SOS Forum Post
Reviews Editor Mike Senior replies: The problem with inputting multiple bits of S/PDIF gear into a single unit is that the digital clocks of all the different pieces of gear need to be synchronised, otherwise you'll get nasty digital clicks. A soundcard with two S/PDIF inputs would either have to have word-clock inputs and outputs to synchonise it with the two input sources, or it would have to incorporate built-in sample-rate conversion. Good sample-rate conversion isn't cheap or easy to implement, so most budget soundcards (and hardware recorders too) won't be able to include it. That's why you will generally find only one-in/one-out S/PDIF interfacing where there's no word-clock I/O.
You can run more than one unit into a computer through a single S/PDIF input, but the signals need to be mixed together first, bearing in mind that there's the possibility of overload from the combined signal. There are a number of different devices available that can do this for you. Two that I can immediately think of are the Roland M1000 digital line mixer and the Mutec Smart Merge router. Both have been reviewed in Sound On Sound, in June 2003 and January 2004 respectively. Although they have slightly different takes on the problem, both are useful problem-solvers, offering all the necessary sample-rate conversion, and should cost you around £500.
If you just want to switch between sources without having to un-plug and re-plug, the Smart Merge also includes this function, but is perhaps a little overspecified for just this task. You might want to look for a simple digital source switcher instead.