What you are looking for is to change digital video and audio to analog which would degrade both your video and your audio but if you have a specific purpose for doing it, I would try contacting Cisco which makes network, modems, gateways and other commercial and non commercial electronic equipment.  If they don't have it, they would know who does but you might try searching for "Digital to Analog" as you should bet a whole lot of indications on where to look or what to buy but look hard at the reviews on whatever you find before buying it.