On Tue, May 31, 2011 at 5:59 AM, Tom Rondeau <trondeau1...@gmail.com> wrote:
> On Tue, May 31, 2011 at 2:13 AM, Colby Boyer <colby.bo...@gmail.com>wrote: > >> Hi All, >> >> Recently I've been using the MMSE interp filter. I found that when I shift >> a signal by a fractional amount of 0 (or anything really), the signal goes >> way off! I would expect SOME difference, but not this much... >> >> Some example output. >> >> Mu:0 In:(-1.67869,0.480381) Out:(0.0418351,-0.16734) >> >> Mu:0 In:(-1.23772,-0.104519) Out:(0.16003,-0.115883) >> >> Mu:0 In:(-1.7598,-0.0618457) Out:(0.0986395,-0.33428) >> >> On the imgur links are two Re-Im scatter plots of a bpsk signal, one with >> the fractional shift of 0 and the other with a fractional shift of 0.1 The >> points with 'x' are the resampled points and points with 'o' is the original >> signal. As you can see, the mmse fractional interp more or less destroys the >> signal! Unless I'm using it wrong!? >> >> Any comments? >> >> imgur: http://imgur.com/a/w98SX top picture is 0 delay, bottom is 0.1 >> delay. >> >> Thanks! >> Colby > > > > Colby, > This block has been around for years without any changes, and I and others > have been successfully using it in various projects, so my guess is that you > have some misconception about what it's doing or what the parameters are. > > You can see how it's used to simulate a timing offset in > gnuradio-core/src/lib/hier/gr_channel_model.cc where it's used inside of the > gr_fractional_interpolator_cc block. We also use it in the > gr_clock_recovery_mm_XX and gr_mpsk_receiver_cc. > > Tom > > Hi Tom, I looked at the issue again and I was not taking in account the group delay of the interp filter (its 5 taps into the future or 3 in the past), so I was sampling a few taps out of phase! After a time shift, it passes the sanity check of '0.0' delay. Thanks for the response! --Colby
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