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Umm.. no. The quality of light matters. Over a certain length (just like any cable) the light degrades/fades. Also, some cables are just shit... foggy, or scratched outta the box. When the lights is disrupted by scrathes or fades, there's a loss in quality. Yes, if it is too disrupted, it will just not work, but there is a loss of quailty with anything less.
The cable would have to be very long to create any kind of loss problems. Youre talking about a 6 to 12 foot cable in most cases. Loss shouldnt be a big problem unless the cable is VERY poor. If there is a scratch then you have a bad cable. Thats a given. If the signal is marginal error correction will kick in on your equiptment. If its too poor for the error correction to overcome then yes, you will hear it but it will be clicks, dropouts or digital noise. There will be audible errors. It wont be the type of situation where you can say one sounds better than another. If the light was distrupted completely or intermittenly youd lose your signal. But if its getting through at least reasonably well you'll never know the difference. If youre hearing clear sound you have a good cable. I still say the biggest difference is going to be duribilty. How it reacts to bends and aging. Its not going to be something like 'this one lets the light through easier so it sounds better." If youre talking copper cabling then theres much more room to argue but with optical any decent quality, undamaged cable is as good as the next. If you dont believe me, try it.