10/2/92 (RK)
Rev. 4/11/96 (RK)
These are a few notes on what features might be desirable in the next version of DSS (Digital Stimulus System) timers. They are based mainly on comments from users of the Auditory Lab.
It is assumed that the timers would include at a minimum what is
possible with the present DSS. Currently available features are not
repeated, but they are summarized in Figure 2 below
(click on the figure to get a larger version).
The following are some additional features that are either not possible in the present DSS, or difficult to do :
(1) It should be possible to have multiple (independent) stimulus
on/off events within a single duration-time cycle. For example, one may
want to present multiple short tone pips with a fixed off time between each,
followed by a longer off period, followed by another "repetition" of the same
series, etc. This is shown in the following figure :
Even though the above example shows only five tone pips, the number possible should be much larger - 100 or more.
(2) A logical extension of the above case is when all the individual
tone pips are of independent duration. This is shown in the following
example :
(3) Another logical extension of the above case is that the timer
events control independent actions. For example, in the case of Figure (1)
above, there may be a need to turn on lights for an arbitrary interval at a
certain point during the stimulus train. This is illustrated below:
It is easy to imagine the case where the number of events to be controlled is more than just one light. This case is often encountered in behavioral experiments.
(4) The max. number of repetitions should not be limited to 65,000. There have been cases where a million or more repetitions would have been desirable. An example of this is when short clicks are to be presented at a fairly fast rate (e.g. 200 clicks/sec). The max limit of 65000 reps means that after 65000/200=325 secs the clicks will stop. In some experiments it is necessary to have more reps (for example, to average evoked potentials when signal to noise ratio is very poor).
(5) The timers should use 32 bit counters. The present design with 16 bit registers means that times greater than 64 millisecs cannot be timed with 1 microsec accuracy as is sometimes needed. An example of this is when two tones are to presented, with one delayed by 100 millisecs or more with respect to the other. If the tones are moderately high frequency, then it necessary to maintain 1 microsec accuracy in the delay timer or the phase of the second tone becomes uncertain with respect to the first one.
(6) In some circumstances (e.g. Nigel and Bill’s laser expts.) it is desirable to be able to change the Number of Reps. after the timers have been started. For example, one could be examining data from each trial in near real time, deciding to discard certain “out of bounds” data, then making up for it by extending the number of reps.
The above suggestions are based on situations that have come up in recent times. Nothing is said here about how they may be implemented in the hardware. It should be noted that the design of the present DSS timers has proven more than adequate for about 95% of all experiments that have been proposed in the Auditory Lab.