[time-nuts] Sound cards
Hal Murray
hmurray at megapathdsl.net
Tue Jan 13 23:24:29 UTC 2009
> The optical link commonly being used for S/P-DIF is TosLink and it
> seems like it can be the cause of many problems. It seems like some
> care in doing the optical link setup is needed. I have never digged
> into why the optical links have that problem. I can only guess, but
> bad optical coupling seems reasonable. The multimode "fiber" seems to
> be leaving one or two things to ask for.
It's been a while since I did any serious work with fibers.
There are 2 limitations.
One is signal to noise. You have to get enough light in the transmit end so
that after attenuation there will be enough coming out for the receiver to be
able to find the bits. Attenuatiion is linear with length with a constant
for getting in and out of the fiber. Add some more for splices/connectors.
The other is dispersion. If you have a multi-mode fiber, some of the photons
bounce around more than others which results in a longer path and increased
transit time. Simple geometry is a good approximation. The net result is
that the photons get smeared in time. If your pulses are too narrow (bitrate
too high), the smearing will cause adjacent bits to overlap and you can't
easily sort things out at the receiver.
Single mode fibers don't have modal dispersion. But they do have chromatic
dispersion. Long distance telco links use very narrow bandwidth lasers.
One characteristic of dispersion is that there is a trade-off between
distance and bandwidth. Fibers have ratings in megabit-miles. Typical
multi-mode fibers were 300-500 megabit-miles.
Single mode fibers are roughly 7-9 microns dia for the active region.
Multi-mode fibers were 50 or 62.5 microns.
Roughly 10 years ago, there was a sweet spot at 155 megabits (OC-3) and 2 km
using LEDs for the transmitter and multi-mode fibers. Since then, they are
using low cost lasers (from CDs) so things have changed. If you wanted
faster or farther, you used a laser and single mode fibers.
The engineering/specsmanship on the overall link was super conservative. It
was essentially impossible to measure the error rate. The trick is to insert
enough attenuation so you get enough errors to measure, then compute what you
would get without the attenuation.
I haven't worked with plastic fibers. I'd expect the engineering to be
conservative so it should just work. If it doesn't the obvious problems are
dirt/mud at the connectors or cracked/broken fibers. (I'm assuming a sane
length.)
One disadvantage of conservative engineering is that a system that's broken
might actually work well enough to act like a flaky system. I'm thinking of
something like a broken fiber that is sometimes held in place close-enough by
the jacket.
--
These are my opinions, not necessarily my employer's. I hate spam.
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