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
_______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
