HighPassFilter.nxc

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/* -*- Mode: C; indent-tabs-mode: t; c-basic-offset: 4; tab-width: 4 -*- */
/*
    HighPassFilter.nxc
    Copyright (C) 2008 Naba Kumar   <naba@gnome.org>

    This program 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.

    This program 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 this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
*/

#ifndef __HIGH_PASS_FILTER_H
#define __HIGH_PASS_FILTER_H

struct HighPassFilterContext
{
   int RC;         
   int alpha1000;  
   int lastInput;  
   int lastOutput; 
};

void
HighPassFilterInit (HighPassFilterContext &context, int RC, int samplingRate)
{
    context.RC = RC;
    /* alpha scaled by 1000 for int ops */
    context.alpha1000 = (RC * samplingRate * 1000)/(RC * samplingRate + 1);
    context.lastOutput = -1;
    context.lastInput = -1;
}

int
HighPassFilter (HighPassFilterContext &context, int sampleInput)
{
    if (context.lastOutput < 0)
        context.lastOutput = sampleInput;
    else
        context.lastOutput = context.alpha1000 * (context.lastOutput + sampleInput - context.lastInput);
    context.lastOutput /= 1000; /* Scaled down from int ops */
    context.lastInput = sampleInput;
    return context.lastOutput;
}

#endif