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GStreamer 0.10 Core Reference Manual | ![]() |
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#include <gst/gst.h> GstClock; GstClockClass; typedef GstClockTime; typedef GstClockTimeDiff; typedef GstClockID; #define GST_CLOCK_TIME_NONE #define GST_CLOCK_TIME_IS_VALID (time) #define GST_SECOND #define GST_MSECOND #define GST_USECOND #define GST_NSECOND #define GST_TIME_AS_SECONDS (time) #define GST_TIME_AS_MSECONDS (time) #define GST_TIME_AS_USECONDS (time) #define GST_TIME_AS_NSECONDS (time) #define GST_CLOCK_DIFF (s, e) #define GST_TIMEVAL_TO_TIME (tv) #define GST_TIME_TO_TIMEVAL (t,tv) #define GST_TIMESPEC_TO_TIME (ts) #define GST_TIME_TO_TIMESPEC (t,ts) #define GST_CLOCK_ENTRY_TRACE_NAME GstClockEntry; gboolean (*GstClockCallback) (GstClock *clock, GstClockTime time, GstClockID id, gpointer user_data); enum GstClockEntryType; #define GST_CLOCK_ENTRY (entry) #define GST_CLOCK_ENTRY_CLOCK (entry) #define GST_CLOCK_ENTRY_TYPE (entry) #define GST_CLOCK_ENTRY_TIME (entry) #define GST_CLOCK_ENTRY_INTERVAL (entry) #define GST_CLOCK_ENTRY_STATUS (entry) enum GstClockReturn; enum GstClockFlags; #define GST_CLOCK_FLAGS (clock) #define GST_CLOCK_BROADCAST (clock) #define GST_CLOCK_COND (clock) #define GST_CLOCK_TIMED_WAIT (clock,tv) #define GST_CLOCK_WAIT (clock) gboolean gst_clock_add_observation (GstClock *clock, GstClockTime slave, GstClockTime master, gdouble *r_squared); gboolean gst_clock_set_master (GstClock *clock, GstClock *master); GstClock* gst_clock_get_master (GstClock *clock); GstClockTime gst_clock_set_resolution (GstClock *clock, GstClockTime resolution); GstClockTime gst_clock_get_resolution (GstClock *clock); GstClockTime gst_clock_get_time (GstClock *clock); GstClockID gst_clock_new_single_shot_id (GstClock *clock, GstClockTime time); GstClockID gst_clock_new_periodic_id (GstClock *clock, GstClockTime start_time, GstClockTime interval); GstClockTime gst_clock_get_internal_time (GstClock *clock); GstClockTime gst_clock_adjust_unlocked (GstClock *clock, GstClockTime internal); GstClockTime gst_clock_unadjust_unlocked (GstClock *clock, GstClockTime external); void gst_clock_get_calibration (GstClock *clock, GstClockTime *internal, GstClockTime *external, GstClockTime *rate_num, GstClockTime *rate_denom); void gst_clock_set_calibration (GstClock *clock, GstClockTime internal, GstClockTime external, GstClockTime rate_num, GstClockTime rate_denom); GstClockTime gst_clock_id_get_time (GstClockID id); GstClockReturn gst_clock_id_wait (GstClockID id, GstClockTimeDiff *jitter); GstClockReturn gst_clock_id_wait_async (GstClockID id, GstClockCallback func, gpointer user_data); void gst_clock_id_unschedule (GstClockID id); gint gst_clock_id_compare_func (gconstpointer id1, gconstpointer id2); GstClockID gst_clock_id_ref (GstClockID id); void gst_clock_id_unref (GstClockID id);
"stats" gboolean : Read / Write "timeout" guint64 : Read / Write "window-size" gint : Read / Write "window-threshold" gint : Read / Write
GStreamer uses a global clock to synchronize the plugins in a pipeline. Different clock implementations are possible by implementing this abstract base class or, more conveniently, by subclassing GstSystemClock.
The GstClock returns a monotonically increasing time with the method
gst_clock_get_time()
. Its accuracy and base time depend on the specific
clock implementation but time is always expressed in nanoseconds. Since the
baseline of the clock is undefined, the clock time returned is not
meaningful in itself, what matters are the deltas between two clock times.
The time returned by a clock is called the absolute time.
