[Difx-users] vex2difx bandwidth-sample rate check

[Adam Deller]

Hi Joe,

I don't believe the current vex standard is actually capable of
representing that a signal is complex sampled (a problem which is one of
the justifications for the long-awaited vex2 standard).  To sidestep this,
the v2d file can state that the data sampling is complex - but the vex
loader doesn't have access to that information.  So the vex file basically
can't represent the full info, and it has to be wrong or at least
incomplete in one way or another.  So if you just change the sample rate in
the vex file (but still tell vex2difx via the v2d file that the sampling is
complex) you should be ok.

Out of interest, how did you get this vex file (with the sampling rate set
to the bandwidth) in the first place?

Cheers,
Adam

On Sat, 12 Jun 2021 at 03:22, Joe Skeens via Difx-users <
difx-users at listmgr.nrao.edu> wrote:

> Hello all,
>
> I've been working with a couple of collaborators on running DiFX with
> complex signals, and we've found something that looks like it may be an
> oversight. In the vex2difx file 'vexload.cpp,' there's a simple if
> statement check which seems to ensure the sample rate is above the Nyquist
> frequency:
>
> if(bandwidth - stream.sampRate/2 > 1e-6)
>
> This causes vex2difx to exit on an error condition for bandwidth greater
> than sample rate divided by 2, but for a complex signal, the signal is
> only undersampled for bandwidth greater than sample rate. Commenting out
> this check and rebuilding DiFX seems to correct the problem. Is there
> something I'm missing with respect to how the bandwidth is processed in
> the channel definitions under $FREQ in the .vex file, or has this check
> been designed with only real signals in mind? Thanks in advance for any
> insights.
>
> Best,
> Joe Skeens
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-- 
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A/Prof. Adam Deller
ARC Future Fellow
Centre for Astrophysics & Supercomputing
Swinburne University of Technology
John St, Hawthorn VIC 3122 Australia
phone: +61 3 9214 5307
fax: +61 3 9214 8797

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