Can PC RCA jacks be used for ADC?
The "DIY EEG" does not show using an RCA jack.  The project uses 3.5mm plug (same as headphone plugs).  

Here is RCA plugs  :  


  .  

Here is what the DIY suggested  :   

 .  

Also,  the DIY stated 


  As we will be using our computer's sound card to get the data in.


.  So they are using the sound card as the digitizer.