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diff --git a/src/fsk.h b/src/fsk.h
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+/*
+    Copyright (C) 2020 Carl Hetherington <cth@carlh.net>
+
+    This file is part of libdcp.
+
+    libdcp is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    libdcp 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 General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with libdcp.  If not, see <http://www.gnu.org/licenses/>.
+
+    In addition, as a special exception, the copyright holders give
+    permission to link the code of portions of this program with the
+    OpenSSL library under certain conditions as described in each
+    individual source file, and distribute linked combinations
+    including the two.
+
+    You must obey the GNU General Public License in all respects
+    for all of the code used other than OpenSSL.  If you modify
+    file(s) with this exception, you may extend this exception to your
+    version of the file(s), but you are not obligated to do so.  If you
+    do not wish to do so, delete this exception statement from your
+    version.  If you delete this exception statement from all source
+    files in the program, then also delete it here.
+*/
+
+
+#include <stdint.h>
+#include <vector>
+
+
+namespace dcp {
+
+
+/** @class FSK
+ *  @brief Create frequency-shift-keyed samples for encoding synchronization signals.
+ *
+ *  An array of data is given to a FSK object using set_data(), and on calling get()
+ *  this data will be returned in a the D-Cinema FSK "format", sample by sample, starting
+ *  with the MSB of the first byte in the data array.
+ *
+ */
+class FSK
+{
+public:
+	FSK ();
+
+	void set_data (std::vector<bool> data);
+
+	/** @return the next sample as a 24-bit signed integer */
+	int32_t get ();
+
+private:
+	std::vector<bool> _data;
+	/** current offset into _data */
+	int _data_position;
+	/** current sample number of the current bit (0-3) */
+	int _sample_position;
+	/** polarity of the last bit to be written (false for -ve, true for +ve) */
+	bool _last_polarity;
+	/** value of the last bit to be written */
+	bool _last_bit;
+};
+
+}
+
+