3.4 Tape Initialization File

The tape initialization file is mostly obsolete. It is possible that the ability to select the media type for particular stations will be of use in the future during transitions between recording systems. But all tape positioning capabilities became obsolete along with the tape systems. The highly unlikely that a tape initialization section is needed in any schedule file. If one is present, it is probably an artifact of the use of old templates and it should be removed.

The tape initialization information can be used to select what type of recording to make at a station and to give details about the tape recordings if tape is used.

During the transition to Mark5, there was a period where both tape and disk systems will be available at stations. SCHED needs a way to select which to use. For each station, a default can be set with MEDIADEF in the station catalog. To override that default, the MEDIA parameter is provided here to tell SCHED whether to assume that the data transmission system is specified by RECORDER or DISK in the station catalog.

The following is retain for historical reasons and should not concern most users.

If you are not using the MEDIA parameter, then you almost certainly should not be using a tape initialization section TAPEINI or tape initialization file. All the parameters have good defaults for nearly all cases so explicitly setting them is more likely to get you into trouble than to help. The capability provided by the file is only retained for special test observations. Even the MEDIA parameter is not normally needed. If your SCHED input file contains a TAPEINI section, you are probably an old time VLBI observer using ancient templates.

When using a tape system, the tape initialization information tells SCHED details about the tapes to be used at each site. In nearly all cases, the defaults are appropriate and very few users need to specify any of this information. Other than for test observations which the user may wish to start in the middle of a tape, the only exceptions are for observations that use thick tapes on Mark III or Mark IV systems. But since such tapes are no longer allowed at most stations, this should be very rare. If tape initialization information is needed, the information can be specified in an external file, the TAPEFILE, or can be imbedded in the main file following the parameter TAPEINI.

The capability to have a separate TAPEFILE was originally part of an attempt to deal with the general case of VLBA tape handling. Such a file could have been used to schedule multiple projects per tape by specifying where each project starts. The file would have one full set of inputs per project. But the automatic tape allocation by the on-line system now deals with this situation in a cleaner manner. Automatic tape allocation and automatic reversals can be requested using SCHED’s AUTOTAPE parameter; however, these options only apply to sites with VLBA control systems and more than one tape drive and can only be used for projects to be correlated on the VLBA correlator. When automatic tape handling is requested, SCHED puts the appropriate commands in the control files. It also attempts to predict what the tapes will do assuming that they start in the place specified in the tape initialization file. Users should be aware that these predictions may be not be accurate because tape lengths can vary and the tape start position may depend on the preceding project.

Note that the number of heads in use at a time (8, 16, or 32), which determines the number of passes per head index position (1, 2, or 4 as specified by TPMODE in the setup file, is not allowed to change during a project at a station, although different stations may use different values for TPMODE. This restriction avoids a the bookkeeping nightmare raised by the more general case. SCHED checks all the requested setups for a station and remembers the largest number of heads (smallest TPMODE) and allocates tape for all scans as if that value were in use. This is wasteful of tape, but allows some tests to be done with mode switching.

The following are some capabilities supported through the tape initialization file:

  1. Different tape lengths at different stations.
  2. Different bit densities, and hence tape speeds, at different stations.
  3. Tape drive and index position at program start specified for each station.
  4. Tape speed, bandwidth, and sample rate can change during run. Beware of operational restrictions on the use of this capability. See the guidelines for preparing observing schedules available from the NRAO WWW home page under VLBA.
  5. Different number of tape drives at each station.

A single set of tape initialization conditions can be specified in the SCHED keyin file using TAPEINI followed by a “/”, in much the same way that in-line catalogs are given. This is the normal way that the tape initialization information is provided. Unlike the source and station catalogs, only one input group will be read and that group will be terminated by a “/”. No ENDCAT or equivalent is required, or even allowed.

The input parameters of the TAPEFILE are:

 

MEDIA:    Which record system to use.

 

NDRIVES:  Number of drives at the site.

 

NHEADPOS: Number of head positions in use at site.

 

OBSCODE:  Experiment code for this observation.

 

TPSTA:    Station name for this group of parameters.

 

TPDRIVE:  The drive to begin the observation on.

 

TPINDEX:  The tape index at which to start the observations.

 

TPLENGTH: The length of the tapes to be used at the site.

 

DENSITY:  Are tapes at sites high or low density.

 

TPTIME:   Mark II tape length in time.

 

HEADMODE: Indicator of head positions to use.

