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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <FE434AB14D2442DA9517B6F7304CE6E3@AGB><FB72BFCD7B564064B0C93A0FD2DA9448@Extensa><4F97E9C2.9030000@iup.uni-heidelberg.de> <CAA8k23REzyvnLSkQGwzh4ckSFTeSFSPQkUMD2z-K9-YTGZfxQw@mail.gmail.com> <7B23411450514A5CB804E6D2C65A1876@JimPC>
Date: Sun, 6 May 2012 17:12:51 +0100
MIME-Version: 1.0
Subject: Re: LF: Generating 8970 Hz carrier with Spectrum Lab ?
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Hi Jim There was a case of a USB framing incompatibility on the Funcube
"dongle" VHF/UFH receiver which necessitated a rewrite of the code in the
dongle. Something to do with slippage between the USB post and the frequency
source in the dongle.
This produced a noticable click on some systems and an almost imperceptible
one on others. It totally destroyed some data decoding.

The Funcude USB is compatible with USB 1.1 I believe.

Alan
G3NYK

----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, May 06, 2012 4:14 PM
Subject: Re: LF: Generating 8970 Hz carrier with Spectrum Lab ?


> Dear Andy, LF Group,
>
> A bit late, but never mind...
>
> > Has anyone tried using an external USB soundcard with a separate
> > locked clock?   Most work from a 12MHz crystal which can be replaces
> > with a GPS locked source without too much effort. But I can't help
> > wondering if there will be subsequent USB synchronisation glitches
> > upsetting the input sampling.
>
>
> I can confirm that glitches do occur with USB sound cards. I have found
this
> to be a perennial problem trying to use such a sound card with the laptops
I
> have available. For 9kHz reception, the relatively rapid temperature
> fluctuations inside the laptop, and resulting cyclic drift of the internal
> soundcard sampling frequency interfere with the operation of DL4YHF's
> ingenious sample rate correction facility in SpecLab, making the internal
> sound card unusable for FFT resolution below a few millihertz. I found my
> USB soundcard solved this particular problem quite well, but introduced
> glitches that made achieving FFT resolution in the uHz range impractical.
>
> Watching Speclab's sample rate correction "history" window, the USB card
> sample rate typically starts off a few hundred ppm low (much larger than
the
> actual clock frequency error), but remaining stable to within a few ppm,
but
> then at unpredictable intervals abrupt jumps in sample rate of a similar
> order of magnitude occur, with corresponding "blips" on the spectrogram
> traces. The reported sample rate is always lower than the nominal value,
> suggesting that some samples are being periodically discarded somehow.
>
> The sound card uses a single-chip integrated audio codec and USB
> transceiver, using a single 12MHz crystal. I can't really believe in "USB
> slippage" in the hardware - surely losing some of the data would either be
> handled quietly by the USB error checking, or result in endless error
> messages. The same sound card seems to work in a glitch-free way when
> plugged into my desktop machine, where the reported sample rate error is
in
> line with the error in the crystal frequency.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>