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A question of jitter..
Short fibre optic links can be designed with low jitter, but no better than cable. Long links using lasers will have mode problems that just don't exist on copper.
I used to use led modulators with a linear pin receiver in a fibre video transmission system. I found that the sync pulse was obviously distorted. The led turned out to have a low frequency distortion problem similar to dielectric absorption - probably thermal in origin - but only affecting lower current levels. In a SPDIF application this would certainly cause jitter. Different manufacturers all had this feature.
TOSlink receivers are another story, they have a crude non linear amplifier and simple slicer, that gives bad pulse width distortion that varies with received signal level.
The short links also have mode problems, as too much light is reflected back to the source.
SM fibre was designed to work at lengths where copper craps out for high-speed transmission. Around 1 km or so. Shorter than that, you are asking for problems. Yes, it will of course pass data, but....
While LED-based fibre systems do not have the same problems associated with SM, it has more than enough of its own to prevent its use in a high-performance SPDIF application.
Optical links are actually worse in almost every way than proper electrical links for this kind of application.
The advantages of fiber are low loss and wide bandwidth. Both are very dependent on the type of the fiber, the optical wavelength used, and the type of optical source used. There's all sorts of non-linear optical effects and system considerations that are way too complex to describe here. Just one example to keep in mind - I've learned through hard experience that you need better than 50 dB of return loss in an optical link to minimize distortions. Think about that. With an electrical link, you can probably get better results with only 35 dB return loss.
The one true advantage of fiber optics for audio applications is galvanic signal isolation. No ground loops with fiber, that's for sure.
A moderately bad optical link might be better than a monumentally bad electrical link, but that's no endorsement. Better to just get it right in the first place.
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Dette er bare ett utdrag av en tråd med flere bidragsytere.
Konklusjonen er at man bør styre unna optisk fiber til jittersensetive formål. Det følger for mange ulemper med den som man ikke sliter med i elektriske linjer.
Mvh
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