Macbook Pro Optical Audio Output Toggles On/Off

My Macbook Pro optical audio output seems to switch off with a loud click when sound is not playing for a few seconds.

Then, when sound resumes, there's a delay as it switches back on.

Is there a way to stop this toggling?

And please don't tell me to check the connections.

I've already tried different sample sizes and bit rates to no avail. It seems as if the system is switching something when there is no audio being played, then switching it on again. Even if the playback is only on pause.