From d2b1bf32b46f1b08ac6e767ccf46cadde2545257 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Fri, 8 Feb 2019 20:21:42 +0100
Subject: [PATCH] added AM demodulator with AGC

---
 README.md |  4 ++++
 amd.hh    | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 amd.hh

diff --git a/README.md b/README.md
index 17cb821..cff171b 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,10 @@ The following [infinite impulse response](https://en.wikipedia.org/wiki/Infinite
 
 [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).
 
+### [amd.hh](amd.hh)
+
+[Amplitude modulation](https://en.wikipedia.org/wiki/Amplitude_modulation) [demodulation](https://en.wikipedia.org/wiki/Demodulation) with [automatic gain control](https://en.wikipedia.org/wiki/Automatic_gain_control).
+
 ### [atan2.hh](atan2.hh)
 
 [atan](https://en.wikipedia.org/wiki/Inverse_trigonometric_functions) and [atan2](https://en.wikipedia.org/wiki/Atan2).
diff --git a/amd.hh b/amd.hh
new file mode 100644
index 0000000..1048686
--- /dev/null
+++ b/amd.hh
@@ -0,0 +1,43 @@
+/*
+AM Demodulation
+
+Copyright 2019 Ahmet Inan <inan@aicodix.de>
+*/
+
+#pragma once
+
+namespace DSP {
+
+template <typename TYPE>
+class AMD1
+{
+	typedef TYPE complex_type;
+	typedef typename complex_type::value_type value_type;
+	value_type avg, att, dec, lo, hi, idx;
+public:
+	AMD1() : avg(0), att(0), dec(0), lo(0.001), hi(1), idx(1)
+	{
+	}
+	void index(value_type modidx)
+	{
+		idx = modidx;
+	}
+	void agc(value_type attack, value_type decay, value_type low = value_type(0.001), value_type high = value_type(1))
+	{
+		att = attack;
+		dec = decay;
+		lo = low;
+		hi = high;
+	}
+	value_type operator()(complex_type input)
+	{
+		value_type amp = abs(input);
+		avg = lerp(amp < avg ? dec : att, avg, amp);
+		avg = min(max(avg, lo), hi);
+		amp /= avg;
+		return (amp * (value_type(1)+idx) - value_type(1)) / idx;
+	}
+};
+
+}
+