The latest version of perian (to be released soon), will choose between using the hack method for passthrough, and Apple’s method. Anywhere labeled “Hack” will never work with Apple’s method, so it is bypassed to avoid their broken code. The hack does not work with all 5.1 receivers. Taking a file format which is not properly framed, and making it into a .mov file will not correct the framing. Likewise, it will not destroy the framing of a properly framed file format.

AppleTV:

Note: I have never seen properly framed avi files, nor improperly framed mkv files.
*1: MKV files will passthrough if the import is allowed to complete. I tested this by making a reference movie out of the mkv file. If the import does not complete, this will result in silence.

Mac:
As far as I know, the hack always works, and real passthrough does not, in all cases. See related post.

Basically, use mkvmerge from mkvtoolnix to convert the avi file to an mkv file. Then, open the file with Perian, and save it as a .mov file. The result is a file which both has the proper framing for AC3 data, and also has the proper frame decode and display information. This means that one can do real AC3 passthrough on avi files. Maybe this will be the ultimate solution.

Now, if only the designers of MKV had any sense when they designed the timecode system. For those that don’t know what I mean, ask yourself this: Which system can give greater precision:

1. value * timescale / 1E9
2. value / timescale

The answer is the second. If you don’t believe me, try to figure out how to do the value of 1/48000 in the first system (both value and timescale are integers). It isn’t possible, yet the MKV designers decided to go with this system for defining time. The latter was designed by Apple over a decade prior, and is still the superior system.

What does this mean for MKV? It means the default timescale used for 48KHz AC3 audio isn’t exact. If one forces it to 1E6, then it works, but this timescale doesn’t work as well for video. Once could use a finer timescale, such as 100, but the real problem still remains. BTW, with AC3 data, no timescale in MKV works 44.1KHz audio. Yet another case where mov is better, because it has a timescale for the movie as a whole, and then separate timescales for each track, in addition to having a more useful timescale system. It’s a shame the MKV people didn’t realize a superior format already existed when they decided to make their own system. At the least, they could have learned from it and not make new mistakes.

My tests did turn up one interesting fact. I spent forever trying to figure out why an .avi file I had would refuse to passthrough using Apple’s method. I eventually concluded that the difference is that the brilliant people who decided to stuff AC3 into a format never designed to handle anything of the sort, also decided that it was acceptable to destroy the whole point of the format. AC3 is comprised of a frame of data, which is entirely self contained, and must be complete to decode. AVI (and other formats), have the concept of a packet of audio data for decode. Now the “brilliant people” decided there was no need to make the packet and frame have a 1 to 1 correspondence. Result, badly packetized AC3 frames, where several frames are in a single packet, and some frames span two packets.

Apple’s passthrough requires that a packet begin with a frame, and a frame not be split across two packets. I haven’t tested multiple frames in a packet, but I really don’t care to since the above is a deal breaker. This means that Apple’s passthrough will never work with AVI files, hence commit r858 to perian. Since I figured I had nothing to loose, I filed a ticket with Apple about this issue, number 5876598. Maybe they’ll care to fix it, but since it has no bearing on the consumer AppleTV, I doubt it. Since they haven’t done anything yet about passthrough on the desktop (see my other post on this topic), I doubt they will do anything about this either.

Guess we can always hope that they will surprise us all someday.

Apple has enabled AC3 passthrough with Quicktime. They have enabled it on the AppleTV to support playback of their HD movies. The code for passthrough is even in the latest version of Quicktime on the desktop, but the celebration ends there. Apple has not exposed a method for enabling passthrough on the desktop either through the user interface, or a hidden preference. They have not even exposed a method for developers to use it through Quicktime (I’ve tried).

So, what is to be done about it? Tell Apple to address this. Go to http://bugreport.apple.com and fill in an enhancement request. You can reference my report id, which is 5799133, and says:

Please enable a method for AC3 passthrough on the desktop using Quicktime. There are no licensing issues for passthrough, and you already have the enabling code in place. The AppleTV proves that your code works, so please add the interface so this can be used on the desktop.