The pipeline uses the clock to calculate the running time. Usually all renderers synchronize to the global clock using the buffer timestamps, the newsegment events and the element's base time, see GstPipeline.
A clock implementation can support periodic and single shot clock notifications both synchronous and asynchronous.
One first needs to create a GstClockID for the periodic or single shot
notification using gst_clock_new_single_shot_id()
or
gst_clock_new_periodic_id()
.
To perform a blocking wait for the specific time of the GstClockID use the
gst_clock_id_wait()
. To receive a callback when the specific time is reached
in the clock use gst_clock_id_wait_async()
. Both these calls can be
interrupted with the gst_clock_id_unschedule()
call. If the blocking wait is
unscheduled a return value of GST_CLOCK_UNSCHEDULED is returned.
Periodic callbacks scheduled async will be repeatedly called automatically
until it is unscheduled. To schedule a sync periodic callback,
gst_clock_id_wait()
should be called repeatedly.
The async callbacks can happen from any thread, either provided by the core or from a streaming thread. The application should be prepared for this.
A GstClockID that has been unscheduled cannot be used again for any wait
operation, a new GstClockID should be created and the old unscheduled one
should be destroyed with gst_clock_id_unref()
.
It is possible to perform a blocking wait on the same GstClockID from multiple threads. However, registering the same GstClockID for multiple async notifications is not possible, the callback will only be called for the thread registering the entry last.
None of the wait operations unref the GstClockID, the owner is responsible for unreffing the ids itself. This holds for both periodic and single shot notifications. The reason being that the owner of the GstClockID has to keep a handle to the GstClockID to unblock the wait on FLUSHING events or state changes and if the entry would be unreffed automatically, the handle might become invalid without any notification.
These clock operations do not operate on the running time, so the callbacks will also occur when not in PLAYING state as if the clock just keeps on running. Some clocks however do not progress when the element that provided the clock is not PLAYING.
When a clock has the GST_CLOCK_FLAG_CAN_SET_MASTER flag set, it can be
slaved to another GstClock with the gst_clock_set_master()
. The clock will
then automatically be synchronized to this master clock by repeatedly
sampling the master clock and the slave clock and recalibrating the slave
clock with gst_clock_set_calibration()
. This feature is mostly useful for
plugins that have an internal clock but must operate with another clock
selected by the GstPipeline. They can track the offset and rate difference
of their internal clock relative to the master clock by using the
gst_clock_get_calibration()
function.
The master/slave synchronisation can be tuned with the "timeout", "window-size" and "window-threshold" properties. The "timeout" property defines the interval to sample the master clock and run the calibration functions. "window-size" defines the number of samples to use when calibrating and "window-threshold" defines the minimum number of samples before the calibration is performed.
Last reviewed on 2009-05-21 (0.10.24)
typedef struct _GstClock GstClock;
GstClock base structure. The values of this structure are protected for subclasses, use the methods to use the GstClock.
typedef struct { GstObjectClass parent_class; /* vtable */ GstClockTime (*change_resolution) (GstClock *clock, GstClockTime old_resolution, GstClockTime new_resolution); GstClockTime (*get_resolution) (GstClock *clock); GstClockTime (*get_internal_time) (GstClock *clock); /* waiting on an ID */ GstClockReturn (*wait) (GstClock *clock, GstClockEntry *entry); GstClockReturn (*wait_async) (GstClock *clock, GstClockEntry *entry); void (*unschedule) (GstClock *clock, GstClockEntry *entry); /* ABI added to replace the deprecated wait */ GstClockReturn (*wait_jitter) (GstClock *clock, GstClockEntry *entry, GstClockTimeDiff *jitter); } GstClockClass;
GStreamer clock class. Override the vmethods to implement the clock functionality.
GstObjectClass parent_class ; |
the parent class structure |
change_resolution () |
change the resolution of the clock. Not all values might be acceptable. The new resolution should be returned. |
get_resolution () |
get the resolution of the clock. |
get_internal_time () |
get the internal unadjusted time of the clock. |
wait () |
perform a blocking wait for the given GstClockEntry. Deprecated,
implement wait_jitter instead.
|
wait_async () |
perform an asynchronous wait for the given GstClockEntry. |
unschedule () |
unblock a blocking or async wait operation. |
wait_jitter () |
perform a blocking wait on the given GstClockEntry and return the jitter. (Since: 0.10.10) |
typedef gint64 GstClockTimeDiff;
A datatype to hold a time difference, measured in nanoseconds.
typedef gpointer GstClockID;
A datatype to hold the handle to an outstanding sync or async clock callback.
