Thank you folks, Jeff, thank you for your snapshot. I modified Firas' MATLAB m file( http://www.nabble.com/file/8506/usrpddc.m) to get similar picture. :-)
Sebastiaan and Matt, thank you for your kind explanation. Now, I get it clearly. :-) Regards, ILKYOUNG. 2009/11/12 Matt Ettus <m...@ettus.com> > > I will explain the RX side, the TX side is basically the same. > > small_hb_dec is the short filter which works at the higher rate. There are > 2 of them instantiated, one for I and one for Q. It has 7 taps. One of > those taps is the center tap which only requires a shift and not a multiply, > and 2 of those taps are zeros. That leaves 4 taps. The taps are symmetric, > which leaves 2 multiplies per output. Since we have at least 2 cycles to > produce each output, we can use a single multiplier. > > hb_dec is 2nd halfband filter and it works at the lower rate. There are 2 > of them instantiated, one for I and one for Q. It has 31 taps. One of > those taps is the center tap which only requires a shift and not a multiply, > and half of the remainder are zeros. That leaves 16 taps. They are > symmetric, so that means we need to do 8 multiplies to produce each output. > There are at least 4 cycles to produce each output, so we need to do 2 > multiplies at a time. > > One of those multipliers does the "outer" coefficients, meaning the ones at > the very beginning and end of the impulse response, and one does the "inner" > coefficients, meaning the ones around the center of the impulse response. > This division is purely an implementation choice, and does not affect the > output. I could have put the odd ones on one mult and the even ones on the > other, or any other split you could imagine. It doesn't matter. > > Matt > > > > Jeff Brower wrote: > >> Ilkyoung Kwoun- >> >> Thank you for your advice. Actually I am aware of basic characteristics of >>> half band filter. It is very well explained in Rick Ryon's "Understanding >>> Digital Signal Processing (2nd Ed.)" ( >>> >>> http://www.amazon.com/Understanding-Digital-Signal-Processing-2nd/dp/0131089897/ref=sr_1_1?ie=UTF8&s=books&qid=1257900094&sr=8-1 >>> ) >>> >>> The thing I do not have any clue is the 'inner' and 'outer' coefficient >>> things in FIR filter. I guess this is something related to the practical >>> implementation issue rather than the fundamental concept. I did a simple >>> googling yesterday and found a paper. ( >>> >>> http://ce.et.tudelft.nl/publicationfiles/1090_509_shahbahrami_prorisc2005.pdf >>> ) >>> I hope I can find a way to get started. :-) >>> >> >> Here is a Hypersignal log-magnitude plot showing the frequency response of >> the two halfband filters (31-tap in blue, >> 7-tap in red): >> >> http://www.signalogic.com/images/gnu_radio_halfband_filters.jpg >> >> In the plots I used an arbitrary sampling rate of 25 MHz -- don't know >> what you're using. Note that both filters have >> an approx -6 dB point at Fs/4 as would be expected due to halfband >> symmetry properties. >> >> As for "inner and outer" mention in the Verilog code, I might guess that >> refers to 2 multiplies needed when filtering >> a complex signal. As for why there are two (2) filters, one is used for >> higher rate up/down conversions and the other >> for lower rate. Firas' documentation has some information on this... also >> here is Q&A exchange between Firas and Matt >> that might help you out: >> >> http://old.nabble.com/Some-USRP2-Questions-td20729711.html >> >> -Jeff >> >> 2009/11/10 Sebastiaan Heunis <sheu...@gmail.com> >>> >>> Hi >>>> >>>> I think Brian is just referring to the fact that you can see that the >>>> HBFs are implemented as symmetric FIR filters (the coefficients). A >>>> HBF is just a special filter that is designed so that the cutoff >>>> frequency will always be at fs/2, so if you filter with it and >>>> decimate by 2 afterwards (which is why you would use an HBF), you >>>> don't get aliasing. Also, the decimation in the CIC filters can be >>>> set to a wide range of values, so the HBF will always have to cut at >>>> fs/2 regardless of the decimation in the CIC. From what I've read, >>>> Goodman and Carey came up with them so you can probably look for some >>>> papers if you're interested. >>>> >>>> Sebastiaan >>>> >>>> -- >>>> Sebastiaan Heunis >>>> Radar Remote Sensing Group, University of Cape Town, South Africa >>>> Tel: +27 72 950 9370 >>>> >>>> _______________________________________________ >>> Discuss-gnuradio mailing list >>> Discuss-gnuradio@gnu.org >>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>> >>> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > >
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