Problem with Macbook Optical Out

[pdm4606]

We tend to make things so complicated.
I have in the past checked for a red glow on the end of the fiber optic/Toslink cable at the end that is supposed to plug into the decoder device.
Even then check for kinks as these fiber optic/Toslink cables are somewhat sensitive to kinks. A minor kink could cut down the signal strength enough to act as an "off".
Have you tried another shorter fiber optic/Toslink cable? Sometimes the overall length will offer so much resistant as shut down the decoder.
I purposely have a 6 ft. fiber optic/Toslink cable just to check for length issues.
Another issue is the size of the fiber optic/Toslink fibers inside the cable. I have a perfectly good fiber optic/Toslink that is very thin(small in diameter) and gives non working issues.
At some point in time the inside diameter has been changed and that causes fits. The heavier, the better. Older fiber optic cables I have are twice the size of current fiber optic cables.
I recently have seen a documentary about under sea fiber optic cables. Those cables are very thin. Larger than a human hair. The bundle size constructed of bunches of fiber cables and made to last at a depth of 3,000 feet deep in a bunch around the size of a mans arm. And they carry an enormous amount of data. Imagine for instance that the cable connecting a good portion of South America to the U.S. that is around 5,000+ miles long that is constructed of bunches of hair size fibers carries a vast amount of telephone and digital data from Chile to Los Angeles. The transmitter end is state of the art with a very high output and built for longevity.

 

[cpt_paranoia]

I recently have seen a documentary about under sea fiber optic cables. Those cables are very thin. Larger than a human hair.

Those are monomode fibres, intended to have very low time dispersal, to ensure the optical pulses do not become smeared. There is only one propagation path in the fibre.

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

Cheap fibres used for TOSlink are so short, and the bit rate so low, that pulse smearing is not an issue. It doesn't need a monomode fibre, and cheap, polymer fibres, where the light can take many paths down the fibre, are perfectly good enough.