Hello Diego and welcome to pink fish.
The Numerik has two S/P-DIF signal INPUTS. Each of these inputs has an accompanying CD SYNC OUTPUT.
The S/P-DIF input connectors are the BNC type.
The SYNC output connectors are the RCA type.
Any digital source device (whether CD transport or computer audio card or music streaming device) that has a S/P-DIF electrical output can be connected directly to an input of the Numerik, using the appropriate lead and connectors. None of these devices will be able to accept a CD SYNC signal from the Numerik (function works only with a Linn CD transport), but that will not prevent the Numerik from operating correctly as a standalone DAC.
If you want to connect a digital source that only has an optical output, then you must use an optical to electrical signal convertor between the source and the Numerik. These convertors are available quite cheaply and seem to work well enough.
Please note that the Numerik is only able to accept "standard resolution" digital audio at 44.1 kHz (CD) and 48 kHz (DAT) and therefore will not work with "high-res" signals.
Hope that answers your questions.
Mr Tibbs