Oh and I don't know which inputs match with which outputs, I'd probably just connect three to '0' and one to white noise, and then check which output is non-zero. Rinse and repeat! Also, test! I've experienced flakiness with casper libraries lately so a zeroth-order test is not a bad idea
On Wed, Jan 14, 2015 at 1:03 PM, Ryan Monroe <[email protected]> wrote: > 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) >> >> >
