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Because doesn't need a narrower filter.
If I want a 4x interpolation in one stage I need a LPF whith f_stop= 4f_in/8, viceversa If I do it in two stage I need f_stop=2f_in/4 for every stage. Right ?
By the way I think that the sample rate to be specified in the fir compiler megawizard is referred as the output sample rate. In fact if you choose interpolation by two and set the sample rate at 30MHz the wizard will set f_stop=7.5MHz.
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you need to cut after 2/60 and gently towards 13/60:
h=firls(20,2*[0 2 12 30]/60,[1 1 0 0]);
freqz(h,1,0:.01:15,60)
only 21 taps for up4
I don't know what tool you are using but I expect sampling rate definition to be defined in the doc.
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