TAPS-1)/2];
+	value_type imco[(TAPS-1)/4];
+	value_type reco;
+	bool sign;
+public:
+	CDC(value_type a = value_type(2)) : sign(false)
+	{
+		Kaiser<value_type> win(a);
+		LowPass2<value_type> hbf(1, 4);
+		reco = win((TAPS-1)/2, TAPS) * hbf((TAPS-1)/2, TAPS);
+		for (int i = 0; i < (TAPS-1)/4; ++i)
+			imco[i] = win(2*i+(TAPS-1)/2+1, TAPS) * hbf(2*i+(TAPS-1)/2+1, TAPS);
+	}
+	complex_type operator()(value_type in0, value_type in1)
+	{
+		sign = !sign;
+		value_type re = reco * real[0];
+		for (int i = 0; i < (TAPS-1)/4-1; ++i)
+			real[i] = real[i+1];
+		real[(TAPS-1)/4-1] = sign ? in1 : -in1;
+		for (int i = 0; i < (TAPS-1)/2-1; ++i)
+			imag[i] = imag[i+1];
+		imag[(TAPS-1)/2-1] = sign ? in0 : -in0;
+		value_type im = imco[0] * (imag[(TAPS-1)/4-1] + imag[(TAPS-1)/4]);
+		for (int i = 1; i < (TAPS-1)/4; ++i)
+			im += imco[i] * (imag[(TAPS-1)/4-1-i] + imag[(TAPS-1)/4+i]);
+		return complex_type(re, im);
+	}
+};
+
+}
+