The first example shows a tapeini section that might be used during the transition to MARK5. Note that BOTH means create a schedule with both types of control information so that operations can pick which to use. This works for the VLBA, but not for stations controlled by the VEX file. For this example, all tape parameter will take their default values.

 
tapeini /  
tpsta    = vlba_sc, vlba_hn, vlba_nl, vlba_fd, vlba_la, vlba_pt,  
           vlba_kp, vlba_ov, vlba_br, vlba_mk, gb_vlba, medicina,  
           noto,    onsala60  
MEDIA    = TAPE,    TAPE,    TAPE,    DISK,    DISK,    DISK,  
           DISK,    TAPE,    DISK,    TAPE,    TAPE,    DISK,  
           TAPE,    DISK  
  /

The second example of a tape initialization group shows the usage for specifying the tape system. Note the NDRIVES is not given so that it is picked up from the station catalog. Note that the example file for VLBI at the VLA contains a tape initialization group to show what it is like. Usually it should be reasonable to take the defaults.

 
obscode =’default’  
tpsta   =’vlba’, ’vla1’, ’vla27’, ’eb_vlba’, ’gb_vlba’, ’default’  
tpdrive =1,      1,      1,       1,         1,         1  
tpindex =1,      1,      1,       1,         1,         1  
tplength=17600,  17600,  17600,   17600,     17600,     8800  
nheadpos=14,     14,     14,      14,        14,        14  
density =H,      H,      H,       H,         H,         L  
        /  

3.4.1 Details of Tape Initialization Parameters

MEDIA

MEDIA is used to specify how the data are to be transmitted to the correlator. This is mainly useful when a station has alternatives, such as both a tape system ( station catalog parameter RECORDER) and a disk system (station catalog parameter DISK) and the default needs to be overridden. Valid arguments are blank (use the default), TAPE (use the tape system), and DISK (use the disk system). Eventually other options such as real time may be added.

If an option is specified that is not consistent with what is available at the station, an error will occur.

Note that most, if not all, stations now have ways to convert tape to disk files so it is essentially never necessary to specify MEDIA, and, in fact, is essentially never necessary to have a tape initialization section.

Argument: An array of character strings of up to 8 characters each, one per station. Not case sensitive.

Options: One of Blank, TAPE, DISK, TAPEDISK, DISKTAPE. Must have corresponding RECORDER or DISK in the station catalog.

Default: TAPEDISK for the VLBA controlled systems, TAPE for others.

Usage: Defaults to the previous group.

Example: MEDIA=DISK, TAPE, TAPEDISK, DISK

NDRIVES

NDRIVES if the number of tape drives at each station. This parameter can also be given in the station catalog and only need be given here for abnormal configurations. The value in the tape initialization information will override the number in the station catalog.

The VLBA sites and the VLA normally have 2 drives. Other sites will usually have 1.

Note that the schedule can be forced to use a single tape drive other than number 1 by setting NDRIVES=1 and TPDRIVE=N where N is the desired drive (usually 2). This is useful if, for example, tape drive 1 is out of service.

For S2 stations NDRIVES indicates the number of transports in the S2 recorder (usually 8).

NDRIVES will not be allowed to exceed the number specified in the station catalog.

Argument: An array of up to 30 integers, one for each station in the TPSTA list.

Options: 1 or 2 are the likely choices.

Default: 2 for the VLBA and VLA, 1 for others.

Usage: Defaults to the previous group.

Example: NDRIVES=1,1,2,2,1,2

NHEADPOS

NHEADPOS is the number of head positions to use at the station. The traditional number for Mark IIIa sites is 12. However, the VLBA uses a head position sequence with 14 positions. This allows more data to be put on a tape. This input is provided mainly to allow the number of head positions to be forced to 12 to match a schedule made to allow time for tape changes every 12 head positions. This is likely to be common for Mark III projects.

The MkIV schedules are currently scheduled with similar transverse tape format as the VLBA; normally the same 14 head positions are used. For recordings made with 2 heads, 6 head positions is the default. For S2 recorders this parameter is ignored, the different groups in an S2 format can be set by TPMODE.

Argument: An array of up to 30 integers, one for each station in the TPSTA list.

Options: 12 or 14 are the likely choices but the program will take any value.

Default: 14

Usage: Defaults to previous group.

Example: NHEADPOS=12,12,14,14,12,14

OBSCODE

OBSCODE is the Project code for the project for which this group of inputs applies.

Argument: Character of length 8.

Options: Any valid project code.

Default: DEFAULT. Implies that this group will give the default information.

Usage: Defaults to previous group, which is not likely to be useful.

