For years, Macs have shipped with headphone/speaker sockets that are actually rather clever: as well as serving traditional analogue audio, they can also pipe out pure digital audio over an optical link. This is good not only for techy reasons – it keeps the audio in your Mac clean and digital until the very last minute when it's converted to analogue for your speakers – but because it disables your Mac's volume control and forces you to set volume on the speakers.



Link: Techradar