[Difx-users] Getting difx working on VDIF files - newbie {External}

[Hugh Garsden]

Hello all,

I'm new to difx and I want to run it on VDIF files. I've seen some recent mails about VDIF files so they are obviously being used, but I was looking at the manual<https://github.com/difx/difx/wiki/vex2difx#vdif-issues> for VDIF information it seems to be incomplete: "TO BE ADDED". I wondered if I could get more info or example files or something.

Here is a much longer story of what I'm up to:

I have difx running on the example files in the vf252 dataset here<https://www.atnf.csiro.au/vlbi/dokuwiki/doku.php/difx/datasets>. I stripped down the vex/v2d files to correlate only one file each of the PARKES and ATCA data. That works. So for PARKES I have this filelist:

    /home/jbcaadmin/work/v252f/v252f_Pk_161_012830.lba

and for ATCA this filelist:

    /home/jbcaadmin/work/v252f/v252f_At_161_012830.lba

and the vex/v2d files contain the minimum neccessary to make everything work.

Now I want to replace the above files with two VDIF files, and make the necessary changes to the vex/v2d files to run difx. There will be changes to timing, sampling rate etc., to accomodate the new files, but the VDIF files don't contain all the information necessary for  vex/v2d files, so I'm going to "pretend" that it some of it is the same as for the LBA files above. (The VDIF files are just test files and not actually from an observation, but they are valid.)

I made all the changes that I think are necessary, but vex2difx gives me this error:

Warning: job y_1 not written since it correlates no data
This is often due to media not being specified, all frequency Ids being
unselected, or too many antennas being explicitly unselected by time.
It is also possible that the vex file is faulty and missing, e.g., an $IF
section, leading to missing polarisation information.
nBaseline=0  minSubarraySize=2

I think this is something to do with the $TRACKS block in the vex file. I have this in the $TRACKS:

$TRACKS;
*
def LBA_.4Ch2bit1to1;
     track_frame_format = VDIF/8032/4;
enddef;

but from the manual I think more is needed, because it says:

As a concrete example, a complete $TRACKS section may resemble:

TOBEADDED

so I'm wondering what I'm missing. I tried adding some fanout_def statements, based on there being a single 4 bit data stream in the VDIF. Then vex2difx was happy, and vex2difx and calcif2 ran successfully, and the correlator difx started ok, but seemed to be quite confused about the input data streams. Also the manual says that fanout_def is not used for VDIF.

I've attached vex/v2d files of the set that works (LBA files) and the set that doesn't work (VDIF files).


Thanks,


               Hugh

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