Re: [Discuss-gnuradio] How to control latency

[Marcus Müller]

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
>
>
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