timefilter.h
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1 /*
2  * Delay Locked Loop based time filter prototypes and declarations
3  * Copyright (c) 2009 Samalyse
4  * Copyright (c) 2009 Michael Niedermayer
5  * Author: Olivier Guilyardi <olivier samalyse com>
6  * Michael Niedermayer <michaelni gmx at>
7  *
8  * This file is part of FFmpeg.
9  *
10  * FFmpeg is free software; you can redistribute it and/or
11  * modify it under the terms of the GNU Lesser General Public
12  * License as published by the Free Software Foundation; either
13  * version 2.1 of the License, or (at your option) any later version.
14  *
15  * FFmpeg is distributed in the hope that it will be useful,
16  * but WITHOUT ANY WARRANTY; without even the implied warranty of
17  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
18  * Lesser General Public License for more details.
19  *
20  * You should have received a copy of the GNU Lesser General Public
21  * License along with FFmpeg; if not, write to the Free Software
22  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
23  */
24 
25 #ifndef AVDEVICE_TIMEFILTER_H
26 #define AVDEVICE_TIMEFILTER_H
27 
28 /**
29  * Opaque type representing a time filter state
30  *
31  * The purpose of this filter is to provide a way to compute accurate time
32  * stamps that can be compared to wall clock time, especially when dealing
33  * with two clocks: the system clock and a hardware device clock, such as
34  * a soundcard.
35  */
36 typedef struct TimeFilter TimeFilter;
37 
38 
39 /**
40  * Create a new Delay Locked Loop time filter
41  *
42  * feedback2_factor and feedback3_factor are the factors used for the
43  * multiplications that are respectively performed in the second and third
44  * feedback paths of the loop.
45  *
46  * Unless you know what you are doing, you should set these as follow:
47  *
48  * o = 2 * M_PI * bandwidth * period_in_seconds
49  * feedback2_factor = sqrt(2) * o
50  * feedback3_factor = o * o
51  *
52  * Where bandwidth is up to you to choose. Smaller values will filter out more
53  * of the jitter, but also take a longer time for the loop to settle. A good
54  * starting point is something between 0.3 and 3 Hz.
55  *
56  * @param time_base period of the hardware clock in seconds
57  * (for example 1.0/44100)
58  * @param period expected update interval, in input units
59  * @param brandwidth filtering bandwidth, in Hz
60  *
61  * For more details about these parameters and background concepts please see:
62  * http://www.kokkinizita.net/papers/usingdll.pdf
63  */
65 
66 /**
67  * Update the filter
68  *
69  * This function must be called in real time, at each process cycle.
70  *
71  * @param period the device cycle duration in clock_periods. For example, at
72  * 44.1kHz and a buffer size of 512 frames, period = 512 when clock_period
73  * was 1.0/44100, or 512/44100 if clock_period was 1.
74  *
75  * system_time, in seconds, should be the value of the system clock time,
76  * at (or as close as possible to) the moment the device hardware interrupt
77  * occurred (or any other event the device clock raises at the beginning of a
78  * cycle).
79  *
80  * @return the filtered time, in seconds
81  */
82 double ff_timefilter_update(TimeFilter *self, double system_time, double period);
83 
84 /**
85  * Evaluate the filter at a specified time
86  *
87  * @param delta difference between the requested time and the current time
88  * (last call to ff_timefilter_update).
89  * @return the filtered time
90  */
91 double ff_timefilter_eval(TimeFilter *self, double delta);
92 
93 /**
94  * Reset the filter
95  *
96  * This function should mainly be called in case of XRUN.
97  *
98  * Warning: after calling this, the filter is in an undetermined state until
99  * the next call to ff_timefilter_update()
100  */
102 
103 /**
104  * Free all resources associated with the filter
105  */
107 
108 #endif /* AVDEVICE_TIMEFILTER_H */
double ff_timefilter_eval(TimeFilter *self, double delta)
Evaluate the filter at a specified time.
Definition: timefilter.c:84
double ff_timefilter_update(TimeFilter *self, double system_time, double period)
Update the filter.
Definition: timefilter.c:68
float delta
void ff_timefilter_reset(TimeFilter *)
Reset the filter.
Definition: timefilter.c:63
double clock_period
Definition: timefilter.c:36
void ff_timefilter_destroy(TimeFilter *)
Free all resources associated with the filter.
Definition: timefilter.c:58
TimeFilter * ff_timefilter_new(double clock_period, double feedback2_factor, double feedback3_factor)
Create a new Delay Locked Loop time filter.
Definition: timefilter.c:46
double feedback3_factor
Definition: timefilter.c:35
double feedback2_factor
Definition: timefilter.c:34