Synopsis of digital surround support in Mac OS X, updated with info from thread:

Mac OS X supports digital audio output, and digital surround pass-through, which any application can be programmed to take advantage of (much like in Windows, Linux, or other Operating Systems). All new Macs include optical digital audio output capabilities, these include the AppleTV, Mac Mini (intel based versions, PPC version did not), MacBook, MacBook Pro, iMac, and Mac Pro.

Several applications use this functionality, including VLC, EyeTV, MythTV Frontend, Media Central, Apple's DVD Player.app, and more.

Apple's primary multimedia tool, QuickTime, does not have support for digital surround pass-through. iTunes, which uses QuickTime, thus does not have surround support either. This lack of support leads to questions on how or if Apple will provide surround sound for movies and other content through the iTunes Music Store for use on AppleTV.

QuickTime and iTunes can be "tricked" into outputting surround audio, by playing DD/DTS encoded WAV files. The audio player treats them as standard stereo WAV files, but since the data is encoded as DD/DTS, the surround receiver interprets the encoded content and outputs DD/DTS surround. But, this is not true surround support. See the following page for information on how to do this with iTunes: http://docs.info.apple.com/article.html?artnum=93870-en

Another point of confusion is support for Decoding DD content. QuickTime can play a file with AC3 encoded audio, and decode it (with appropriate plugins) - converting to stereo for playback. This loses the full surround capability, but allows the audio to be played on any system. This is pretty much the opposite of what most HTPC users are looking for, which is output of true 5.1 channel digital surround.


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I see a sticky thread was added regarding 5.1 output from a Mac. But, it's locked, so I cannot add some more info.

The Core Audio APIs allow for digital audio pass-through, so 5.1 audio is possible with other apps. There are a few apps that I know of which implement it, and probably more that I don't know of.

- MythTV Frontend. It supports digital output from HDTV recordings, and supports DD5.1 for DVDs via its internal video player.

- Media Central, from Equinux. I think it was version 2.5 that added Dolby Digital surround sound.

I just did another test on my Core Duo Mini, and found that QuickTime actually DOES support Dolby Digital 5.1.

I downloaded a DD wav file, and played it in Quicktime (and Finder). It sounds great, and my receiver lights the surround sound indicator and shows "Dolby Digital 3/2".

I tried to play the same wav file in iTunes and only got the "static" sound from trying to play DD encoded data as normal stereo PCM.

The same applies to DTS files. I downloaded a DTS wav file, and it plays fine in Quicktime, and my receiver shows "DTS 3/2.1". This one also did not play in iTunes, although the static sounded different because of the different encoding.


Try it yourself. Here is one link a Google search for "DTS Sample files" turned up: http://www.kellyindustries.com/sounds.html

I assume the original questions about DD5.1 came up with all the discussion around AppleTV lately.

There is no reason AppleTV and the iTMS movies could not support surround sound. All they need to do is add the AC3 encoded track to the files. The hardware on the AppleTV is perfectly capable of AC3 pass-through, which I believe was already proven by people using VLC.

I am assuming Apple will start offering 720p movies with surround sound in the near future. The AppleTV device does not make sense without content to take advantage of its capabilities.

EyeTV also supports DD passthrough and output to external cards.

Okay, yet another update on this...

iTunes also DOES support playback of DD5.1 and DTS files. It just doesn't support them very well.

You need to make sure all the audio modifications (soundcheck, sound enhancer, cross fade, volume adjustments for that file) are disabled, and your iTunes application volume must be 100%, otherwise you get static. Once all that stuff was turned off, iTunes played the audio files just fine.


Here is an Apple support page describing how to enable it:
http://docs.info.apple.com/article.html?artnum=93870-en


So, basically DD5.1 / DTS is completely supported in Mac OS X, but no iTMS content provides a surround sound track yet.

This also leads me to believe that surround sound content may not be as imminent as I thought. If it were coming soon, Apple would have better support for it in iTunes (i.e. it would automatically switch those settings off to allow DD file playback and never expose the user to static).

There is no reason AppleTV and the iTMS movies could not support surround sound. All they need to do is add the AC3 encoded track to the files. The hardware on the AppleTV is perfectly capable of AC3 pass-through, which I believe was already proven by people using VLC.
.........
So, basically DD5.1 / DTS is completely supported in Mac OS X, but no iTMS content provides a surround sound track yet.

This also leads me to believe that surround sound content may not be as imminent as I thought. If it were coming soon, Apple would have better support for it in iTunes (i.e. it would automatically switch those settings off to allow DD file playback and never expose the user to static).
I think there is less concern here about 5.1 sound in Apple's measly movie selection, as there is for a practical way to play ripped DVDs over Apple TV while preserving the 5.1 sound. I guess it would be up to Handbrake developers to keep the AC3 or DTS soundtrack while converting the video to H.264. Is that technically feasible?

I just did another test on my Core Duo Mini, and found that QuickTime actually DOES support Dolby Digital 5.1.

I downloaded a DD wav file [...]