#define GST_CLOCK_TIME_NONE ((GstClockTime) -1)
Constant to define an undefined clock time.
#define GST_CLOCK_TIME_IS_VALID(time) (((GstClockTime)(time)) != GST_CLOCK_TIME_NONE)
Tests if a given GstClockTime represents a valid defined time.
time : |
clock time to validate |
#define GST_SECOND (G_USEC_PER_SEC * G_GINT64_CONSTANT (1000))
Constant that defines one GStreamer second.
#define GST_MSECOND (GST_SECOND / G_GINT64_CONSTANT (1000))
Constant that defines one GStreamer millisecond.
#define GST_USECOND (GST_SECOND / G_GINT64_CONSTANT (1000000))
Constant that defines one GStreamer microsecond.
#define GST_NSECOND (GST_SECOND / G_GINT64_CONSTANT (1000000000))
Constant that defines one GStreamer nanosecond
#define GST_TIME_AS_SECONDS(time) ((time) / GST_SECOND)
Convert a GstClockTime to seconds.
time : |
the time |
Since 0.10.16
#define GST_TIME_AS_MSECONDS(time) ((time) / G_GINT64_CONSTANT (1000000))
Convert a GstClockTime to milliseconds (1/1000 of a second).
time : |
the time |
Since 0.10.16
#define GST_TIME_AS_USECONDS(time) ((time) / G_GINT64_CONSTANT (1000))
Convert a GstClockTime to microseconds (1/1000000 of a second).
time : |
the time |
Since 0.10.16
#define GST_TIME_AS_NSECONDS(time) (time)
Convert a GstClockTime to nanoseconds (1/1000000000 of a second).
time : |
the time |
Since 0.10.16
#define GST_CLOCK_DIFF(s, e) (GstClockTimeDiff)((e) - (s))
Calculate a difference between two clock times as a GstClockTimeDiff.
The difference is calculated as e
- s
.
s : |
the first time |
e : |
the second time |
#define GST_TIMEVAL_TO_TIME(tv) (GstClockTime)((tv).tv_sec * GST_SECOND + (tv).tv_usec * GST_USECOND)
Convert a GTimeVal to a GstClockTime.
tv : |
the timeval to convert |
#define GST_TIME_TO_TIMEVAL(t,tv)
Convert a GstClockTime to a GTimeVal
t : |
The GstClockTime to convert |
tv : |
The target timeval |
#define GST_TIMESPEC_TO_TIME(ts) (GstClockTime)((ts).tv_sec * GST_SECOND + (ts).tv_nsec * GST_NSECOND)
Convert a struct timespec (see man pselect) to a GstClockTime.
ts : |
the timespec to convert |
#define GST_TIME_TO_TIMESPEC(t,ts)
Convert a GstClockTime to a struct timespec (see man pselect)
t : |
The GstClockTime to convert |
ts : |
The target timespec |
#define GST_CLOCK_ENTRY_TRACE_NAME "GstClockEntry"
The name used for tracing clock entry allocations.
typedef struct { gint refcount; } GstClockEntry;
All pending timeouts or periodic notifies are converted into an entry.
gint refcount ; |
reference counter (read-only) |
gboolean (*GstClockCallback) (GstClock *clock, GstClockTime time, GstClockID id, gpointer user_data);
The function prototype of the callback.
