Q: Thanks! Excellent and helpful article. Is there any chance to play movies with AC3 passthru on my Mac connected optically to a receiver?
A: Yes, AC3 passthru is supported in iTunes provided your Mac has an optical or HDMI connection available. Keep in mind that Mac OS X is able to detect what type of audio connection is connected to your Mac; the AC3 track will only be available if an appropriate optical or HDMI connection is detected.
You can check your output connections by going to Sound in your Mac OS X System Preferences. If nothing is connected, Internal Speakers will be displayed here; this changes according to what you have plugged in, and will show Digital Out if an appropriate digital audio output is connected.
Provided a digital output is available and selected in your System Preferences, you should be able to select the AC3 track while playing a video either from the Controls, Audio & Subtitles menu in iTunes, or by clicking the speech balloon icon that appears in the on-screen playback controls.
The AC3 passthru track will be listed as “Surround” versus the normal two-channel “Stereo” track.
In fact, with a digital audio connection in place iTunes should normally select the “Surround” track for you by default when playing back any video which includes one, in much the same way as the Apple TV does.
Keep in mind that not all Mac models provide a digital audio output port, and many use a single port to provide both analog and digital output.