[Difx-users] complex support

[Jayce Dowell]

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
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> fax: +61 3 9214 8797
> 
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