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
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