From: Shenzhi on 6 Mar 2010 03:49 Hi, Friends! Recently, I have some questions about Fractional Decimation. I think about them for a lot of time, but can't find a good solution. It comes from my reading Douglas W.Barker's paper "Efficient Resampling Implementations " presented on "Tips&Tricks" IEEE Signal Processing Magazine,2008. I found some interrelated materials in Mr. Fred Harris's book << Multirate signal processing for communication systems >> . This book gives me excellent analysis in how to determine the Interpolation factor. But the polyphase interpolation does not only contain the interpolation, but also the decimation. I have following questions about fractional decimation ******************************************************************* when decimation, how the decimated stopband attenuation will be affect by the decimation. I think for the reason that the original outband noise is folded into the decimated stopband and passband, they will both be degraded. Can we *Accurately* analyze their degradations with mathematic model? ******************************************************************* And there are some more questions Question One: On page 145 of Chapter 7 in his book, he said: "If we up sample by N and down sample by Q the worst case cumulative gain due to the folding is (Q/(Q�C1), which results in worst case folded spectral levels of Q/(Q�C1) * 1/(2N). " (1/2N is a calculated noise, he is analyzing the decimation affect to the interpolated signal) **********How to get the (Q/(Q�C1)? And is there any paper about it? Question Two: On page 53, Chapter 3 (He designs a low-pass filter with the equiripple method, the passband is about normalized 0.025 Fs, the transition is very narrow, and the stopband attenuation is 60dB and equiripple) For a specific example, the filter presented in Figure 3.20 designed for 60dB side-lobe levels is used in a 32-to-1 down sampling application. If the side-lobes are equiripple at 60-dB the integrated side-lobe level is -36.1dB which, when distributed over the remaining bandwidth of 1/32 (-15.1 dB) of input sample rate, results in an effective alias side-lobe suppression of -51 .2dB, equivalent. ********** Where does the "integrated side-lobe level is -36.1dB " come from? How can we get it? *********************************** Shenzhi Ph.D candidate EE Engineering Huazhong Uni.Sci.Tech. Wuhan,China email: markkknd(a)hotmail.com ***********************************
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