Hey ross, this is all going off memory, but... fft_biplex takes two arbitrary complex-valued input streams and computes a separate FFT on each one. In general, the output is also complex-valued.
fft_biplex_real_4x uses a clever trick with FFTs, where for any given FFT, you can instead do two (real-valued input) FFTs instead of one (complex-valued input) fft. Because the input is real, the output is conjugate-symmetric. So, half of the data coming from each FFT is redundant in a sense. fft_biplex_real_2x does the same operations as fft_biplex_real_4x, only it does not generate the conjugate-symmetric part of each FFT. Therefore, there are half as many samples as before, and you can stuff all the data into two streams instead of four. Doing FFTs with a demux factor usually involves doing a separate FFT on each demuxed stream, followed by an FFT across streams. The ways we do that vary across library versions, but the math all looks pretty much the same. If you're doing a fft_biplex_real implementation, you'll almost certainly be using the fft_biplex_real_4x version for that. Give me a call if you need some help, you have my number ;-) --Ryan On Tue, Jan 13, 2015 at 2:58 PM, Ross Williamson < [email protected]> wrote: > I think Glenn touched on this in an earlier post but I still have some > questions. > I'm using the ska-sa fork of mlib_devel (last pull probably 1 month ago). > > 1) Is the output labeling still wrong? i.e. First output claims to be > pol02 and second is pol13. Is it in fact pol01 then pol23 or am I > missing how the FFT works? > > 2) If I have 500 Mhz sampled data that is demuxed by two (i.e. two > 250Mhz streams interleaved), can I simply connect them up to pol0 and > pol1 and the output of pol02 (pol01?) would be the FFT? I could then > do the same with a second channel on pol2 and pol3. > > 3) If the above is correct what is the channel ordering? for example > if I set NFFT to 13 I have 2^12 or 4096 channels. Does each clock > cycle output in sequential order 0-4096 on each clock cycle the repeat > back to 0? > > Cheers, > > Ross > -- > Ross Williamson > Research Scientist - Sub-mm Group > California Institute of Technology > 626-395-2647 (office) > 312-504-3051 (Cell) > >
