#69 dspsr - folding filterbank containing Stokes.

[Ramesh]

Hi Willem, DSPSR experts,

Note sure if this is a bug - this is more a question regarding dspsr usage.

How do I tell dspsr that a filterbank file contains the four Stokes instead of the four coherence products? I have dspsr built with SIGPROC support, and the header of the filterbank file looks sane. However if I create a filterbank from baseband files using digifil, the resulting filterbank folds nicely.

Thanks in advance for your help.

Cheers,
Ramesh

[Paul Demorest]

hi Ramesh,

I don't think there is any way to distinguish stokes vs coherence in the
filterbank headers. If you are using dspsr for folding I think the easiest
thing to do is add "-j 'e state=Stokes'" to the dspsr command line. This
will update the headers in the folded output file to say Stokes params.

I think the only tricky thing to look out with multi-poln filterbank files
are issues regarding signed vs unsigned ints. For 8 and 16-bit data, the
sigproc reader in dspsr currently assumes all data are unsigned and
subtracts an offset equal to half the full integer range from polns 2 and 3
only. For less than 8 bits I'm not sure what happens but I'd be surprised
if it works correctly. If you are using floating-point (32-bit) filterbank
files you don't need to worry about any of this.

Cheers,
Paul

[Ramesh]

Hi Paul,

Thanks - that does the trick. I am 32-bits - so no overflows or scaling issues. While this folds correctly (I believe!), 'pacv -d' shows weird plots on the resulting folded archive (the filterbank contains a 1Hz noise diode signal and I adjusted the archive's header to have "type=PolnCal" and correct receiver basis).

The problem could have been in the way data was created in hardware (Stokes IQUV are computed under the assumption of linear feeds, but the signal came from a feed sensing the two hands of the circular polarisation). I tried to rectify this by rearranging the data so as to reflect circular feeds before folding with 'dspsr -j "-e state=Stokes"' - this shows (with 'pacv -d') nothing similar to the baseband signal from the same receiver, but processed by digifil/dspsr.

Am I overseeing something?

Cheers,
Ramesh