clock : |
The clock that triggered the callback |
time : |
The time it was triggered |
id : |
The GstClockID that expired |
user_data : |
user data passed in the gst_clock_id_wait_async() function
|
Returns : | TRUE or FALSE (currently unused)
|
typedef enum { GST_CLOCK_ENTRY_SINGLE, GST_CLOCK_ENTRY_PERIODIC } GstClockEntryType;
The type of the clock entry
#define GST_CLOCK_ENTRY(entry) ((GstClockEntry *)(entry))
Cast to a clock entry
entry : |
the entry to cast |
#define GST_CLOCK_ENTRY_CLOCK(entry) ((entry)->clock)
Get the owner clock of the entry
entry : |
the entry to query |
#define GST_CLOCK_ENTRY_TYPE(entry) ((entry)->type)
Get the type of the clock entry
entry : |
the entry to query |
#define GST_CLOCK_ENTRY_TIME(entry) ((entry)->time)
Get the requested time of this entry
entry : |
the entry to query |
#define GST_CLOCK_ENTRY_INTERVAL(entry) ((entry)->interval)
Get the interval of this periodic entry
entry : |
the entry to query |
#define GST_CLOCK_ENTRY_STATUS(entry) ((entry)->status)
The status of the entry
entry : |
the entry to query |
typedef enum { GST_CLOCK_OK = 0, GST_CLOCK_EARLY = 1, GST_CLOCK_UNSCHEDULED = 2, GST_CLOCK_BUSY = 3, GST_CLOCK_BADTIME = 4, GST_CLOCK_ERROR = 5, GST_CLOCK_UNSUPPORTED = 6 } GstClockReturn;
The return value of a clock operation.
typedef enum { GST_CLOCK_FLAG_CAN_DO_SINGLE_SYNC = (GST_OBJECT_FLAG_LAST << 0), GST_CLOCK_FLAG_CAN_DO_SINGLE_ASYNC = (GST_OBJECT_FLAG_LAST << 1), GST_CLOCK_FLAG_CAN_DO_PERIODIC_SYNC = (GST_OBJECT_FLAG_LAST << 2), GST_CLOCK_FLAG_CAN_DO_PERIODIC_ASYNC = (GST_OBJECT_FLAG_LAST << 3), GST_CLOCK_FLAG_CAN_SET_RESOLUTION = (GST_OBJECT_FLAG_LAST << 4), GST_CLOCK_FLAG_CAN_SET_MASTER = (GST_OBJECT_FLAG_LAST << 5), /* padding */ GST_CLOCK_FLAG_LAST = (GST_OBJECT_FLAG_LAST << 8) } GstClockFlags;
The capabilities of this clock
#define GST_CLOCK_FLAGS(clock) GST_OBJECT_FLAGS(clock)
Gets the GstClockFlags clock flags.
clock : |
the clock to query |
#define GST_CLOCK_BROADCAST(clock) g_cond_broadcast(GST_CLOCK_COND(clock))
Signal that the entries in the clock have changed.
clock : |
the clock to broadcast |
#define GST_CLOCK_COND(clock) (GST_CLOCK_CAST(clock)->entries_changed)
Gets the GCond that gets signalled when the entries of the clock changed.
clock : |
the clock to query |
#define GST_CLOCK_TIMED_WAIT(clock,tv) g_cond_timed_wait(GST_CLOCK_COND(clock),GST_OBJECT_GET_LOCK(clock),tv)
Wait on the clock until the entries changed or the specified timeout occurred.
clock : |
the clock to wait on |
tv : |
a GTimeVal to wait. |
#define GST_CLOCK_WAIT(clock) g_cond_wait(GST_CLOCK_COND(clock),GST_OBJECT_GET_LOCK(clock))
Wait on the clock until the entries changed.
clock : |
the clock to wait on |
gboolean gst_clock_add_observation (GstClock *clock, GstClockTime slave, GstClockTime master, gdouble *r_squared);
The time master
of the master clock and the time slave
of the slave
clock are added to the list of observations. If enough observations
are available, a linear regression algorithm is run on the
observations and clock
is recalibrated.
If this functions returns TRUE
, r_squared
will contain the
correlation coefficient of the interpolation. A value of 1.0
means a perfect regression was performed. This value can
be used to control the sampling frequency of the master and slave
clocks.
clock : |
a GstClock |
slave : |
a time on the slave |
master : |
a time on the master |
r_squared : |
a pointer to hold the result |
Returns : | TRUE if enough observations were added to run the
regression algorithm.
MT safe.
|
gboolean gst_clock_set_master (GstClock *clock, GstClock *master);
Set master
as the master clock for clock
. clock
will be automatically
calibrated so that gst_clock_get_time()
reports the same time as the
master clock.
A clock provider that slaves its clock to a master can get the current
calibration values with gst_clock_get_calibration()
.
master
can be NULL
in which case clock
will not be slaved anymore. It will
however keep reporting its time adjusted with the last configured rate
and time offsets.
clock : |
a GstClock |
master : |
a master GstClock |
Returns : | TRUE if the clock is capable of being slaved to a master clock.
