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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  * @return a pointer to a TimeFilter struct, or NULL on error
62  *
63  * For more details about these parameters and background concepts please see:
64  * http://www.kokkinizita.net/papers/usingdll.pdf
65  */
67 
68 /**
69  * Update the filter
70  *
71  * This function must be called in real time, at each process cycle.
72  *
73  * @param period the device cycle duration in clock_periods. For example, at
74  * 44.1kHz and a buffer size of 512 frames, period = 512 when clock_period
75  * was 1.0/44100, or 512/44100 if clock_period was 1.
76  *
77  * system_time, in seconds, should be the value of the system clock time,
78  * at (or as close as possible to) the moment the device hardware interrupt
79  * occurred (or any other event the device clock raises at the beginning of a
80  * cycle).
81  *
82  * @return the filtered time, in seconds
83  */
84 double ff_timefilter_update(TimeFilter *self, double system_time, double period);
85 
86 /**
87  * Evaluate the filter at a specified time
88  *
89  * @param delta difference between the requested time and the current time
90  * (last call to ff_timefilter_update).
91  * @return the filtered time
92  */
93 double ff_timefilter_eval(TimeFilter *self, double delta);
94 
95 /**
96  * Reset the filter
97  *
98  * This function should mainly be called in case of XRUN.
99  *
100  * Warning: after calling this, the filter is in an undetermined state until
101  * the next call to ff_timefilter_update()
102  */
104 
105 /**
106  * Free all resources associated with the filter
107  */
109 
110 #endif /* AVDEVICE_TIMEFILTER_H */