That's not so much supporting AC3 / DTS in QuickTime but rather a natural extension of playing bit-exact digital audio through the optical port. QuickTime and iTunes think that they're playing 2-channel uncompressed PCM audio like the wav file claims it contains (thus the static over analogue outputs or if they try manipulating the audio) but recievers are designed to look for AC3/DTS sync codes in audio and they detect that it's compressed audio and treat it as such.

That's not so much supporting AC3 / DTS in QuickTime but rather a natural extension of playing bit-exact digital audio through the optical port. QuickTime and iTunes think that they're playing 2-channel uncompressed PCM audio like the wav file claims it contains (thus the static over analogue outputs or if they try manipulating the audio) but recievers are designed to look for AC3/DTS sync codes in audio and they detect that it's compressed audio and treat it as such.


Yeah, you're right. It doesn't appear to be changing the audio-out port settings to put the port in pass-through mode.

I'm quite willing to modify the sticky if there are other programs that can really pass 5.1. Reading this thread so far, QT remains invalid, iTunes is possible (but apparently not on an AppleTV), EyeTV can (but only on it's own recordings) and MythTV and MediaCentral both can.

Which brings up another question: has anyone succeeded in getting passthrough in MPlayer? Also, does MediaCentral use MPlayer and have people gotten 5.1 passthrough from MediaCentral?

The sticky was made, btw, simply because several people seemed convinced that the AC3 codec for QT passed AC3 via the digital audio port.

QT and iTunes are the same situation. They don't "support" DD5.1, but they can be tricked into outputting surround sound by playing a pre-encoded DD/DTS audio file.

Beyond that, the situation on Mac OS X is just like Windows.. The operating system supports digital audio out, the Core Audio API has hooks for it, it is up to the application developer to support those digital audio hooks and correctly set the port configuration based on the audio output type.

At this point, for whatever reason, Apple has chosen not to support digital audio pass-through in QT and iTunes. This, I believe, is the main issue people are dealing with. Since Apple doesn't support it on their main apps, it won't work with AppleTV, and their ability to do digital surround sound on iTMS content is unclear. But, there is no inherent limitation in Mac OS X that makes digital audio or surround sound any more difficult.

People have not mentioned the obvious app that supports it Apple DVD Player. It supports both DD and DTS.

iTunes also DOES support playback of DD5.1 and DTS files. It just doesn't support them very well.

Here is an Apple support page describing how to enable it:
http://docs.info.apple.com/article.html?artnum=93870-en


Tji, according to that page, iTunes can only playback DTS files. It makes no mention of DD or Dolby. Do you have other information or is Dolby not supported?

It's the same process for both. I tested DD5.1 and DTS files, and as yuvi mentioned, it's just dumping raw encoded DTS/DD data to the audio port as if it were PCM. In effect, it's tricking it into sending surround sound to your receiver. So, while it does result in your audio receiver playing true surround sound, it's still short of actual support for DD/DTS in iTunes.

It's exactly the same situation as QuickTime, I can play that same file in QT and get surround sound output. But, QT does not really support DD/DTS.

My two cents:
In my case, when I play video with 5.1 DD audio (encoded as .AVI (xvid+ac3) or .MOV (divx+ac3) or something similar)) with QuickTime/iTunes/Frontrow using the optical audio out to my audio reciever, the reciever sees it as Dolby Prologic. Its a simulated surround sound that takes 2 stereo channels and splits it up into 4 channels.
(Dolby Surround/Pro Logic is based on basic Matrix technology. When a Dolby Surround soundtrack is created, four channels of sound are matrix-encoded into an ordinary stereo (two channel) sound track by using phase shift techniques. A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range mono rear channel—while systems lacking the decoder play back the audio as standard Stereo.)

So I believe QT doesn't support 5.1 channel AC3

VLC will output the encoded AC3 audio in these videos, which my receiver recognizes and decodes as Dolby Digital.

--sdm.

Yes, that's consistent with what has been discussed here. In some threads, there is confusion between AC3 decoding, pass-through, and encoding. In your case, the AC3 encoded audio data is being decoded into stereo (apparently along with dolby pro logic) because QT doesn't do AC3 pass-through.

Other apps, like DVD Player, VLC, Myth Frontend, etc. see the AC3 audio data and pass that data to the receiver - resulting in DD/DTS digital surround.

Apple's HD trailers have 5.1 discrete audio channels, so you need an encoder to convert that to AC3 format to send over the S/PDIF connection. These are becoming fairly common now, such as in the new video game consoles, allowing them to generate surround sound game audio.


The other scenario we're talking about with the DD/DTS wav files is sort of a hacked pass-through. The applications think it's just a plain uncompressed PCM WAV file, so they send it to the audio-out. In reality, it's a DD/DTS encoded data file, so when the receiver gets the audio stream, it interprets the surround encoding and plays it back as surround sound.

