Recording
Recording of data to disk is done by the program vsib_record. This should be in the path of the user account used for general running of the recorder (usually "vlbi"). If not consult your local expert and get them to set this up. You might want to consider using the GUI front end to vsib_record "disco".
vsib_record initialises the vsib.o kernel module and instructs it to start reading data from the vsib card into main memory. It then write the data to disk using a normal Linux file system (ie local disks or even an network mounted disk, but this is not recommended). Files are limited to a specific size, by default 10 seconds worth of data. Normally the user only has to specify the output filename and the recording time, all other parameters have sensible defaults. Data is recorded into the local directory, so it it important to "cd" to one of the large data disks before recording, otherwise you will very quickly fill up the system disk. If this happens remember to remove the data from system disk.
For a simple recording session, you would need to specify the recording time and a sensible filename. It is safe to request a longer than necessary recording time and use ^C (control-C) to stop recording when done.
> vsib_record -t 2h -o vt02a_Pa
Will record for 2 hours and add the prefix "vt02a_Pa" to the filename to identify the experiment and antenna where the data was recorded. There are a number of extra options needed for various experiments
- Parallel S2 recording. The disks recorders always record 4 data channels, but S2 can only output two useful channels. To preserve disk space, these extra channels can be discarded. The compress mode (-c) is used for this purpose. For S2 "x" modes (e.g. 32x4-2) only the first two channels are active. Use the compress mode -c ooxx. If an S2 "a" mode is being used, use the compress mode -c oxox as the first and third channels are active. In this form the "o" channels are discarded and the "x" channels are saved (reading right to left just for added confusion). The output data rate will then be 128 Mbps (for 16 MHz bandwidth data).
> vsib_record -t 2h -o vt02a_Pa -c ooxx # For S2 "x" modes
> vsib_record -t 2h -o vt02a_Pa -c oxox # For S2 "a" modes
- Scheduled disk based observations. For experiments which are destined for full software correlation (ie not fringe tests or simple tests) it is best to synchronize the recorders to all start at the same time. Previously the "dstart" command has been used for this but this has now need added as an option to vsib_record (-s). Time must be specified as a fully qualified (year month day hour minute second) UT time. Setting of special environment variables to to force the time to be interpreted as UT is not needed (unlike dstart). A range of date formats are possible, including yyyy-mm-ddThh:mm:ss.s and yyyymmddThhmmss.s. E.g.
> vsib_record -t 2h -o vt01i1_Mp -s 2005-05-11T01:38:35
> vsib_record -t 2h -o vt01i1_Mp -s 20050511T013835
- 64 MHz or Huygens observing. Both these modes use 512 Mbps data rates. At the normal 32 MHz clock rates used for the DAS, this requires 16 bits of data per clock sample (eight 2 bit data streams for Huygens, multiplexed data for 64 MHz mode). This is enabled by changing the vsib mode (-m). The default is 3 (8 bit recording) while mode 2 is 16 bit recording.
> vsib_record -t 2h -o vt01i1_Mp -m 2 # Record at 512 Mbps (16 bit recording mode)
- Narrow bandwidth recording. The default bandwidth recording is 16 MHz. For recording narrow bandwidths, use the -w option to specify the bandwidth the DAS is configured to.
> vsib_record -t 2h -o vt01i1_Mp -w 4 # To set to 4 MHz bandwidth. 1, 2 and 8 work also