The Clock object

This class is to help with abstracting code that needs to timestamp events. Subclasses of this implement timestamps using different clocks (e.g. using MATLAB's now, Psychtoolbox's GetSecs, or a DAQ timing clock).

During an experiment all times are recorded using a Clock object. This object is stored by a number of different objects, ensuring that anything that records a time does so using the same clock. The Clock object is always stored in an object's Clock property. The following classes (and their subclasses) use a Clock:

Contents

Using the Clock

Below are some examples of how to use a Clock object. hw.Clock is an abstract class with each subclass implementing absoluteTime. We will instatiate hw.ptb.Clock, Rigbox's default Clock. This uses the Psychtoolbox function GetSecs in its absoluteTime method.

clock = hw.ptb.Clock

clock =

  Clock with properties:

    ReferenceTime: 0

Timestamps are returned by calling the now method, which must return the time in seconds since some reference time, as counted by whatever clock the subclass uses:

clock.now()

ans =

    1.3128e+06

This class also allows you to 'zero' the reference time at some moment. Time is then counted up from that moment on (and is negative for times before that point). Code that needs to track time can use this class to remain agnostic about what timing clock is acutally used. You could even use this e.g. as a neat way to run an experiment at a different speed.

zero(clock);
t = clock.now()

t =

    3.3180e-04

The Clock class also provides some ways to interconvert timestamps, for instance fromMatlab converts from a MATLAB serial date number to the same time but expressed in this clocks reference frame:

yesterday = now-1;
t = clock.fromMatlab(yesterday)

t =

   -8.6400e+04

There are also toPtb and fromPTB methods for subclasses that don't use GetSecs.

The Experiment Clock

When running an experiment via srv.expServer, the clock is retrieved via hw.devices. If there is no 'clock' field in your hardware file, an instance of hw.ptb.Clock is returned. If Timeline is enabled, a hw.TimelineClock instance is used instead. For more info, see the Timeline guide.

The clock is zero'd as soon as an expRef is received (e.g. when a new experiment is started in mc and the messeage is received by expServer). This happens in srv.expServer/runExp. All experiment times are therefore relative to this moment.

Etc.

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Author: Miles Wells

v0.1.0

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Published with MATLAB® R2019a