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MINPAUSE is used to specify the minimum time a recording will be stopped between scans. Its effects are basically ignored by the wideband systems (RDBE, WIDAR with MARK5C recording) on the VLBA, VLA GBT, EB_VLBA and perhaps others so users of those systems can ignore the parameter, taking the default of zero.

The topic of controlling scan timing is discussed in detail in the Scan Times section.

If the interval between scans is shorter than the minimum specified with MINPAUSE, the recording will be be kept going between the scans. The need to prevent the recording from stopping for a short interval was most pronounced with tapes which took several seconds to accelerate and decelerate. With disks, that is not an issue so the only reason would be if there is a finite time to get the recording organized and the playback synchronized. Those are issues, at a low level with the Mark5A systems, but not with the Mark5C, which is in use and will become the dominent recording during 2013. With Mark5C, MINPAUSE should be set to a very small value or zero. Zero is the default for all systems except the legacy VLBA DAR and anything controlled by the Field System (may change for the DBBC). For the exceptions, 10 seconds is used.

For the VLA and sites with the VLBA control system, the recordings are started at the data-good time as reflected in the individual station lines of the VEX file. Those times account for any antenna slew and the additional time requested through TSETTLE and other such parameter in the station catalog. MINPAUSE only affects the media start time which is the nominal start time minus PRESTART, not the data good time, so it is basically ignored for such VLBA and VLA

MINPAUSE can be specified for each scan, but usually will be set to one value for the whole schedule. If the media start of a scan is less than MINPAUSE (seconds) 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 stopped, 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 old 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.

next up previous contents
Next: MONTH Up: Details of SCHED Parameters Previous: MAPLIM   Contents
Craig Walker 2014-06-17