timefilter.h File Reference

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Typedefs
typedef struct TimeFilter	TimeFilter
	Opaque type representing a time filter state. More...

Functions
TimeFilter *	ff_timefilter_new (double clock_period, double feedback2_factor, double feedback3_factor)
	Create a new Delay Locked Loop time filter. More...
double	ff_timefilter_update (TimeFilter *self, double system_time, double period)
	Update the filter. More...
double	ff_timefilter_eval (TimeFilter *self, double delta)
	Evaluate the filter at a specified time. More...
void	ff_timefilter_reset (TimeFilter *)
	Reset the filter. More...
void	ff_timefilter_destroy (TimeFilter *)
	Free all resources associated with the filter. More...

Typedef Documentation

typedef struct TimeFilter TimeFilter

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.

Definition at line 36 of file timefilter.h.

Function Documentation

void ff_timefilter_destroy

(

TimeFilter *

)

Free all resources associated with the filter.

Definition at line 58 of file timefilter.c.

Referenced by ff_alsa_close(), ff_timefilter_eval(), and stop_jack().

double ff_timefilter_eval	(	TimeFilter *	self,
		double	delta
	)

Evaluate the filter at a specified time.

Parameters

delta

difference between the requested time and the current time (last call to ff_timefilter_update).

Returns

the filtered time

Definition at line 84 of file timefilter.c.

Referenced by convert_timestamp().

TimeFilter* ff_timefilter_new	(	double	clock_period,
		double	feedback2_factor,
		double	feedback3_factor
	)

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_in_seconds 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.

Parameters

time_base	period of the hardware clock in seconds (for example 1.0/44100)
period	expected update interval, in input units
brandwidth	filtering bandwidth, in Hz

For more details about these parameters and background concepts please see: http://www.kokkinizita.net/papers/usingdll.pdf

Definition at line 46 of file timefilter.c.

Referenced by audio_read_header(), ff_timefilter_eval(), init_convert_timestamp(), and start_jack().

void ff_timefilter_reset

(

TimeFilter *

)

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()

Definition at line 63 of file timefilter.c.

Referenced by audio_read_packet(), and xrun_callback().

double ff_timefilter_update	(	TimeFilter *	self,
		double	system_time,
		double	period
	)

Update the filter.

This function must be called in real time, at each process cycle.

Parameters

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.

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 occurred (or any other event the device clock raises at the beginning of a cycle).

Returns

the filtered time, in seconds

Definition at line 68 of file timefilter.c.

Referenced by audio_read_packet(), convert_timestamp(), ff_timefilter_eval(), and process_callback().