There are two common types of digital audio out from TVs: coax and optical. Video is not transferred over this link, so you don't need to worry about any affect to your TVs picture.
What you need is something like this. If you Google "toslink rca" you should find similar examples. Toslink is the industry name for the digital optical audio output.
my dad bought a Zenith flatscreen tv and wants to hook it up to his surround sound system but it doesn't have rca output jacks only a optical digital audio output jack.i found converters online but they all have a coaxial input plug that is like an rca jack.he doesn't need a video feed only audio but what is the coaxial input for and if i buy this will it put the audio thru to the surround sound and still have the video or do you have to have the coaxial video feed too?