You need a Toslink optical cable.
http/www.amazon.com/Mediabridge-Toslink-Cable-Feet-Optical/dp/B004LSNF04
Thank you that, it helps. Now, with some knowledge I've come to the second part of the problem. I'm connecting an older surround sound that has RCA input. I've looked at adapters but some say they are not compatible with 5.1 channel output. Do you know of any I can use?