#80 digifil downsampling factor is not applied properly

[Ryan Lynch]

Description: I am trying to use digifil to produce sigproc filterbank
data from data in the GUPPI RAW format. When I specify a downsampling
factor with the -t option the output filterbank files have a sampling
time that is actually tt. The raw data have 256 coarse channels and
a native sampling time of 1.28 us. If I specify a downsampling factor
of 32 I would expect the output data to have sampling time of 40.96
us. Instead it is 40.9632=1310.72 us. If I specify a downsampling
factor of 6 I would expect the output to have a sampling time of 7.68
us. Instead it has a sampling time of 7.68*6=46.08 us.

Command: digifil -b 8 -d 4 -F 256:D -D 11.217356 -K -o guppi_58626_J1125+7819_0013.fil -t 32 ../guppi_58626_J1125+7819_0013.0000.raw

Output:

digifil: using coherent dedispersion
digifil: using convolving filterbank

Result of running 'readfile guppi_58626_J1125+7819_0013.fil':
Assuming the data is a SIGPROC filterbank file.

1: From the SIGPROC filterbank file 'guppi_58626_J1125+7819_0013.fil':
Telescope = GBT
Source Name = J1125+7819
Obs Date String = 2019-05-23T11:27:53.0025
MJD start time = 58626.47769678830809
RA J2000 = 11:25:00.1920
RA J2000 (deg) = 171.2508
Dec J2000 = 78:18:59.7600
Dec J2000 (deg) = 78.3166
Tracking? = True
Azimuth (deg) = 0
Zenith Ang (deg) = 0
Number of polns = 2 (summed)
Polarization order =
Sample time (us) = 1310.72
Central freq (MHz) = 1500.390625
Low channel (MHz) = 1400.78125
High channel (MHz) = 1600
Channel width (MHz) = 0.78125
Number of channels = 256
Total Bandwidth (MHz) = 200
Beam = 0 of 1
Beam FWHM (deg) = 0.137
Spectra per subint = 2400
Starting subint = 1191
Subints per file = 285015082
Spectra per file = 5160340
Time per subint (sec) = 3.145728
Time per file (sec) = 6763.7608448
bits per sample = 8
bytes per spectra = 1024
samples per spectra = 1024
bytes per subint = 2457600
samples per subint = 2457600
zero offset = 0
Invert the band? = False
bytes in file header = 351

Note the value of sampling time. This may be related to ticket #79.

[Andrew Jameson]

Hi Ryan,

Digifil performs the temporal downsampling after the channelisation stage, and so the -t 32 should apply to the sample rate coming out of the channeliser. Since you have chosen a channeliser with the same number of output channels (-F 256:D) as the number of input channels (256), that should not have made a differenced. I would have expected an output sampling time of 40.96 us as you mentioned.

I did find a couple of other issues with digifil that needed to be fixed whilst looking into this. I don't think they are directly realted to this, issue, but I've checked them into the master anyway. Please update to git commit 168468 and perhaps try it again.

If you still find the same behaviour, then perhaps you could make available a (small) data file that exhibits the problem so I can try to reproduce the issue?

[Ryan Lynch]

I've tested on commit 168468 and the downsampling factor seems to be
applied properly now.

On 11/14/19 8:05 PM, Andrew Jameson wrote:

Hi Ryan,

Digifil performs the temporal downsampling after the channelisation
stage, and so the -t 32 should apply to the sample rate coming out of
the channeliser. Since you have chosen a channeliser with the same
number of output channels (-F 256:D) as the number of input channels
(256), that should not have made a differenced. I would have expected
an output sampling time of 40.96 us as you mentioned.

I did find a couple of other issues with digifil that needed to be
fixed whilst looking into this. I don't think they are directly
realted to this, issue, but I've checked them into the master anyway.
Please update to git commit 168468
https://sourceforge.net/p/dspsr/code/ci/1684684424d90b52740608131a84ab6059c4e6e6/
and perhaps try it again.

If you still find the same behaviour, then perhaps you could make
available a (small) data file that exhibits the problem so I can try
to reproduce the issue?

---

[bugs:#80] https://sourceforge.net/p/dspsr/bugs/80/ digifil
downsampling factor is not applied properly

Status: open
Group: version_1.0
Created: Thu Nov 14, 2019 06:13 PM UTC by Ryan Lynch
Last Updated: Thu Nov 14, 2019 06:13 PM UTC
Owner: nobody

Description: I am trying to use digifil to produce sigproc filterbank
data from data in the GUPPI RAW format. When I specify a downsampling
factor with the -t option the output filterbank files have a sampling
time that is actually t/t. The raw data have 256 coarse channels and
a native sampling time of 1.28 us. If I specify a downsampling factor
of 32 I would expect the output data to have sampling time of 40.96
us. Instead it is 40.96/32=1310.72 us. If I specify a downsampling
factor of 6 I would expect the output to have a sampling time of 7.68
us. Instead it has a sampling time of 7.68*6=46.08 us.

Command: |digifil -b 8 -d 4 -F 256:D -D 11.217356 -K -o
guppi_58626_J1125+7819_0013.fil -t 32
../guppi_58626_J1125+7819_0013.0000.raw|

Output:

digifil: using coherent dedispersion
digifil: using convolving filterbank

Result of running 'r|eadfile guppi_58626_J1125+7819_0013.fil|':
Assuming the data is a SIGPROC filterbank file.

1: From the SIGPROC filterbank file 'guppi_58626_J1125+7819_0013.fil':
Telescope = GBT
Source Name = J1125+7819
Obs Date String = 2019-05-23T11:27:53.0025
MJD start time = 58626.47769678830809
RA J2000 = 11:25:00.1920
RA J2000 (deg) = 171.2508
Dec J2000 = 78:18:59.7600
Dec J2000 (deg) = 78.3166
Tracking? = True
Azimuth (deg) = 0
Zenith Ang (deg) = 0
Number of polns = 2 (summed)
Polarization order =
Sample time (us) = 1310.72
Central freq (MHz) = 1500.390625
Low channel (MHz) = 1400.78125
High channel (MHz) = 1600
Channel width (MHz) = 0.78125
Number of channels = 256
Total Bandwidth (MHz) = 200
Beam = 0 of 1
Beam FWHM (deg) = 0.137
Spectra per subint = 2400
Starting subint = 1191
Subints per file = 285015082
Spectra per file = 5160340
Time per subint (sec) = 3.145728
Time per file (sec) = 6763.7608448
bits per sample = 8
bytes per spectra = 1024
samples per spectra = 1024
bytes per subint = 2457600
samples per subint = 2457600
zero offset = 0
Invert the band? = False
bytes in file header = 351

Note the value of sampling time. This may be related to ticket #79.

---

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--
Ryan Lynch

[Andrew Jameson]

Marked as closed as issue is reported as resolved by the commit to master