I'll start by saying I can do it, but I don't know why and I want to know how.
By default, the Mac cannot decode AC3, although it can 'pass-thru' the audio stream to e.g. an AV amp that CAN decode AC3. But I have some MP4 video files of h264 video and ONLY AC3 2.0 audio and these should not play on a standard Mac, normally QuickTime will just show a single frame, but won't actually play when there is only AC3 audio. But on my Mac, these video files play perfectly with full audio (analogue stereo output to computer speakers) in QT Player or iTunes. How come?
I also have installed Flip4Mac, VLC and a couple of other players, but do any of these actually install codecs usable by e.g. QuickTime? I thought they had their own 'built-in' and not available for third parties to use, unlike Perian (not installed) which is/was ONLY a bunch of codecs for other players to use.
Can anyone suggest what my Mac might be using to play videos with AC3 audio?
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VLC is built with its own internal codecs so you avoid the Windows "codec hell" scenario where multiple codecs interfere with each other. As far as I know you cannot make the VLC internal codecs available for external programs to use. I have no idea what Flip4Mac installs.
I find your assertion to be strange because Macs come with a DVD software player and it MUST be able to decode AC3 to be compliant. So there's no reason that QT couldn't use those libraries once they are on the Mac itself. QT may not do so, but again, there's no reason it couldn't. Maybe you are familiar with older Macs and older versions of OSX and perhaps things were different then.
It appears to be a well known issue that QuickTime etc. CANNOT decode AC3. Google it and you'll see what I mean. Since DVDs also come with a regular stereo soundtrack, that's what DVD Player would use to play on the Mac itself. Of course if you're just sending the audio to an AV amp via optical, then the Mac doesn't have to do any decoding, so DVDs would always seem to play.
However I have now found the 'culprit'. It's A52Codec.component. I have it on my Mac, but I've no idea where it comes from. It's not on a different plain vanilla Mac that's had no video players installed, so I'm pretty sure it didn't come from Apple. It appears Perian included a version of this, but that it doesn't work in the latest OSX. There is this other newer/smaller one floating around the web and I seem to have that, but I don't know how. Still, gift horses mouth, don't look etc
So I've installed that component on the Mac that didn't have it and now the videos are playing just as I want (which they were not previously).
So all's well that end's well.
DVD specifications REQUIRE that AC3 be supported. It is NOT an option. Maybe it's different in the UK, but in the USA/Canada most DVDs do NOT come with a "regular stereo soundtrack". That would be LPCM anyway, which takes up a ton of valuable space on the DVD, which is why only some music DVDs have it. Maybe you don't know this, but DTS support on DVD is 100% optional, which is why the spec requires that the DVD have either LPCM or AC3 audio in addition to DTS - players MUST support both AC3 and LPCM. Apple can't use the name "DVD" in "DVD software player" if it's not compliant. And I can tell you that I have tested a ton of DVDs that ONLY contain AC3 soundtracks on my iMac and they all play fine.
QT is a POS anyway so it wouldn't surprise me if it can't decode AC3, but there's no reason that if properly written it can't just use the DVD software AC3 library. It may not do so, but it COULD. I'm hardly a Windows bigot either as WMP is also a POS, but QT is pretty crappy. Not sure why you'd care about what it can do when you can just use VLC.