Example: OBSCODE=’BW005B’

TPSTA

TPSTA is an array of up to 30 station names. One can be ’DEFAULT’ which means that the corresponding parameters will be used for stations not explicitly in the list. A station VLBA will match any VLBA stations that are not specified explicitly. Actually it matches any stations with the first four characters “VLBA”. A station VLA matches any station with the first 3 characters “VLA” that is not otherwise specified explicitly. For example, it matches “VLA1” and “VLA27”.

Argument: Character of length 8.

Options: Any station name.

Default: Blank

Usage: Defaults to previous value if none are specified. This is likely to be the normal mode of use with the station list only given for the first observation. If any stations are given, it is assumed that a complete new list is being given.

Example: TPSTA=’VLBA_PT’,’VLBA_KP’,’VLA27’

TPDRIVE

TPDRIVE is the tape drive on which to start the project. Each element in the array applies to the station in the corresponding element in the TPSTA array. If NDRIVES=1, the schedule will use the tape drive specified by TPDRIVE for all scans. This is useful for cases when tape drive 1 is out of service.

Argument: An array of 30 integers.

Options: 1 or 2 for VLBA sites. 1 for most others.

Default: 1

Usage: Defaults to previous value.

Example: TPDRIVE=2,1,1

TPINDEX

TPINDEX is the head index position on which to start. It will be assumed that there are no previously recorded data at this index position. A value for each station in TPSTA. With the new head position sequence all forward passes start on odd numbered index positions. SCHED will require TPINDEX be odd and will add 1 to any even value.

Argument: An array of 30 odd integers.

Options: Integer between 1 and 13.

Default: 1

Usage: Defaults to previous value.

Example: TPINDEX=5,6,2

TPLENGTH

TPLENGTH is the length of the tapes in use at each site. For VLBA, Mark III, and Mark IV tapes, the length is in feet. Thick tapes, which are still in use within the EVN but are not allowed at the VLBA correlator, have roughly 8800 feet of usable length(see the section on tape lengths). Thin tapes used for most VLBA and Mark IV observations usually have 17600 feet of usable tape. The VLBA operations staff tries to make sure that all thin tapes have at least 17400 feet of usable tape by splicing in extra when needed. Note that the 200 ft difference amounts to 30 seconds at 80 ips (the speed for speed-up-factor 2 observations).

Note that changing the length of the tape is not sufficient to switch from thick to thin tape. In that case one usually also needs to modify the recording density

S2 tape length should be given in seconds, corresponding to Standard Play in a US VCR. A tape with label ST-120 (or SE-180) should be scheduled with TPLENGTH = 7200

Argument: An array of 30 integers giving tape lengths in feet.

Options: Any value, but 8800 and 17600 are likely to be the true values.

Default: 17600

Usage: Defaults to previous value.

Example: TPLENGTH=17600,00,8800

DENSITY

DENSITY specifies whether the tape is to be written at high or low density. The tape speeds corresponding to each density can be specified in the setup file using TPSPEEDH SP:TPSPEEDH and TPSPEEDLSP:TPSPEEDL or, better, they can be allowed to default.

For S2 tapes, density “H” refers to SLP and “L” to LP.

Argument: An array of character strings of up to 8 characters each, of which only the first character will be used.

Options: The first letter of each string must be “H” or “L” (not case sensitive).

Default: H for the VLBA and VLA, L for others

Usage: Defaults to the previous value.

Example: DENSITY=H,L,L

TPTIME

TPTIME. This is for Mark II. It specifies the reference time for tape changes at each site. The tapes at the station will be changed an integral number of 4 hour intervals before or after this time. In effect, it is a station dependent TPREF. This value, if specified, will override TPREF for the corresponding station.

Argument: An array of 30 times in hh:mm:ss format.

Options: Any valid time between 0 and 24 hours.

Default: -1 which causes it not to be used. TPREF or the first scan of the project will be used instead.

Usage: Defaults to previous value.

Example: TPTIME=2:0:0,3:0:0,2:0:0

HEADMODE

HEADMODE. This allows the user to set the head usage mode. Such a mode includes the head positions for each pass and the sequence of head positions to use for passes. There are two options and a default. Use of anything except the default (blank) is only recommended for system tests. The first option is VLBA14 which is the standard VLBA/MARKIV sequence with 14 head positions. The other option is MKIV2H which is appropriate for Mark IV wide band observations that use two headstacks. If MKIV2H is set by the user, or more likely by the program when in TWOHEAD mode, NEADHDPOS is forced to 6.

Argument: A character string of up to 8 characters

Options: ’ ’, ’VLBA14’, or ’MKIV2H’

Default: ’ ’, which tells the program to figure it out.

Usage: Defaults to previous value.

Example: HEADMODE=MKIV2H