Hi Daniele, alignment requirements for the FFTw aren't that strict, but you'll have to wait for a decimation*ntaps of samples to accumulate before you can do the FFT.
Greetings, Marcus On 05/13/2015 05:47 PM, Daniele Nicolodi wrote: > On 01/04/15 00:30, Matt Ettus wrote: >> Daniele, >> >> GNU Radio tries to maximize the size of the chunks of data it deals >> with. Clearly that works well for high rate data, but not low rate >> data. There are some handles to control buffer sizes and things within >> GNU Radio, but you may have better luck just using a much higher sample >> rate. If you just decimate less, and have a sample rate of about 10 kHz >> you won't have this problem. You'll use more CPU, but it will still be >> a tiny amount. > Hello Matt, > > thank for your answer. Sorry if I come back to this so late. > > The low samples rate are unfortunately necessary in my application, or I > don't know how to avoid them. What I'm implementing is a control loop > and the communication with the actuator is rather slow (commands send > through a RS232 connection). > > However, I found where the problem was coming from: I was using gnuradio > fft filter implementation, and it looks like that this block has a > minimum number of output elements that increases rapidly with increasing > number of taps. I suspect this is due to alignment requirements of the > underlying fftw library, but I haven't check that. Indeed, using higher > hardware sampling rates required longer filters and made things worst. > > Switching from fft filter to simple fir filter blocks, the problem of > too large buffers goes away and at those modest sampling rates the > higher computational cost is not a problem. > > Cheers, > Daniele > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
