[Difx-users] complex support

[Jayce Dowell]

Hi Adam & Chris,

I've moved on to the VLA+LWA data and am running into a new problem.  I 
have my frequency setups as:

# FREQ TABLE #######!
FREQ ENTRIES:       2
FREQ (MHZ) 0:       71.09999999441
BW (MHZ) 0:         9.80000000000
SIDEBAND 0:         U
NUM CHANNELS 0:     392
CHANS TO AVG 0:     1
OVERSAMPLE FAC. 0:  1
DECIMATION FAC. 0:  1
PHASE CALS 0 OUT:   0
FREQ (MHZ) 1:       71.99999999441
BW (MHZ) 1:         8.00000000000
SIDEBAND 1:         U
NUM CHANNELS 1:     320
CHANS TO AVG 1:     1
OVERSAMPLE FAC. 1:  1
DECIMATION FAC. 1:  1
PHASE CALS 1 OUT:   0

In my data streams table I have the LWA stations recording complex 
double sideband with frequency setup 0 and one zoom band of setup 1. 
The VLA antennas are set real upper sideband with frequency setup 1 and 
no zoom band.  I think this is what I want and mpifxcorr doesn't 
complain when I run it.  However, difx2fits immediately segfaults when I 
try to build a FITS-IDI file.  Any suggestions?

Jayce
--
On 3/10/21 4:19 PM, Adam Deller wrote:
> *  [EXTERNAL]*
> 
> **
> Glad to hear it!
> 
> On Thu, 11 Mar 2021 at 09:58, Jayce Dowell <jdowell at unm.edu 
> <mailto:jdowell at unm.edu>> wrote:
> 
>     Hi Chris,
> 
>     After some digging I was able to get to the bottom of this and I am now
>     getting results that I am happy with.  It was a combination of
>     switching
>     over to trunk and fixing a bad assumption in my VDIF conversion that
>     everyone represents four-bit complex integers in the same way.
> 
>     Jayce
>     --
>     On 2/4/21 3:08 PM, Phillips, Chris (CASS, Marsfield) wrote:
>      >    [EXTERNAL]
>      >
>      > 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
> !=============================================================!