fplanque: dev blog

So your hooked up your Mac to your 5.1 receiver and all you get is lousy stereo? Worry no more, decent AC-3 support is finally available for Mac OSX!

In my case I have an Intel Mac Mini from 2006 hooked up to a Denon AVR-3801 via a fiber optic S/PDIF connection. On most (fairly recent) macs, an SPDIF fiberoptic cable will be the best solution to get the digital 5.1 signal from your mac to your home theater equipment.

Once you make that connection, there is one thing you should be able to do immeditately: play back DVDs in 5.1 Dolby Digital (or DTS or whatever surround track is on the DVD). If not, remove any DVD from the drive, then launch the DVD player app (not Front Row), go to Settings and tell it to output the DVD sound directly to the digital output instead of using the Mac Mini sound processing system.

If still not, the problem is either the cable or the mode selected on your receiver

Now, what if you want to convert your DVDs to files on your terrabyte harddrive? (Private use copy is legal here but I can't tell you about your country). So for the video you'll probably use a divx or xvid codec and for the 5.1 audio you'll use an AC-3 codec... and package all that into a nice AVI file.

Now in order to play that back on your mac, you need a bunch of codecs that are now conveniently packaged in a single downloadable package called Perian.

Once you install Perian, you will get playback but the sound will be stereo, or at best, some variation of Dolby Pro-Logic depending on what you set on the Perian preferences pane. But what you actually want is to get the AC-3 audio from the AVI file straight out to the SPDIF digital out without your Mac doing any alterations (especially not a stereo downmix!)

In order to do that you first need to go to your Applications/Utilities folder and launch the "Audio MIDI Setup" app.

At the lower right, you want to set the Digital Out format to 48000.0 Hz. (See screenshot) That's the frequency used by AC3. Some say you also want to set it to 2ch-16bit but I haven't found this to be necessary so I leave it at 2ch-24bit.

Once that is done, you need to launch the Terminal app that you will also find in Applications/Utilities. In the terminal window type the following in order to enable passthrough of the AC3 signal to the digital output:

Follow up:

defaults write com.cod3r.a52codec attemptPassthrough 1

If you ever want to disable pass through of AC-3 to SPDIF, type this:

defaults write com.cod3r.a52codec attemptPassthrough 0

Now open your AVi file in Quicktime and just make sure the volume slider is at the max! Again you don't want your Mac to interfere with the AC3 signal. From there it should just work If not, are you sure your AVI was encoded in 5.1? (hit command-I in QuickTime and check the audio encoding )