Trying to set a master on a clock without the
GST_CLOCK_FLAG_CAN_SET_MASTER flag will make this function return FALSE .
MT safe.
|
GstClock* gst_clock_get_master (GstClock *clock);
Get the master clock that clock
is slaved to or NULL
when the clock is
not slaved to any master clock.
GstClockTime gst_clock_set_resolution (GstClock *clock, GstClockTime resolution);
Set the accuracy of the clock. Some clocks have the possibility to operate with different accuracy at the expense of more resource usage. There is normally no need to change the default resolution of a clock. The resolution of a clock can only be changed if the clock has the GST_CLOCK_FLAG_CAN_SET_RESOLUTION flag set.
clock : |
a GstClock |
resolution : |
The resolution to set |
Returns : | the new resolution of the clock. |
GstClockTime gst_clock_get_resolution (GstClock *clock);
Get the accuracy of the clock. The accuracy of the clock is the granularity
of the values returned by gst_clock_get_time()
.
clock : |
a GstClock |
Returns : | the resolution of the clock in units of GstClockTime. MT safe. |
GstClockTime gst_clock_get_time (GstClock *clock);
Gets the current time of the given clock. The time is always monotonically increasing and adjusted according to the current offset and rate.
clock : |
a GstClock to query |
Returns : | the time of the clock. Or GST_CLOCK_TIME_NONE when given invalid input. MT safe. |
GstClockID gst_clock_new_single_shot_id (GstClock *clock, GstClockTime time);
Get a GstClockID from clock
to trigger a single shot
notification at the requested time. The single shot id should be
unreffed after usage.
clock : |
The GstClockID to get a single shot notification from |
time : |
the requested time |
Returns : | A GstClockID that can be used to request the time notification. MT safe. |
GstClockID gst_clock_new_periodic_id (GstClock *clock, GstClockTime start_time, GstClockTime interval);
Get an ID from clock
to trigger a periodic notification.
The periodic notifications will start at time start_time
and
will then be fired with the given interval
. id
should be unreffed
after usage.
clock : |
The GstClockID to get a periodic notification id from |
start_time : |
the requested start time |
interval : |
the requested interval |
Returns : | A GstClockID that can be used to request the time notification. MT safe. |
GstClockTime gst_clock_get_internal_time (GstClock *clock);
Gets the current internal time of the given clock. The time is returned unadjusted for the offset and the rate.
clock : |
a GstClock to query |
Returns : | the internal time of the clock. Or GST_CLOCK_TIME_NONE when given invalid input. MT safe. |
GstClockTime gst_clock_adjust_unlocked (GstClock *clock, GstClockTime internal);
Converts the given internal
clock time to the external time, adjusting for the
rate and reference time set with gst_clock_set_calibration()
and making sure
that the returned time is increasing. This function should be called with the
clock's OBJECT_LOCK held and is mainly used by clock subclasses.
This function is the reverse of gst_clock_unadjust_unlocked()
.
clock : |
a GstClock to use |
internal : |
a clock time |
Returns : | the converted time of the clock. |
GstClockTime gst_clock_unadjust_unlocked (GstClock *clock, GstClockTime external);
Converts the given external
clock time to the internal time of clock
,
using the rate and reference time set with gst_clock_set_calibration()
.
This function should be called with the clock's OBJECT_LOCK held and
is mainly used by clock subclasses.
This function is the reverse of gst_clock_adjust_unlocked()
.
clock : |
a GstClock to use |
external : |
an external clock time |
Returns : | the internal time of the clock corresponding to external .
|
Since 0.10.13
void gst_clock_get_calibration (GstClock *clock, GstClockTime *internal, GstClockTime *external, GstClockTime *rate_num, GstClockTime *rate_denom);
Gets the internal rate and reference time of clock
. See
gst_clock_set_calibration()
for more information.
internal
, external
, rate_num
, and rate_denom
can be left NULL
if the
caller is not interested in the values.
MT safe.
clock : |
a GstClock |
internal : |
a location to store the internal time |
external : |
a location to store the external time |
rate_num : |
a location to store the rate numerator |
rate_denom : |
a location to store the rate denominator |
void gst_clock_set_calibration (GstClock *clock, GstClockTime internal, GstClockTime external, GstClockTime rate_num, GstClockTime rate_denom);
Adjusts the rate and time of clock
. A rate of 1/1 is the normal speed of
the clock. Values bigger than 1/1 make the clock go faster.
internal
and external
are calibration parameters that arrange that
gst_clock_get_time()
should have been external
at internal time internal
.
