[Difx-users] complex support

[Adam Deller]

Hi Jayce,

Yep, so if you give the LWA 392 channels and an LO offset of -0.00000588,
and the VLA 320 channels, then you should have everything matched up.

Cheers,
Adam

On Tue, 9 Feb 2021 at 04:18, Jayce Dowell <jdowell at unm.edu> wrote:

> Hi Adam and Chris,
>
> The VLA is outputting 8 MHz of bandwidth centered at 76 MHz and the LWA
> is 9.8 MHz of bandwidth centered at (76 MHz - 5.588 mHz).  To answer
> Chris' earlier question, yes, both data sets are in VDIF format (if I
> can figure out how to correctly convert the LWA data).
>
> Jayce
> --
> On 2/7/21 3:33 PM, Adam Deller wrote:
> > *  [EXTERNAL]*
> >
> > **
> > Hi Jayce,
> >
> > Just to expand on what Chris wrote, if you give the exact centre
> > frequency and bandwidth for the LWA and VLA, it would be easy to let you
> > know how DiFX can be set up to support correlation of the over-lapping
> > portion of the bands.  As Chris said, this would be implemented using
> > "zoom" bands (which select a subset of channels) after configuration the
> > spectral resolution of the VLA and LWA to match.  If the centre
> > frequencies are convenient, then this can be done pretty easily.  If
> > they are separated by an unfortunate amount, though, you need to either
> > go to very high spectral resolution (in order to get the centre
> > frequencies of the channels to match, in addition to their widths) or
> > alternatively one can use the "LO offset" functionality in DiFX to
> > effectively tweak the centre frequency of one of the bands slightly to a
> > more convenient value, enabling lower spectral resolution to be used.
> >
> > Cheers,
> > Adam
> >
> > On Fri, 5 Feb 2021 at 09:11, Phillips, Chris (CASS, Marsfield)
> > <Chris.Phillips at csiro.au> wrote:
> >
> >     Hi Jayce
> >
> >     I'm working the data from the Long Wavelength Array and I am trying
> >     to see if we can move from our homegrown correlator to DiFX.  The
> >     two modes I need to support are a LWA-only mode with homogeneous
> >     data and an "eLWA" mode with heterogeneous data that combines the
> >     LWA stations with the VLA 4-band system.  For the LWA-only mode I
> >     have 4+4-bit complex voltage data from two 19.6 MHz wide spectral
> >     windows.  I am currently converting these data from the LWA format
> >     into two VDIF files, one per window, for correlation with DiFX.  My
> >     .input file describes the data as:
> >      >
> >      > DATA FORMAT:        INTERLACEDVDIF/0:1
> >      > QUANTISATION BITS:  4
> >      > DATA FRAME SIZE:    7872
> >      > DATA SAMPLING:      COMPLEX_DSB
> >      > DATA SOURCE:        FILE
> >      >
> >
> >     I have never personally tested complex_dsb data with interlaced
> >     vdif. I have no idea if there could be any “interaction” with the
> >     VDIF interleaving and the specific data type (I would have thought
> >     not, but you never know).
> >
> >     The issue could also be with interpreting the interlaced VDIF, not
> >     the complex double sideband.
> >
> >     Once you have tested using trunk, not the released version, I would
> >     try and use some of the vdifio utilities to try and convert the
> >     interlaced VDIF into plain VDIF - either extracting out one thread,
> >     or merging into multichannel single thread VDIF. You would obviously
> >     need to update the DIFX .input (and or .v2d) file and vexfile to
> >     match the new setup.
> >
> >     I would also be using the m5access tools to do things like make the
> >     autocorrelation (both before and after fiddling with the data
> >     layout). Check DIFX gives the same auto shape as you get with
> >     m5spec.  If you use a modulated noise source, folding the data
> >     (m5fold) on the period of the noise cal can be very informative
> >     also.  Also look at the headers and check that the # frames/sec etc
> >     are what you expect (before and after merging threads etc). If you
> >     decide it is the interlacing is the issue, I suspect you will need
> >     to discuss with Walter.
> >
> >
> >      > I am less sure about DiFX supporting our eLWA mode.  For this we
> >     have the same 4+4-bit complex voltage data from the LWA stations but
> >     with a bandwidth of 9.8 MHz.  We are combining this with 4-bit real
> >     voltages from the VLA with 8 MHz of bandwidth and a slightly
> >     different frequency setup.  We currently process these data by
> >     shifting the center frequency of the LWA data to match that of the
> >     VLA, running two F-engines with different channelization to arrive
> >     at the same channel width for both data sources, and then cross
> >     correlate what overlaps.  Is this mode something that DiFX supports?
> >      >
> >
> >     Can you represent this data as VDIF? If you you can probably handle
> >     the different setup using zoom bands. However if this can be done
> >     will really depend on the EXACT setup. Basically you need to be able
> >     to channelise the two setups with the same final frequency
> >     resolution (ie different FFT sizes will be needed, with the ratio of
> >     the FFT size matching the ratio of the bandwidth difference. But the
> >     sky frequency of the frequency points out of the FFT need to match
> >     exactly (ie even if you could achieve the same frequency resolution,
> >     if the fine channels are offset by 1/3 of a channel you will have
> >     problems (if that offset is a simple integer ratio, you can just use
> >     a higher frequency resolution)
> >
> >     I hope this makes sense.
> >
> >     Cheers
> >     Chris
> >
> >
> >
> > --
> > !=============================================================!
> > 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
> >
> > office days (usually): Mon-Thu
> > !=============================================================!
>
>

-- 
!=============================================================!
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

office days (usually): Mon-Thu
!=============================================================!
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