On Fri, Aug 7, 2015 at 2:47 PM, Tom Cook <tom.k.c...@gmail.com> 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) > You've just strung together 5 filters in a row here. You can do your decimation step in the low pass filter, and the WBFM stage has a filter in it, as well. That 4k transition in the initial LPF at 768 kHz will make for a long filter; if that's also doing 4x down sampling, you'll remove a lot of processing requirements there. Think about all of these filter stages and how you might be able to combine them and reduce their overall power requirement.s > * Multiply > * Low pass filter (192k sample rate, 15k cutoff, 2k transition) > * Add, subtract > * 2 x rational resampler (4x decimation) > * Audio sink (sample rate 48k) > Again, you can do the resampling and LPF together; that resampler implements its own filter. You can also integrate the multiply stage into the LPF by setting the gain of the taps to your multiply value. > 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 > It sounds like you're close to what you need to get this working. Just think more about the filter stages and values. Use gr_filter_design to plot them and see how many taps you're using to help you reduce the computational requirements. Tom
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