Hi Richard, I will let someone else say how the PFB block is actually implemented, but the correct method is as you have described, so that each sample in a 4-tap PFB will be used 4 times. I do not see how it could work otherwise, and indeed the whole point of the PFB is to give the effect of a window 4 times as wide as the sample window without lowering the sample rate.
Regards, Dale On Mon, Dec 5, 2016 at 9:39 AM, Richard Prestage <rpres...@nrao.edu> wrote: > Hi All – > > > > [ Long time CASPER groupie, first time posting. J ] > > > > I am trying to understand the VEGAS implementations of the PFB/FFT > technique, both in the FPGA and the CPU. For simplicity, I will use the > example on the CASPER wiki page, https://casper.berkeley.edu/ > wiki/The_Polyphase_Filter_Bank_Technique, and specifically Figure 3, to > pose my question. > > > > In the FFT-only approach, I would take the FFT of 256 samples, running > from i = 384 to 639. Let us refer to the time this operation takes as T. > > > > In this case, the PFB/FFT approach makes use of 1024 samples, but still > only produces the equivalent of a 256 point FFT. > > > > My initial assumption was that VEGAS would slide the 1024 window by 256 > points, each time. Thus, every T seconds, it would generate a new FFT, and > each one would correspond to the conventional approach, but with the window > function centered on it, hence reducing spectral leakage. This is how Joe > Brandt believes the GPU code works. > > > > But, Randy McCullough understands that in the FPGA implementation, each > sample is only used once. That would imply that one FFT comes out every P x > T seconds. > > > > These two approaches are not the same. For example, imagine a large RFI > spike at i = 896. If we do not use a sliding window, then the value of the > window function w(896) will be quite small, and the RFI will be attenuated. > The next spectrum will correspond to I = 1024 – 2047, and there will be no > RFI present. If we use a sliding window, keeping the same origin for I, > then when the window runs from I = 512 to 1535, w(896) will have a high > value, and so the RFI would be obvious in the corresponding spectrum. This > seems intuitively correct to me. > > > > Can someone definitively explain what is actually implemented in the FPGA > (HBW modes), and the GPU (LBW modes) for VEGAS? If it does not use a > sliding window, then is my above example correct, and the RFI would be > attenuated? If not, can you explain that also? > > > > Thanks, > > Richard > > > > Richard Prestage > > Scientist > > Green Bank Observatory > > Green Bank, WV 24944, USA > > > > > > >
