if you happen to do the same filter for a larger set of subbands, look into the polyphase filters:
http://www.trondeau.com/examples/2014/1/23/pfb-channelizers-and-synthesizers.html
The question which method is the computationally best cannot be answered universally; in essence: go ahead and try.
This is a really large sampling rate factor, so I assume a direct FIR filter will have a shipload of taps. You can try and compare the FFT-Filter with a cascade of smaller ones.
The question of "what filter is right for me" is as scientific as it is religious. There's no way to know unless you know the spectrum you want to get.
Greetings,
Marcus
On 06.10.2014 21:08, khalid.el-darymli wrote:
Hi, I would like to downsample a signal from *390,625* Hz to *125 *Hz. *What is the best way to do this in GNURadio? * Please find below some issues I am having with this. First, I used the Low Pass Filter block, in GNURadio, I set my Decimation to 3125, my Cutoff Freq to 125/2. But I am not sure about *the recommended value for the Transition Width parameter*? On another note, to obtain accurate results for downsampling in Matlab (and for DSP generally) it is often recommended to split the big decimation factor into a cascade of smaller ones. If I was to do the downsampling from 390625 to 125 Hz in Matlab I would do something like 5*5*5*5*5 (i.e., five separate downsamplers). *How to optimally do this in GNURadio?* By 'optimally' I mean with less computational resources given that I need to do this for multiple channels simultaneously. Finally, in another experiment for testing downsampling in GNURadio, I tried to use the Rational Resampler block with the *default settings*. However, my results were aliased. I think Low-Pass Filtering is not included in the Rational Resampler by default. *Is this right?* *How do I enable the low-pass filer? *and *what kind of filter is recommended?* Thanks in advance for your help. Best regards, Khalid
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