This internal time should not be in the future; that is, it should be less
than the value of gst_clock_get_internal_time()
when this function is called.
Subsequent calls to gst_clock_get_time()
will return clock times computed as
follows:
time = (internal_time - internal) * rate_num / rate_denom + external
This formula is implemented in gst_clock_adjust_unlocked()
. Of course, it
tries to do the integer arithmetic as precisely as possible.
Note that gst_clock_get_time()
always returns increasing values so when you
move the clock backwards, gst_clock_get_time()
will report the previous value
until the clock catches up.
MT safe.
clock : |
a GstClock to calibrate |
internal : |
a reference internal time |
external : |
a reference external time |
rate_num : |
the numerator of the rate of the clock relative to its internal time |
rate_denom : |
the denominator of the rate of the clock |
GstClockTime gst_clock_id_get_time (GstClockID id);
Get the time of the clock ID
id : |
The GstClockID to query |
Returns : | the time of the given clock id. MT safe. |
GstClockReturn gst_clock_id_wait (GstClockID id, GstClockTimeDiff *jitter);
Perform a blocking wait on id
.
id
should have been created with gst_clock_new_single_shot_id()
or gst_clock_new_periodic_id()
and should not have been unscheduled
with a call to gst_clock_id_unschedule()
.
If the jitter
argument is not NULL
and this function returns GST_CLOCK_OK
or GST_CLOCK_EARLY, it will contain the difference
against the clock and the time of id
when this method was
called.
Positive values indicate how late id
was relative to the clock
(in which case this function will return GST_CLOCK_EARLY).
Negative values indicate how much time was spent waiting on the clock
before this function returned.
id : |
The GstClockID to wait on |
jitter : |
A pointer that will contain the jitter, can be NULL .
|
Returns : | the result of the blocking wait. GST_CLOCK_EARLY will be returned
if the current clock time is past the time of id , GST_CLOCK_OK if
id was scheduled in time. GST_CLOCK_UNSCHEDULED if id was
unscheduled with gst_clock_id_unschedule() .
MT safe.
|
GstClockReturn gst_clock_id_wait_async (GstClockID id, GstClockCallback func, gpointer user_data);
Register a callback on the given GstClockID id
with the given
function and user_data. When passing a GstClockID with an invalid
time to this function, the callback will be called immediately
with a time set to GST_CLOCK_TIME_NONE. The callback will
be called when the time of id
has been reached.
The callback func
can be invoked from any thread, either provided by the
core or from a streaming thread. The application should be prepared for this.
id : |
a GstClockID to wait on |
func : |
The callback function |
user_data : |
User data passed in the callback |
Returns : | the result of the non blocking wait. MT safe. |
void gst_clock_id_unschedule (GstClockID id);
Cancel an outstanding request with id
. This can either
be an outstanding async notification or a pending sync notification.
After this call, id
cannot be used anymore to receive sync or
async notifications, you need to create a new GstClockID.
MT safe.
id : |
The id to unschedule |
gint gst_clock_id_compare_func (gconstpointer id1, gconstpointer id2);
Compares the two GstClockID instances. This function can be used as a GCompareFunc when sorting ids.
id1 : |
A GstClockID |
id2 : |
A GstClockID to compare with |
Returns : | negative value if a < b; zero if a = b; positive value if a > b MT safe. |
GstClockID gst_clock_id_ref (GstClockID id);
Increase the refcount of given id
.
id : |
The GstClockID to ref |
Returns : | The same GstClockID with increased refcount. MT safe. |
void gst_clock_id_unref (GstClockID id);
Unref given id
. When the refcount reaches 0 the
GstClockID will be freed.
MT safe.
id : |
The GstClockID to unref |
"stats"
property"stats" gboolean : Read / Write
Enable clock stats (unimplemented).
Default value: FALSE
"timeout"
property"timeout" guint64 : Read / Write
The amount of time, in nanoseconds, to sample master and slave clocks.
Default value: 100000000
"window-size"
property"window-size" gint : Read / Write
The size of the window used to calculate rate and offset.
Allowed values: [2,1024]
Default value: 32