EN: Yup, most regular desktop computers are safety class 1 devices, that require a connection to safety ground. So if one integrates such a computer into one's audio or AV system, which happens to consist of safety class II devices and to be connected to an antenna or cable radio/TV wall socket, chances are pretty high, that this will introduce hum due to different ground potentials, so that one will need to provide galvanic isolation, most usually either with an audio transformer/ground loop isolator between the computer and the audio/AV system or with an antenna ground loop isolator between the wall socket and the system. Both of these approaches may be not entirely non-lossy, though - in which case optical SPDIF can be a welcome alternative - especially if the receiving device isn't an older design, which might still derive its clock from the SPDIF input and thus react less benign to the not rarely somewhat increased jitter (compared to electrical SPDIF) than a modern design, that relies on its own master clock...
And of course the same would apply, if a CD transport (or CD player, that's merely used as a transport) would happen to be the only safety class I device in a system that otherwise only consists of safety class II devices.
Greetings from Munich!
Manfred / lini