Related. It seems that this delay is not taking in account for the MM clock recovery block? It some cases, this delay will not matter (2 or 4 samples per symbol), but for other rates it will.
--Colby On Tue, May 31, 2011 at 1:29 PM, Tom Rondeau <[email protected]> wrote: > On Tue, May 31, 2011 at 4:25 PM, Colby Boyer <[email protected]>wrote: > >> >> >> On Tue, May 31, 2011 at 5:59 AM, Tom Rondeau <[email protected]>wrote: >> >>> On Tue, May 31, 2011 at 2:13 AM, Colby Boyer <[email protected]>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 >> > > Excellent, glad that worked out. I was pretty sure it works, but you never > know... good to have extra confirmation. > > Tom > > > >
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