The hack in use by Perian isn’t perfect. It does not work with all receivers where the true passthrough will. Audio outputted using the encoded digital audio output of the optical interface works, as evident by the DVD player, but this method is not available to Quicktime in any documented manner.

If enough people fill in reports, Apple should realize this is important to people and enable the interface. Please use your own words in your own reports; it says more to them if the reports are not copy/paste.

Update: Apple has marked my ticket as a duplicate of 4557501.

1. Install Perian
   Just go to Perian.org and click the big download link. Double click on the pref pane, and it’ll install itself.
2. Connect your receiver
   The audio connection must be a digital connection. In all cases that I know of, this means an optical cable from your mac to the receiver. In the case of the AppleTV, this is certainly the case. I suppose this could work with the coaxial digital connection, but I don’t know of the audio hardware that provides this connection.
3. Change the sample rate to 48KHz
   • On the mac, open /Applications/Utilities/AudioMidiSetup.app
   • In the lower right, change the format to 48000.0 Hz, and 2ch-16bit
     Note: If, in the off situation, you are playing a file with 44.1KHz audio, you will need to set it to 44100.0 Hz instead.
   • On the AppleTV, this is much more difficult. I recommend using Sapphire instead for playback. It’ll do this step for you
4. Enable AC3/DTS passthrough.
   This is easy when done correctly. For AC3 passthrough, open /Applications/Utilities/Terminal.app and type

   defaults write com.cod3r.a52codec attemptPassthrough 1

   To turn off AC3 passthrough, you type this instead:

   defaults delete com.cod3r.a52codec attemptPassthrough

   DTS passthrough is done through:

   defaults write org.perian.Perian attemptDTSPassthrough 1

   To turn off DTS passthrough, you type this instead:

   defaults delete org.perian.Perian attemptDTSPassthrough

5. Launch your application to play with passthrough. This means if it was already running, quit it and start it again. You can quit Frontrow by entering Frontrow, and pressing Cmd-Opt-Esc.
6. Play with 100% volume
   This does not mean modify the movie to play with greater gain than the original movie file in the properties, but it does mean you shouldn’t change the volume level of the movie file. If it has been changed, set it to the top with the little volume slider. Most likely, you will not have to do anything with this step.
7. Check Channel Layout
   Note: Since Apple refuses to do the right thing and fix their bugs, A52Codec now attempts to detect this layout and adjust itself accordingly. This is yet another hack to work around Apple’s refusals to correct their errors. It may not work correctly in all circumstances.
   Due to bugs in Apple’s Quicktime components, (rdar:5799879 reported in March of 2008, not fixed), some methods of reading AC3 will have the wrong channel layout. Furthermore, due to issues related to how multi-channel audio is handled, the decoder is unaware of the order QuickTime expects (rdar:6921431). Make sure the channel layout is ordered: L, R, C, LFE, Ls, Rs. This can be verified by the “Show Movie Properties” within QuickTime Player. Select the audio track, and “Audio Settings”. It should look like this:
   
   Note: I’ve been arguing with Apple about this one for almost 2 years now! For those within the company, see to it that rdar 5799879 and 6921431 get more attention than a single comment every 3-18 months! Either one would resolve this, but neither are getting response consistent with documentation. What’s worse is that Apple’s chosen layouts are even used anywhere!
8. Check Receiver’s Settings
   As Dominik Pich pointed out, some receivers have an encoding setting. In his example, some Sony receivers need to be changed from DEC PCM to DEC AUTO

That’s it. Play you movies with AC3/DTS audio and enjoy your 5.1 system.

Edit: Added volume information which was missing.
Edit: Added restarting the app which was missing.

I’ve re-enabled comments on this post for the purposes of constructing a list of receivers which work and those that don’t. Please reply with the make/model of your receiver and whether it works. Comments disabled due to spammers.