[sdiy] Isolating an external DAC from computer Soundcard

[Linium]

Hello,

After months and months spent around my highspeed computerized DAC CV 
generator with MUX, recent tests are showing that it is mostly working (i 
just need to sync well the INHIBIT signal for the 4051, cf my other post).

Today whereas i was playing bass guitar along with my sequencer, my computer 
crashed weirdly and i suspect that my new invention caused this.

To sum up quicly, i take spdif TTL signal and 24 mhz clock from a computer 
SoundBlaster LIVE soundcard "AUD_EXT" connector and connect it to a external 
circuit containing a DIR1703 (spdif->i2s), a TDA1543 (i2s DAC), some AOP, and 
various CMOS chip (counter/Mux/).

This external circuit has its own power supply and i suspect that there is a 
need for proper isolation between the signals taken from the soundcard 
connector and the external circuit.

So i guess i would need to isolate the 3 lines linking the 2 sides:
-GND 
-SPDIF (TTL)
-24 MHZ CLOCK (TTL) 

I thought about an opto-coupler but so far i don't find a high speed one able 
to support 24 mhz.  

Do you think that my trouble could mainly come from linking the 2 grounds 
(computer card and external supply) together ? And may be a simple 
counter-mesure would exist to somehow isolate grounds (caps) ?

Or should i insist on finding an optocoupler, if so, any ref for a high speed  
one would be great :)

Thx, for the reading :)  

Linium