Jan 25, 2016

Noisy Converters - Adventures with Optical Audio, Part I

I'm quite positive that everyone who once tried to transmit analog audio over a long distance encountered sound quality issues in some form. When I wanted to hook up a pair of speakers across the room to my computer, I went for a digital solution right away: TOSLINK. A simple standardized audio interface using a fiber optic cable. Its signals are identical to S/PDIF, an elektrical digital audio interface. Both TOSLINK and S/PDIF have gotten pretty common nowadays in all kinds of audio equipment such as TVs, CD players, HiFi systems and even PC sound cards. No interference, crystal clear sound, no path loss. Sounds great so far? (Pun intended)
I bought a cheap sound card for my PC with optical output, a 10 m fiber cable and a digital to analog converter.

However, whenever the converter doesn't receive an optical input, such as when the sender is turned off, it started to output ugly noise. There wasn't really much you could do against it except shutting the converter off when my PC was off. So I made a little device with a monostable circuit triggered by the optical signal that switched to power for the converter on whenever there was an active input and shut it back of when the input was inactive.