Keep anti aliasing filter notes from APERTIF.

libraries/dsp/doc/anti_aliasing_filter_notulen.txt

+Apertif Redmine #709 anti-aliasing filter
+
+Problem:
+The subband filterbank (Fsub) is critically sampled. This causes aliasing at the edges of the subbands
+and that shows in the channels that are created by subsequent the channel filterbank (Fchan). The
+Matlab two_pfb.m simulation of the Fsub and Fchan shows this by simulating a chirp input through the
+frequency band:
+
+  https://svn.astron.nl/UniBoard_FP7/RadioHDL/trunk/applications/apertif/matlab/two_pfb.m
+
+Erik van der Wal has invented a solution that can compensate for this aliasing. This we refer to as
+Anti Aliasing Filter (AAF). Stefan has cleaned up Erik's original Matlab code:
+
+  https://svn.astron.nl/UniBoard_FP7/RadioHDL/trunk/applications/apertif/matlab/unripple/unripple_v2_matlab.m
+
+into:
+
+  https://svn.astron.nl/UniBoard_FP7/RadioHDL/trunk/applications/apertif/matlab/unripple/AAF_APERTIF.m
+
+Bas can spend time on this, Stefan is too busy with other projects. Action Eric: discuss with Bas and 
+Stefan. Agnes has set up a meeting between Eric, Bas and Stefan in week 24 - 2017.
+
+Discussion with Bas and Stefan is to clarify:
+
+1) what will be gained scientifically by avoiding or solving the problem?
+  - how does this relate to the L0 requirements of Apertif and Arts
+  
+2) identify possible solutions:
+  a) flag and ignore the affected channels (Lofar approach)
+  b) avoid the problem by using an oversampled subband filterbank (SKA Low CBF approach)
+    - the oversampling factor increases the processing load for Fsub, BF and Fchan
+    - the oversampling factor also increases the load on the fiber links, which are already fully loaded
+  c) correct the affected channels using the AAF 
+  
+3) does this AAF algorithm work in the case of APERTIF?
+  - with beamforming and fringe stopping in between Fsub and Fchan
+  
+4) where in the processing chain should this be implemented
+  - for APERTIF correlator in firmware after the channel filterbank or in software after integration on the visibilities
+  - for ARTS tied array beamforming
+  
+5) can it be implemented
+  - hardware limitations like processing power, are the neighbouring channels available at the node
+  
+Action points:
+- SJW : Start "ASTRON_RP_1549_Apertif_Anti_Aliasing_Filter.tex" (AAAF) document in Latex to document our ongoing work
+- SJW/BT : Obtain relevant spectra from astronomers, apply the aliasing and then check the impact
+- BT : Get familiar with AAF alorithm and add AAF description to the AAAF document
+- EK : Organize that Stefan and Bas get access to Apertif firmware repository in SVN and to Apertif Redmine for AAF issue
+- EK : Add overview of possible solutions and Apertif/Arts processing chain to the AAAF document
+- EK : Update AAF redmine issue #709
+       
+
+  Apertif BF @ dish                              Apertif X in central building
+  
+  ADC --> Fsub --> BF  ---> fiber links ---+---> Fchan --+--> Correlator (X) --> network --> Data writer
+                       compound beam data  |             |                      visibilities
+                                           |             |
+                                           |             |    Arts BF for SC3,4 in central building
+                                           |             +--> Tied array BF  --> network --> Transient Search pipeline (SC3,4)
+                                           |                                    IUQV TABs
+                                           |
+                                           |    Arts BF for SC1 and SC2 in the central building
+                                           +--> Tied array BF  --> network --> Pulsar Timing (SC1), VLBI (SC2)
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