Further: I modified the first post in this thread, with my synopsis of how I understand the situation for digital surround support in Mac OS X. I think something like that could be good for the sticky FAQ. I also think a good summary of the DVD Player.app issues would also be good there.

good summary tji, I think you should specifically mention Front Row, where appropriate. Not mentioning it at all--and specifically mentioning iTunes, Quicktime and DVD player.app--might confuse some readers.

good summary tji, I think you should specifically mention Front Row, where appropriate. Not mentioning it at all--and specifically mentioning iTunes, Quicktime and DVD player.app--might confuse some readers.

Yeah, good point. I will have to test a few things from within Front Row before adding that in...

Apple's HD trailers have 5.1 discrete audio channels, so you need an encoder to convert that to AC3 format to send over the S/PDIF connection. These are becoming fairly common now, such as in the new video game consoles, allowing them to generate surround sound game audio.tji, do you know if the discrete 5.1 Apple uses in its AAC audio tracks for downloaded video is the original sound file? In other words, does Apple take an original DVD or film AC3 soundtrack for a movie and "process" it to get the AAC? Or is it just something like decoding it and taking the original digital data and repackaging it in a lossless manner?

The reason I ask, is if Apple processes the signal going from AC3 -> AAC (if in fact that is what they are doing), and then we have to re-encode the AAC back to AC3 in order to pass it off to a receiver, then how much signal degradation has occurred? Is it possible to do the two conversions and still have a bit perfect signal? Or is it just a third generation signal?

Lots of questions, but I think our goal is that we want the orignal 5.1 AC3 audio track delivered to our receiver untouched, however Apple chooses to do this. Whether through adding AC3 to downloaded content and full support to OS X and software, or coming up with a method for using AAC to pass bit perfect through the two process it needs to go through to get to our receivers.

Make sense? I don't think most people here at AVS will be totally satisfied until we get that. And I don't think that it is unreasonable for us to expect it. I'm tired of hearing people justify the dumbing down of audio quaity for downloaded video content as an acceptable tradeoff for just getting the system going.

I don't know about how Apple encodes their content -- or whoever they contract to do the actual encoding. I would assume they have access to the high quality sources, but that's just guessing.

As for the conversions, I don't think there is anyway around conversion losses when switching between lossy compression algorithms. It's just a question of what is acceptable. And, it will always be better when going from a higher quality version and going to lower quality (i.e. if Apple's AAC is higher quality than what AC3 can do, the result of going 5.1 AAC -> AC3 should be very good).


It will be interesting to see what Apple does. I have been surprised that they have not already offered HD videos on iTMS, and some indication of how they will do surround sound. If they don't do surround, or only do analog surround encoding, that could be a big weakness for AppleTV.

Tji, the reason I made the sticky about 5.1 was because several people new to the forum had installed the AC3 QT codec expecting and believing that it would get AC3 into their amplifier. The point of the sticky was not to create a 5.1 FAQ, it was to prevent the constant repeating of the same AC3 codec discussion (much like the next sticky "which flat-screen displays work with a Mac").

You've done a nice write-up, but please don't be offended if I only use parts of it. The main purpose is still to avoid having repeats of the same discussion. It would be nice, though, if Apple did something soon so that we could just delete it!

right, but the same questions are repeated over and over again for several different devices and software, so, that 5.1 FAQ should encompass:

1) the Mac and now the aTV, since both play back video
2) Apple software like iTunes, Apple dvd player.app and alternatives, like VLC
3) Front Row and any alternatives which don't downconvert to two channel
4) high def, a la EyeTV
and
5) it should explain the Quicktime ramifications.

Anything less is a disservice, especially since we're 97% there already.

Right now a lot of the questions have had to do with aTV conversions, but we've had too many of these AC-3 codec canards going back two years not to do it right, right now. That's because we're always going to have new users everytime there's an update to any given software or app.

At least link to this thread from your FAQ and we'll keep taking care of the rest.

right, but the same questions are repeated over and over again for several different devices and software, so, that 5.1 FAQ should encompass:...

Some of these touch on more general issues as well. But to avoid FAQ clutter, perhaps the main FAQ should be re-written, with clearly defined sections addressing the major HTPC software options, strengths and weaknesses of each, and solutions to the most common problems. 5.1 would be one such clearly defined section.

I would also suggest that someone familiar with Apple TV also writes a FAQ, which can then keep us abreast of all the new developments and work-arounds pertaining to ATV.

I think its also important when talking about AC3 support to note the difference between players that support AC3 in a Video_TS or VOB or DVD and those that support AC3 contained in an .AVI, .MKV or .MOV.

ie. Media Central claims 5.1 AC3 support, but that is only for the DVD-like sources. Which is something I'm not looking for in particular.

--sdm.

followed these instructions- http://docs.info.apple.com/article.html?artnum=93870-en
and attempted to play a .mov (xvid+ac3) (not DTS) from iTunes. The audio is still seen as Dolby Prologic by my reciever.

too bad.
--sdm.

As long as you're going to have this post, please note that Media Central only supports surround sound for DVDs. Nothing else.

I've updated the sticky with the information from this thread. If there are any more suggestions, please post.