User Reference:Timing

Real-Time Operation

Timing is a critical issue in a system that processes data in real time. Such a system must process data faster than they are acquired; this is the real-time constraint. In BCI2000, data is acquired and processed in sample blocks. Ideally, these sample blocks are acquired in regular intervals. To work in real-time, the system needs to finish processing, user display, and data storage within a block duration, i.e., the duration corresponding to a single sample block.

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 fulfill 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. By this time, video memory should have been updated, given that the module enforces a display update at the end of its Process() function; generally, this is done by calling the GUI::GraphDisplay::Update() function for the module's display window.

The DataIOFilter then 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 VisualizeTiming 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 overhead -- this is represented by the difference between roundtrip and stimulus curves.

See also

Technical Reference:Core Modules, User Reference:DataIOFilter