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diff --git a/src/lingot-filter.h b/src/lingot-filter.h
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+/*
+ * lingot, a musical instrument tuner.
+ *
+ * Copyright (C) 2004-2011  Ibán Cereijo Graña, Jairo Chapela Martínez.
+ *
+ * This file is part of lingot.
+ *
+ * lingot 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.
+ *
+ * lingot 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 lingot; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifndef __LINGOT_FILTER_H__
+#define __LINGOT_FILTER_H__
+
+#include <fcntl.h>
+#include <stdlib.h>
+
+#include "lingot-defs.h"
+
+/*
+ digital filtering implementation.
+ */
+
+typedef struct _LingotFilter LingotFilter;
+
+struct _LingotFilter
+  {
+
+    FLT* a;
+    FLT* b; // coefs
+    FLT* s; // status
+
+    unsigned int N;
+
+  };
+
+LingotFilter
+    * lingot_filter_new(unsigned int Na, unsigned int Nb, FLT* a, FLT* b);
+
+/**
+ * Design a Chebyshev type I low pass filter with Rp dB of pass band ripple
+ * with cutoff pi*wc radians.
+ */
+LingotFilter* lingot_filter_cheby_design(unsigned int order, FLT Rp, FLT wc);
+
+void lingot_filter_destroy(LingotFilter*);
+
+// Digital Filter Implementation II, in & out overlapables. Vector filtering
+void lingot_filter_filter(LingotFilter*, unsigned int n, FLT* in, FLT* out);
+
+// sample filtering
+FLT lingot_filter_filter_sample(LingotFilter*, FLT in);
+
+#endif