User Reference:Timing
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.
Timing Display
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.
See also
Technical Reference:Core Modules, User Reference:DataIOFilter