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diff --git a/gr-atsc/include/atsc/fs_checker_impl.h b/gr-atsc/include/atsc/fs_checker_impl.h
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+/* -*- c++ -*- */
+/*
+ * Copyright 2002 Free Software Foundation, Inc.
+ *
+ * This file is part of GNU Radio
+ *
+ * GNU Radio 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 3, or (at your option)
+ * any later version.
+ *
+ * GNU Radio 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 GNU Radio; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+#ifndef _ATSC_FS_CHECKER_H_
+#define _ATSC_FS_CHECKER_H_
+
+#include <atsc/api.h>
+#include <atsc/syminfo_impl.h>
+
+/*!
+ * \brief abstract base class for ATSC field sync checker
+ *
+ * Processes input samples one at a time looking for
+ * an occurence of either the field sync 1 or field sync 2 pattern.
+ *
+ * Note that unlike atsci_fs_correlator, this class uses the symbol_num in
+ * input_tag to avoid having to test each symbol position.
+ *
+ * For each sample processed, an output sample and an output tag are produced.
+ * The output samples are identical to the input samples but are delayed by
+ * a number of samples given by \p delay().  The output tag associated with
+ * the the given output sample indicates whether this sample is the beginning
+ * of one of the field syncs or is an ordinary sample.  The tags are defined in
+ * atsci_sync_tag.h.
+ *
+ * For ease of use, the field sync patterns are defined to begin with the
+ * first symbol of the 4 symbol data segment sync pattern that immediately
+ * proceeds the actual PN 511 code.  This makes it easier for downstream code
+ * to determine the location of data segment syncs merely by counting.  They'll
+ * occur every 832 samples assuming everything is working.
+ */
+
+class ATSC_API atsci_fs_checker {
+
+public:
+
+  // CREATORS
+  atsci_fs_checker ();
+  virtual ~atsci_fs_checker () = 0;
+
+  // MANIPULATORS
+  virtual void reset () = 0;
+  virtual void filter (float input_sample, atsc::syminfo input_tag,
+		       float *output_sample, atsc::syminfo *output_tag) = 0;
+
+  // ACCESSORS
+
+  //! return delay in samples from input to output
+  virtual int delay () const = 0;
+};
+
+#endif /* _ATSC_FS_CHECKER_H_ */