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/*
* Copyright (C) 2013 Mark Hills <mark@xwax.org>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* version 2, as published by the Free Software Foundation.
*
* 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 version 2 for more details.
*
* You should have received a copy of the GNU General Public License
* version 2 along with this program; if not, write to the Free
* Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
* MA 02110-1301, USA.
*
*/
#ifndef PITCH_H
#define PITCH_H
/* Values for the filter concluded experimentally */
#define ALPHA (1.0/512)
#define BETA (ALPHA/256)
/* State of the pitch calculation filter */
struct pitch {
double dt, x, v;
};
/* Prepare the filter for observations every dt seconds */
static inline void pitch_init(struct pitch *p, double dt)
{
p->dt = dt;
p->x = 0.0;
p->v = 0.0;
}
/* Input an observation to the filter; in the last dt seconds the
* position has moved by dx.
*
* Because the vinyl uses timestamps, the values for dx are discrete
* rather than smooth. */
static inline void pitch_dt_observation(struct pitch *p, double dx)
{
double predicted_x, predicted_v, residual_x;
predicted_x = p->x + p->v * p->dt;
predicted_v = p->v;
residual_x = dx - predicted_x;
p->x = predicted_x + residual_x * ALPHA;
p->v = predicted_v + residual_x * BETA / p->dt;
p->x -= dx; /* relative to previous */
}
/* Get the pitch after filtering */
static inline double pitch_current(struct pitch *p)
{
return p->v;
}
#endif
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