The profusion of recording systems described in the last section is further complicated by the the possibility of mixing elements of the different systems. There are three elements of each system that must be specified in order to make proper schedules. These are specified in the SCHED station catalog with the paramters CONTROL, DAR, RECORDER and DISK. The first element is the control system -- what software (and hardware) is used to control the recording system. The options that SCHED can handle for wide band VLBI observations are VLBA and VEX. Formally, these actually refer to the control file type, although each type currently implies a specific computer and software system. Note that the CONTROL variable has other options, but they imply one of the above two, plus some specific other telescope control files.
The VLBA control system runs on VME computers using software mainly written for the VLBA. A few other sites (VLA, Green Bank, and one of the systems at Effelsberg) have such systems to control their VLBI hardware. Systems controlled by the VLBA software always consist of VLBA data aquisition racks (DAR -- BBCs, formatter etc) and VLBA tape recorder(s) or Mark5 disks. During 2008, new Digital Backends (DBE) will be introduced on the VLBA, but the implications for SCHED are not yet clear.
The VEX option for CONTROL causes SCHED to generate a schedule file in the VEX format, as does the DOVEX main schedule input parameter. This is the schedule distribution file format developed mainly for the PC Field System (PCFS) and the Mark IV correlators, although it could be used by other systems. The PCFS, running on Linux PCs, is used to control VLBA, VLBA4, Mark IV, S2, and MARK5 VLBI hardware at many non-VLBA stations including all of the EVN. It is the descendent of, and replacement for, the control system for Mark III systems. Sometimes this system is refered to as FS9, for ``Field System 9'', refering to the earliest version that could handle VEX input. Use of the VEX file to control the new VLBA software correlator is being considered.
Systems controlled by the PCFS can consist of a VLBA, VLBA4 or Mark IV DAR connected to a VLBA4, VLBA, Mark IV, S2, or Mark5 recorder. Each combination has slightly different capabilities and requirements. SCHED understands a considerable amount about what these capabilities and requirements are and attempts to insure that the requested observations are possible. However, when doing a large observation with many diverse telescopes, it is best to stick to standard, well tested, observing modes and use the frequencies selected using the setup file parameter BAND.
During the transitions between recording systems, it may not be obvious when the schedules are made which system a station will have. Or stations may have both and want to run observations on one or the other depending on the supply of media. Because of this situation, a separate input, called DISK can be specified in addition to RECORDER in the station catalog. It tells SCHED that the station has a disk system and schedules can be made for it. There is another new parameter, MEDIADEF, which can be TAPE or DISK, that can be used to set the default medium for the station while both are available. The MEDIA parameter in the TAPEINI section can be used to override the default. From this description, it is clear that this scheme was designed for the transition from tape to disk. As new varieties of disk or real-time systems are implemented, changes are likely.