AT&T cables with ST-BNC connectrs does it work ?

i could be wrong, but the ST connector uses a fiber optic cable inside while a BNC cable uses a copper/silver wire inside. the fiber optic cable sends data as modulations in light while the BNC bales sends data as voltage down the wire. i can't imagine how one could change light modulations to voltage midstream inside the cable, but that may be while i'm a social worker and not an engineer.

well such cables ST-BNC exist (Brand Siecom) in the tele communications industry, therefore I just wonder whether it will work out with audio CD transports and DAC or not ?

What you're probably thinking of is a BNC-to-multimode AT&T ST fiber optic media converter. It's an external powered device with an integrated transceiver that, depending on implementation, may or may not be compatible end-to-end with the FO transceiver inside your component.

FWIW, I've generally found that both ST and toslink fiber interfaces in transports and DACs sound inferior to PDIF or BNC with a good cable.

thx yes correct Dgarretson, so probably it will not work out in audio ?

You might be OK if you can determine whether you have multi-mode or single-mode ST, and then select a transceiver with the correct orientation(i.e. transmitter or receiver.) However my knowledge is not strong enough to know whether all FO transceivers pass systems clock transparently.

Another approach would be to have a BNC interface installed on the component and go BNC end-to-end. Check with a tech, but I believe this merely entails replacing RCA with BNC. BNC is a high-quality true 75ohm interface. Multiple electrical to FO conversions with serial cables may introduce jitter.