User Reference:Timing: Difference between revisions

Revision as of 15:24, 8 November 2007

Roundtrip time is the time needed for a sample block to traverse the core modules. Starting with the acquisition of a sample block, a block's roundtrip includes the time spent on signal processing and stimulus display. The roundtrip finishes when the block enters the data acquisition module again.

To fulfil the real-time constraint, roundtrip time may not exceed the physical duration of a sample block. For stable system operation, a weaker condition is sufficient; only the roundtrip's average value needs to stay below a sample block duration.

Measuring Block Duration

Immediately after data block has been acquired from hardware, the DataIOFilter writes a 16-bit millisecond-resolution time stamp into the SourceTime state. Block duration is measured as the difference between two consecutive time stamps.

Measuring Roundtrip Time

Roundtrip is measured by subtracting a data block's time stamp from the current time when it enters the data acquisition module coming from the application module.

Measuring Source-to-Stimulus Delay

In the StimulusTime state, the application module stores a time stamp when the module's Process() function has finished. The DataIOFilter subtracts the source time stamp from the stimulus time stamp to compute the source-to-stimulus delay. Unlike roundtrip time, measurement of the source-to-stimulus delay requires that data acquisition and application modules share a common time base; when distributed over multiple machines in a network, source-to-stimulus delay cannot be measured.

Roundtrip Display

File:RoundtripDisplay.png

When switched on via the VisualizeRoundtrip parameter, timing data are displayed in a visualization window. In this window, theoretical sample block length is indicated with a tickmark on the y axis, with scaling such that a range of 0..2 physical sample block lengths is displayed.

Timing values are displayed separately for actual block duration, roundtrip time, and source-to-stimulus delay. Using this graph, the experimentator can keep track of

regularity of data acquisition -- the block duration curve should be a straight line;
real-time operation -- roundtrip time should stay below theoretical sample block length;
module communication (networking) overhead -- this is represented by the difference between roundtrip and stimulus curves.

@@ Line 4: / Line 4: @@
 To fulfil the real-time constraint, roundtrip time may not exceed the physical duration of a sample block.
 For stable system operation, a weaker condition is sufficient; only the roundtrip's average value needs to stay below a sample block duration.
+==Measuring Block Duration==
+Immediately after data block has been acquired from hardware, the [[User Reference:DataIOFilter|DataIOFilter]] writes a 16-bit millisecond-resolution time stamp into the ''SourceTime'' state.
+Block duration is measured as the difference between two consecutive time stamps.
 ==Measuring Roundtrip Time==
-Roundtrip is measured using the millisecond timestamp stored in the ''SourceTime'' state. When, coming from the application module, a block enters the data acquisition module, the [[User Reference:DataIOFilter|DataIOFilter]] subtracts the time stamp from the current system time to obtain roundtrip time.
+Roundtrip is measured by subtracting a data block's time stamp from the current time when it enters the data acquisition module coming from the application module.
 ==Measuring Source-to-Stimulus Delay==
@@ Line 15: / Line 19: @@
 [[Image:RoundtripDisplay.png|frameless]]
-When switched on via the ''VisualizeRoundtrip'' parameter, roundtrip time and source-to-stimulus delay are displayed in a visualization window.
+When switched on via the ''VisualizeRoundtrip'' parameter, timing data are displayed in a visualization window.
-In this window, physical sample block length is indicated with a tickmark on the y axis, with scaling such that a range of 0..2 physical sample block lengths is displayed.
+In this window, theoretical sample block length is indicated with a tickmark on the y axis, with scaling such that a range of 0..2 physical sample block lengths is displayed.
-This display allows to easily observe the system's timing behavior, and whether real-time operation is possible.
+Timing values are displayed separately for actual block duration, roundtrip time, and source-to-stimulus delay.
+Using this graph, the experimentator can keep track of
+*regularity of data acquisition -- the block duration curve should be a straight line;
+*real-time operation -- roundtrip time should stay below theoretical sample block length;
+*module communication (networking) overhead -- this is represented by the difference between ''roundtrip'' and ''stimulus'' curves.
 ==See also==
 [[Technical Reference:Core Modules]], [[User Reference:DataIOFilter]]