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difx:highspecres [2016/02/23 01:57]
walterbrisken created
difx:highspecres [2016/02/23 02:12] (current)
walterbrisken
Line 8: Line 8:
     * Output data could be larger than input data!     * Output data could be larger than input data!
   * Individual FFTs become long (in time), butting up against   * Individual FFTs become long (in time), butting up against
 +  * Due to timekeeping mechanisms in mpifxcorr, an FFT cannot span more than 2^31-1 nanoseconds
 +    * A hair over 2 seconds is the longest allowed transform
 +    * 0.5 Hz is thus the finest frequency resolution allowed
  
 Some general tips to consider: Some general tips to consider:
Line 17: Line 20:
   * Make use of zoom band to only save part of the spectrum, if consistent with the science   * Make use of zoom band to only save part of the spectrum, if consistent with the science
   * To minimize memory usage, numBufferedFFTs should probably be set to 1 (may greatly hurt performance)   * To minimize memory usage, numBufferedFFTs should probably be set to 1 (may greatly hurt performance)
 +
 +Some example .v2d file content leading to 0.5 Hz resolution:
 +
 +<code sh>
 +# Might need to increase the largest read size to accomodate long FFTs
 +maxReadSize = 40000000
 +
 +# Correlate a 1 kHz portion of the input band centered on 4101.3 MHz
 +ANTENNA FD { toneSelection=none filelist=fd.filelist addZoomFreq=freq@4101.2995/​bw@0.001 }
 +ANTENNA KP { toneSelection=none filelist=kp.filelist addZoomFreq=freq@4101.2995/​bw@0.001 }
 +ANTENNA NL { toneSelection=none filelist=nl.filelist addZoomFreq=freq@4101.2995/​bw@0.001 }
 +ANTENNA OV { toneSelection=none filelist=ov.filelist addZoomFreq=freq@4101.2995/​bw@0.001 }
 +ANTENNA PT { toneSelection=none filelist=pt.filelist addZoomFreq=freq@4101.2995/​bw@0.001 }
 +
 +SETUP default
 +{
 +  tInt = 2 # this is the shortest integration allowed for half Hz
 +  fftSpecRes = 0.0000005
 +  specRes ​   = 0.0000005
 +  doPolar = False
 +  numBufferedFFTs = 1 # When 1 FFT per integration,​ this is the only legal value
 +  maxNSBetweenACAvg = 2000000000 # change to be integration time; avoid warnings
 +}
 +</​code>​
 +
 +Note that the above 5 station correlation of a pair of 4 MHz channels requires 12 GB of RAM for each core process.
difx/highspecres.txt · Last modified: 2016/02/23 02:12 by walterbrisken