Recording and Accessing Event Times

TCA depends crucially on recording the absolute time in the n-tuple. The standard way absolute times are stored in the event data structure is as 3 integers: Julian date, Universal secs, Universal nsecs. However with standard (row-wise) n-tuples, each variable is stored in a single precision variable which limits accuracy to about 7 decimal digits. This is sufficient for the first two words (date and sec) but falls down on the last (nsec). So TCA uses a new time format: N-TUPLE TIME consisting of the 4 real numbers:-

• Julian date
• Universal secs
• Universal microsecs
• Universal nsecs

The DQF NTIME is used to store time in this format e.g.:-

time_jdy     ntime       ev+$kev_jdy;
time_sec     reserved               ;
time_msec    reserved               ;
time_nsec    reserved               ;

while the DQF NTIME_DIFFERENCE is used to compute differences in nsec e.g.:-

dtime     ntime_difference  time_jdy_1, time_jdy_2;

This DQF could be used to define a time gate which would appear to make the symbolic command $TCA_TIME_GATE superfluous. However, if TCA is extended to deal with multiplicities of 3 or higher, it will be easier to deal with a single time gate centered on the primary, rather than a number of time gates defined between pairs of members of the association. There is one case where you do need to place a cut using NTIME_DIFFERENCE. The time gate has to overlap the primary so if you require that the secondary occurs between say 100 and 300 nsec AFTER the primary then the time gate must be set:-

$TCA_TIME_GATE 0 300

and the constraint fine tuned with the TCAR line:-

dtime     ntime_difference  time_jdy_2, time_jdy_1: 100 300;

Every input n-tuple that TCA processes has to have n-tuple time recorded in it. TCA assumes that the first variable name to containing the string 'TIME' is that variable.