[ntp:questions] Re: Audio refclock+linux+soundcard = yuck

David L. Mills mills at udel.edu
Tue Sep 30 23:46:00 UTC 2003


Adam,

All the audio drivers sample at the same rate: 8 kHz, 8-bit, mu-law, on
the assumption even the worst audio codec can handle that.

However, your comment about the CHU/WWV drivers has merit. The CHU
driver is really a gloriously stupid 300-bps modem and majority decoder
which exploits the considerable redundancy in the decoded character
stream. It works best here on the right coast where the CHU signals are
robust.

The WWV/H driver is by comparison a sleek racehorse which squeezes out
just about every grain of information possible. It is in fact a
theoretically optimal maximum a-priori probability (MAP)
demodulator/decoder matched precisely to the WWV/H signal design. It
works most impressively when the signals are almost too weak to be heard
by ear with a good communications receiver.

There is a price to pay for this. In order to get going, the WWV/H
driver needs to phase-lock on both the seconds pulse and minute
subcarrier. While doing that it is not in a theoretically optimal
configuration. It gets better and better as it refines the phase of both
signals and refines the frequency offset of the codec sample clock, but
getting started can be a drag. However, once the driver has reliable
phase and frequency it's as solid as a rock. Operating the WWV driver
for weeks at a time when signals are often noisy, polluted by
broadcasters and generally beneath the waves here, the WWV/H driver
clearly outperforms the CHU driver hands down.

Dave

Adam Myrow wrote:
> 
> Have you tried the CHU audio driver?  My experience was that if CHU was
> putting in a decent signal, I often got better performance from it than
> the WWV driver.  Actually, for me, the CHU driver worked while the WWV
> driver did not.  The best I ever got from WWV was the date and part of
> the time code.  Since the CHU driver samples less often, I suspect it's
> less sensitive to buggy drivers or hardware.  My gut feeling is that
> sound drivers in Linux were never designed with WWV or any precise
> timekeeping in mind, but only playing and recording sounds without
> breaking up.



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