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3073aadf2d
The feedback factors for the timefilter are directly computed from the expected period. This commit changes the init function to accept the period itself and compute the feedback factors internally, rather than having all client code duplicate the formulas. This commit also actually fixes the formulas: the current code had sqrt(2*o), but the correct formula, both theoretically and according to experimental testing, is sqrt(2)*o. Furthermore, it adds an exponential to feedback factors larger than 1 with large periods.
100 lines
3.5 KiB
C
100 lines
3.5 KiB
C
/*
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* Delay Locked Loop based time filter prototypes and declarations
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* 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 FFmpeg.
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*
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* FFmpeg is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* FFmpeg is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with FFmpeg; 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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*/
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#ifndef AVDEVICE_TIMEFILTER_H
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#define AVDEVICE_TIMEFILTER_H
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/**
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* Opaque type representing a time filter state
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*
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* The purpose of this filter is to provide a way to compute accurate time
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* stamps that can be compared to wall clock time, especially when dealing
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* with two clocks: the system clock and a hardware device clock, such as
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* a soundcard.
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*/
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typedef struct TimeFilter TimeFilter;
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/**
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* Create a new Delay Locked Loop time filter
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*
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* feedback2_factor and feedback3_factor are the factors used for the
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* multiplications that are respectively performed in the second and third
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* feedback paths of the loop.
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*
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* Unless you know what you are doing, you should set these as follow:
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*
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* o = 2 * M_PI * bandwidth * period_in_seconds
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* feedback2_factor = sqrt(2) * o
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* feedback3_factor = o * o
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*
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* Where bandwidth is up to you to choose. Smaller values will filter out more
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* of the jitter, but also take a longer time for the loop to settle. A good
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* starting point is something between 0.3 and 3 Hz.
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*
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* @param time_base period of the hardware clock in seconds
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* (for example 1.0/44100)
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* @param period expected update interval, in input units
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* @param brandwidth filtering bandwidth, in Hz
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*
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* For more details about these parameters and background concepts please see:
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* http://www.kokkinizita.net/papers/usingdll.pdf
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*/
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TimeFilter * ff_timefilter_new(double clock_period, double feedback2_factor, double feedback3_factor);
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/**
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* Update the filter
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*
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* This function must be called in real time, at each process cycle.
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*
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* @param period the device cycle duration in clock_periods. For example, at
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* 44.1kHz and a buffer size of 512 frames, period = 512 when clock_period
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* 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,
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* 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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*
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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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/**
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* Reset the filter
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*
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* This function should mainly be called in case of XRUN.
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*
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* Warning: after calling this, the filter is in an undetermined state until
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* the next call to ff_timefilter_update()
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*/
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void ff_timefilter_reset(TimeFilter *);
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/**
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* Free all resources associated with the filter
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*/
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void ff_timefilter_destroy(TimeFilter *);
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#endif /* AVDEVICE_TIMEFILTER_H */
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