"Best" sampling rate?

James Peroulas james at peroulas.com
Thu Sep 6 20:16:21 UTC 2012

Message: 1

> Date: Thu, 6 Sep 2012 05:22:25 +1000
> From: Ben Ryan <benryanau at hotmail.com>
> To: <osmocom-sdr at lists.osmocom.org>
> Subject: "Best" sampling rate?
> Message-ID: <SNT139-W176A4A72E42D16D41CA612B6A90 at phx.gbl>
> Content-Type: text/plain; charset="iso-8859-1"
> Come to think of it, I've also found some samplerates to be better than
> others. No further info to share beyond the fact that I've noticed it too.
> Drops at higher samplerates are probably due to your CPU, IO latency, etc
> (esp if virtualised). You should be able to run 2.048Msps with AF rate of
> 44.1khz with next to no drops and smooth carrier audio. If not, and your
> CPU isn't maxed out,  likely you have a latency or bandwidth issue.
> What's the rig you're running on?
> There appears to be an interaction between AF samplerate and IQ
> samplerate.. under HDSDR/Win32 anyways.
> It might well reduce skips and cpu load if the two rates were cleanly
> divisible but I haven't tried to work the theoreticals out..
> I just set 3.2Msps for spectrum scoping, then narrow to around 1.2Msps for
> FM/AM work. The audio rate is usually 44.1khz although sometimes whendoing
> an "audio print" of a signal using DRM mode I'll up it to 48khz. On the
> Core2Duo I get quite a few dropped samples at 3.2Msps/48khz though - it's
> hitting the limits for this system as-is (it's running VMWare MS plus it's
> got layers of drivers on it).
> Another thing to consider if under Win32: differing versions of libusb,
> ExtIO_RTL, ExtIO_USRP, rtl2832u++.dll files all behave differently wrt cpu,
> samplerates and drops.
> I haven't documented my findings yet but there's a complex relationship
> between the above elements and tuner range.. some RTL libs will permit
> 50mhz-~2000mhz, others puke at various spots on the dial etc.

Thanks for the inspiration. I actually re-wrote my code to use async mode
and now I'm getting reliable captures up to 1.92MSPS (my desired sampling
rate). I was using sync mode but apparently that's not fast enough to
capture all samples!

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