Is AT&T a Good digital cable connection ?

There is a difference between Toslink optical and ATT/ST. The ATT/ST is reputed to be a high quality link and many older high end products offered it, at least as an option. Few do now, tho. Have not compared, but is reported to be way better than Toslink.

Swampwalker is right, ST-glass is much much better than TosLink.
The implementation of AT&T sender and receiver-units are rather expensive....but the results are quite special (at least to my ears).

Jayt- Not all glass fiber transceivers are created (or implemented) equal. With certain manufacturers, Goldmund being one of them, the AT&T input is simply amazing. On C-J DAC, it was not! Copper does provide more "tuning" ability because again, not all coax sounds the same. However, with the gear I'm running, glass is the way to go. Just my .02.

It seems each manufacturer has there own preference for digital connections. Perhaps it is in the design of the DAC or transport that makes one connection inherently better than another in certain gear. EAD always used to recommend AT&T glass. Ed Meitner, noted digital designer, recommends AT&T because it breaks the ground between components. The late Howard Mandel of Altis Audio also recommended AT&T, and all his high end stuff came with it standard. So is it a good connection? Absolutely. But we are talking about AT&T, not toslink, which is an inferior connection. Perhaps toslink is the connection Mgl123 is talking about when he says that "high end stuff doesn't even offer glass". I think you are referring to AT&T by your reference to "ST", which is another designation for it. Not only does some high end gear come with AT&T, but Theta had there own dedicated optical transmission system called "laser link", I believe.

I experimented with ST cables and connections with every DAC or transport I had that supported it. I found the ST connection to be dead quiet, very black background, lots of detail, but somewhat sterile. That was in the system I had at that time. It's sure worth experimenting with if your digital gear supports that connection.

I've found AT&T ST to always sound equal or better than coax or aes/ebu. If the DAC and Transport are designed around this method of data delivery, that's what will sound best. I think a lot of the newer gear doesn't include it do to cost and the fact that aes/ebu has come a long way. Plus I2S seems to be the new "hot" item. The one reward that I have found, is that it does not have to cost you an arm and a leg for a "glass" cable. I had one made for me at my local supplier for less than $15. You just need to know which fiber, single or multi mode. It's all in the termination of the ST connector. I've compared it to the D-60 and Orchid. It equals the $350 Orchid at a fraction of the price.