I don't know that much about the digital speaker since I never have one yet. All I wonder is if digital and analog signal are really different when they all are from computers which use digital signal as a base. If it's really output as digital, then the digital speaker must have DAC too. Does that sound weird? and why should we have to have a sound card anyway--just to have a digital connector?
That's what I have to figure out next. Surely, I will get one of those soon.
However, what I want to say today is I had a chance to fix one of my friends' computer; he had Sony VAIO with SoundMax AC'97 onboard. There are 3 common connectors--line in, speaker, and mic in. Moreover there is one SPDIF as well. The strange thing is he had plugged the RCA coax + converter to stereo plug to line in connector. He claimed that it worked randomly. I really had no idea how it works; I thought it might be the same as my creative audigy ZS which one connector can handle both digital and analog output. By the way, I have no luck to make it work. All I can find is plain drivers which allow sound card to work properly--there is no option to tweak only a bit. At that time, I'm almost pretty sure of that's his mistake. But I don't know really because I never have this experience. All a guide book, whatever documents from sony.com don't help.
After having no hope to tweak those connector to work digitally, I asked him that he had to have a new cable. Fortunately, he gathered all stuffs from his cabinet and found old computers. What I found is Creative SoundBlaster Audigy 2. Yes! it has S/PDIF output as well and that port is not optical but electrical which is working very well for his cable. :-D Ultimately I can solve his problem then.
Only the question left in my mind: if it's an electrical connector which needs electrons to carry the signal--is it digital really? or it's analog signal with digital encoded inside. If so, it should consider as digital or just analog as same as telephone signal?
Oh I don't believe that I'm an electrical engineer :'( because I think everything is analog by the way :-P Digital is just for any calculations or processes in circuits. Whenever signal-to-noise ratio does matter, thus digital is all the way. (analog is just what we don't realize it's underneath in every single steps)