timefilter.h 3.41 KB
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/*
 * Delay Locked Loop based time filter prototypes and declarations
 * Copyright (c) 2009 Samalyse
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 * Copyright (c) 2009 Michael Niedermayer
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 * Author: Olivier Guilyardi <olivier samalyse com>
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 *         Michael Niedermayer <michaelni gmx at>
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 *
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 * This file is part of Libav.
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 *
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 * Libav is free software; you can redistribute it and/or
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 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
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 * Libav is distributed in the hope that it will be useful,
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 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
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 * License along with Libav; if not, write to the Free Software
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 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

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#ifndef AVDEVICE_TIMEFILTER_H
#define AVDEVICE_TIMEFILTER_H
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/**
 * Opaque type representing a time filter state
 *
 * The purpose of this filter is to provide a way to compute accurate time
 * stamps that can be compared to wall clock time, especially when dealing
 * with two clocks: the system clock and a hardware device clock, such as
 * a soundcard.
 */
typedef struct TimeFilter TimeFilter;


/**
 * Create a new Delay Locked Loop time filter
 *
 * feedback2_factor and feedback3_factor are the factors used for the
 * multiplications that are respectively performed in the second and third
 * feedback paths of the loop.
 *
 * Unless you know what you are doing, you should set these as follow:
 *
 * o = 2 * M_PI * bandwidth * period
 * feedback2_factor = sqrt(2 * o)
 * feedback3_factor = o * o
 *
 * Where bandwidth is up to you to choose. Smaller values will filter out more
 * of the jitter, but also take a longer time for the loop to settle. A good
 * starting point is something between 0.3 and 3 Hz.
 *
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 * @param clock_period period of the hardware clock in seconds
 *        (for example 1.0/44100)
 *
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 * @return a pointer to a TimeFilter struct, or NULL on error
 *
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 * For more details about these parameters and background concepts please see:
 * http://www.kokkinizita.net/papers/usingdll.pdf
 */
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TimeFilter * ff_timefilter_new(double clock_period, double feedback2_factor, double feedback3_factor);
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/**
 * Update the filter
 *
 * This function must be called in real time, at each process cycle.
 *
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 * @param period the device cycle duration in clock_periods. For example, at
 * 44.1kHz and a buffer size of 512 frames, period = 512 when clock_period
 * was 1.0/44100, or 512/44100 if clock_period was 1.
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 *
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 * system_time, in seconds, should be the value of the system clock time,
 * at (or as close as possible to) the moment the device hardware interrupt
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 * occurred (or any other event the device clock raises at the beginning of a
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 * cycle).
 *
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 * @return the filtered time, in seconds
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 */
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double ff_timefilter_update(TimeFilter *self, double system_time, double period);
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/**
 * Reset the filter
 *
 * This function should mainly be called in case of XRUN.
 *
 * Warning: after calling this, the filter is in an undetermined state until
 * the next call to ff_timefilter_update()
 */
void ff_timefilter_reset(TimeFilter *);

/**
 * Free all resources associated with the filter
 */
void ff_timefilter_destroy(TimeFilter *);

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#endif /* AVDEVICE_TIMEFILTER_H */