Re: [Discuss-gnuradio] Processing expectations

[Martin Braun]

With a processor like that, you should be able to clock your dongle at 2
Msps all the time, and then resample. I remember faintly that there was
some issue with rtlsdr dongles not working will with lower sampling
rates, maybe that's what it is.

On a side note, this allows you to offset-tune and avoid DC spurs, which
can be pretty ugly on the RTLSDR.

M

On 07.08.2015 11:47, Tom Cook wrote:
> I'm brand new to SDR and I'm just trying to calibrate my expectations of
> what will be possible in real time.  I'm hoping someone here can give me
> a pointer or two.
> 
> I'm running GNU Radio 3.7.2 on Ubuntu 14.04, having installed from the
> Ubuntu repositories.  The machine I'm using has an Intel Core i5-2520M
> running at 2500MHz.
> 
> I'm constructing flowgraphs in GRC and using an RTL-2832U-based USB
> dongle as a receiver.
> 
> I've built a fairly simple broadcast FM stereo receiver as an example. 
> It has the following components:
> * RTL-SDR source (768k sample rate).
> * Low pass filter (768k sample rate, 96k cutoff, 4k transition)
> * Rational resampler (4x decimation)
> * WBFM receiver (quadrature rate 192k)
> * Low pass filter (1x decimation, 192k sample rate, 15k cutoff, 2k
> transition)
> * 2x Band pass filter (1x decimation, 192k sample rate, differing cutoff
> and transition)
> * Multiply
> * Low pass filter (192k sample rate, 15k cutoff, 2k transition)
> * Add, subtract
> * 2 x rational resampler (4x decimation)
> * Audio sink (sample rate 48k)
> 
> My system is not able to run this in realtime, producing regular
> underruns (about two per second).  Decreasing the receiver sample rate
> to 384k and decreasing the decimation factor in the first resampler to 2
> allows it to run in without underruns, from which I deduce that the CPU
> is right on the edge of being able to process the flowchart in realtime.
> 
> Are my expectations of what I'll be able to do in realtime unreasonably
> high?  Is there something I'm missing, that I can do to make this run
> much faster?
> 
> Thanks,
> Tom
> 
> 
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