The maximum bit rate that can be normally be recorded on a Mark IV or VLBA tape is 256 Mbps. But both systems have the capability to record wider bandwidths. On the VLBA, the capability is provided by using both tape recorders simultaneously to obtain 512 Mbps total bit rate. On the Mark IV systems, 512 Mbps can be obtained by using two head stacks on the tape drive. A 1 Gbps mode for MarkIV had been planned but the transition to Mark5, with its inherent 1 Gbps capacity overtook that project.
SCHED can make schedules for the 512 Mbps modes. See the examples eg512.key for a VLBA only case and eg2head.key for a Mark IV case. For the user, 512 Mbps modes are not much different from other modes. The station catalog must indicate that the stations involved have either 2 heads or 2 drives. The VLBA telescope schedules indicate use of the second drive simply through the specification of track numbers above 64. Note that the two examples do either only VLBA or only Mark IV, but it is ok to mix them.
When using the 512 Mbps mode on the VLBA, the user cannot use autoallocation of tape resources. Scans must be coordinate with tapes and a long gap, on the order of half an hour, should be left for any tape change. This will not usually be important because 512 Mbps projects are rarely allocated more than one tape pair.
The complication of using two drives or two heads has been relieved by the Mark5 systems which can take 2 drives/heads worth of inputs. But the high bandwidth observations are still scheduled as if they were writing many tracks. Eventually, with Mark5B, the track concept should disappear. The EVN has switched primarly to Mark5 as of late 2004 (they still send tape to the VLBA correlator). The VLBA began the transition to Mark5 in late 2004 and will be totally Mark5 a year or two later. Mark5B will not be available until mid 2005 or later.