"Best" sampling rate?

Ben Ryan benryanau at hotmail.com
Wed Sep 5 19:22:25 UTC 2012

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.

> I was wondering if certain sampling rates were better than others for
> the RTL2832 dongles? I'm finding that if the sampling rate is below
> 1.1MSPS, I get few samples being dropped (but they still get dropped).
> Above this rate, randomly, I get a multiple of 94 samples being
> dropped.
> Are there certain sampling frequencies that are sweet-spots for the
> RTL2832 that produce very stable performance? Perhaps frequencies that
> are a fraction of DVB sampling frequencies?
> JP
> -- 
> Integrity is a binary state - either you have it or you don?t. - John Doerr

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