MINPAUSE is used to specify the minimum time a recording will be stopped between scans. It can be specified for each scan, but usually will be set to one value for the whole schedule. If the start of a scan is less than MINPAUSE (seconds) times the speedup factor from the end of the last recording scan, the recording will be left running between the scans. This action is station dependent -- different stations can have the recording started at different times except when writing a VEX file, in which case simultaneity is enforced. The nominal start time of the scan as displayed in the summary is not affected by MINPAUSE. You can use the option TPSTART in the SUMITEM list to display in the summary how long before the scan start time the recording starts. The recording start time is also given in the sch file.
MINPAUSE is mainly used now to prevent excessive recording scans on the Mark5A system. The system does not allow more than 1024 data scans on the disks. There is one scan for each period during which the recording does not stop, regardless of the number of source scans during that period. This limitation can be an issue with large disk packs and fast-switching phase-referencing projects. The default value is meant to prevent recording stops during fast switching.
While too many recording scans are a problem, so are too few. If something goes wrong with playback, the correlators cannot recover until the start of the next recording scan. Thus it is not wise to have recording scans more than about an hour long. Note that the MarkIV systems will not stop the recording for gaps of less than 10 seconds so a gap inserted to break a recording scan should be longer than that.
Short recorder stoppages could cause problems for playback in the era of tapes. Every time the recording stoped, it must be resynced, which takes 10-20 seconds for the old VLBA hardware correlator. This is not much of an issue for the DiFX software correlator or the MarkIV correlators.
There are a variety of ways to prevent recordings from being stopped between scans. The simplest is to schedule using DUR or explicit times with no specification of an interval between scans. In such cases, SCHED will not schedule any sort of pause in scans or recordings. One can use MINPAUSE to keep the recording going through short gaps. For longer gaps when using MINPAUSE, the recording start time will not be affected. PRESTART, or a negative value for the obsolete parameter PRESCAN, can be used to shift the scan start time forward from the time set by other criteria (such as antennas on-source when using DWELL). The start time is moved by the same amount for all stations and is not moved past the stop time of the most recent scan at any station.
For PCFS (VEX) controled stations, the user should bear in mind that all start times currently (Oct. 2001) must be equal. SCHED will issue a warning if this condition is violated and will try to synchronize recording starts when using MINPAUSE.
In the tape era, it was possible for MINPAUSE to confound your attempts to leave gaps required by VEX files for tape reversals. In such cases, you would need to adjust MINPAUSE on the offending scans. The default should not cause this problem because it does not request continuous tape motion through scans long enough to keep the VEX check routines happy for a reversal.
Note that MINPAUSE used to be multiplied by the speedup factor to determine the actual length of a pause at record time. That made it actually a time of the pause at playback time on the old VLBA correlator. That concept is no long relevant so the multiplication by the speedup factor has been removed.