> 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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
