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diff --git a/arts/modules/synth/synth_fm_source_impl.cc b/arts/modules/synth/synth_fm_source_impl.cc
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+++ b/arts/modules/synth/synth_fm_source_impl.cc
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+    /*
+
+    Copyright (C) 2000 Jeff Tranter
+                       tranter@pobox.com
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Library General Public
+    License as published by the Free Software Foundation; either
+    version 2 of the License, or (at your option) any later version.
+  
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Library General Public License for more details.
+   
+    You should have received a copy of the GNU Library General Public License
+    along with this library; see the file COPYING.LIB.  If not, write to
+    the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+    Boston, MA 02111-1307, USA.
+
+    */
+
+#include "artsmodulessynth.h"
+#include "stdsynthmodule.h"
+
+using namespace Arts;
+
+// This is used for frequency modulation. Put your frequency to the
+// frequency input and put another signal on the modulator input. Then
+// set modlevel to something, say 0.3. The frequency will be modulated
+// with modulator then. Just try it. Works nice when you put a
+// feedback in there, that means take a combination of the delayed
+// output signal from the Synth_FM_SOURCE (you need to put it to some
+// oscillator as it only takes the role of Synth_FREQUENCY) and some
+// other signal to get good results. Works nicely in combination with
+// Synth_WAVE_SIN oscillators.
+
+class Synth_FM_SOURCE_impl : virtual public Synth_FM_SOURCE_skel,
+							virtual public StdSynthModule
+{
+protected:
+	static const int SAMPLINGRATE = 44100;
+	float posn;
+ 
+public:
+	void streamInit() { posn = 0; }
+
+	void calculateBlock(unsigned long samples)
+	{
+		for (unsigned long i=0; i<samples; i++)
+		{
+			float pinc = frequency[i] / (float) SAMPLINGRATE;
+			posn += pinc;
+			if (posn > 1)
+				posn -= 1;
+			pos[i] = posn + modulator[i] * modlevel[i];
+		}
+	}
+};
+
+REGISTER_IMPLEMENTATION(Synth_FM_SOURCE_impl);