No. And yes. In that order. This article gives the full detail behind what that really means.

A little while back, I posted an article discussing the possibilities for the Apple TV supporting 5.1 audio playback. It’s well worth reading that article first, as it talks a little bit about what “5.1 playback” actually means. Now that the Apple TV is available, and I’ve been able to run some tests, I’ve been able to see what it can and can’t do. It’s a mixed bag of news; not as good as expected, and a little surprising, too.

There are a few different scenarios here, based on what kind of “5.1″ you’re trying to play:

1) You might have a movie file with a Dolby Pro Logic I (that’s I for one, not II for two) soundtrack. DPL I supports the matrix-encoding / decoding of 4.0 channels of audio into a stereo source. Note that this isn’t a four-channel (4.0) soundtrack. Rather, it is a stereo (2.0) soundtrack, with a set of 4.0 audio information matrix-encoded into the stereo track. The four channels you get are Left, Center, Right and Rear (where Rear is a mono “surround” channel for a rear speaker).

Play a DPL I track through a 2-channel device, and you’ll hear the audio, just in 2-channels only. Play it through a DPL I-savvy amp, and the 4 tracks will be un-matrix-encoded into their four separate channels.

Importantly, this is the audio format used for movies from the iTunes Store. So right now, iTunes Store movies aren’t even 5.1 – they’re 4.0.

2) You might have a movie file with a Dolby Pro Logic II soundtrack. Note that similar to DPL I above, this isn’t a six-channel (5.1) soundtrack. Rather, it is a stereo (2.0) soundtrack, with a set of 5.1 audio information matrix-encoded into the stereo track. The six matrix-encoded channels are made up of five normal channels (Left, Center, Right, Left Surround, Right Surround), and one LFE (Low Frequency Effects) channel. These are the 5 and the 1 in 5.1.

Play a DPL II track through a 2-channel device, and you’ll hear the audio, just in 2-channels only. Play it through a DPL II-savvy amp, and the 6 tracks will be un-matrix-encoded into their six separate channels. (Actually, the LFE is really just the low frequencies in the Center channel, but that’s too much detail for here.)

You’re unlikely to have a movie with a DPL II soundtrack, unless you’ve ripped an existing one from a DVD using a tool such as Handbrake.

3) You might have a movie file with a Dolby Digital soundtrack (also known as an AC-3 soundtrack). (Note that AC-3 and AAC are not the same thing.) It’s unlikely you’ll have an AC-3 soundtrack, as QuickTime can’t play these tracks (see my previous article for more information on this). However, DVD conversion tools such as Handbrake can copy Dolby Digital AC-3 tracks straight from a DVD into an avi file, so you may have got one this way.

4) You might have a movie file with a 6-channel AAC soundtrack – such as one of Apple’s high-definition movie trailers. These soundtracks have six discrete audio channels – one each for Left, Center, Right, Left Surround, Right Surround, and LFE (Low Frequency Effects). Each channel is encoded in AAC format.

To play a 6-channel AAC file on a Mac, you would need an audio device with six discrete audio outputs (such as the Griffin FireWave). This will only work with a Mac, as you can’t add external audio devices such as the FireWave to the Apple TV.

It’s unlikely you’ll have one of these soundtracks, unless you’ve downloaded one of the Apple HD trailers. (However, a future version of Handbrake is rumoured to be able to extract a DVD’s 5.1 AC-3 to 6-channel AAC.)

That covers our starting points. On to the tests.

The first major observation is this: it seems that whatever the Apple TV can do, QuickTime can do too. This perhaps isn’t a major surprise – after all, I’d assumed that the Apple TV would be running some flavour of QuickTime 7.1 – but it does make the observations more interesting. More on this later.

So, onto 5.1. According to Gizmodo, Apple claims that the Apple TV “supports 5.1″. Whether this claim is a genuine claim from Apple (or not) is neither here nor there. What is clear from my tests is that the only form of 5.1 supported by the Apple TV is Dolby Pro Logic II – and it isn’t really working too hard to provide this support.

If Apple are saying “we support 5.1″ to mean “we support Dolby Pro Logic II”, what they really mean is:

“If you have an existing QuickTime-friendly movie with a stereo soundtrack, which happens to have Dolby Pro Logic II 5.1 information matrix-encoded into it, then we’ll happily play that stereo soundtrack to your amplifier. If your amplifier then happens to detect and decode the Dolby Pro Logic II encoding, then you’ll get 5.1 sound out of your speakers.”

Or, to put it another way:

“If you’ve got a stereo soundtrack, we’ll play it for you.”

Thanks, Apple TV!

The same principle goes for the 4.0 audio encoded into a Dolby Pro Logic I soundtrack, and this is how iTunes Store movies are providing their “surround sound” audio right now. All the Apple TV is doing is playing the stereo signal to your amplifier, over the Apple TV’s optical output. Your amplifier then decodes the 4.0 DPL I audio from this stereo signal.

To refer back to that QuickTime reference above – this is also true of QuickTime Player, iTunes, and Front Row on a Mac, too. Plug your Mac’s optical output into an amplifier, and the same thing holds true.

So that’s scenarios 1) and 2) covered. What about 3), where we have a proper Dolby Digital track? Simply put, the Apple TV won’t play these encoded Dolby Digital AC-3 tracks any more than QuickTime on a Mac will. So true, pure Dolby Digital 5.1 just isn’t there right now.

But here’s an interesting thing. In scenario 4), the Apple TV does something unexpected. Rather than downmix the six discrete AAC channels to a simple stereo mix – splitting Center between Left and Right, mixing Left Surround into left (likewise Right Surround into right) and ignoring LFE – it actually seems to be doing some matrix encoding of its own. It’s not quite Dolby Pro Logic I, and seems to be more of a 3.0 mix rather than a 4.0 mix. But it’s definitely more than stereo.

Here’s what the AppleTV (and also a Mac with QuickTime 7.1.5) outputs when playing a 5.1 AAC track via an optical cable, with my amplifier set to expect Dolby Pro Logic I:

The Left channel plays through the Left speaker, and (quietly) through the Left and Right Surround speakers
The Right channel plays through the Right speaker, and (quietly) through the Left and Right Surround speakers
The Center channel plays through the Center speaker only
The Left Surround channel plays through the Left speaker, and (quietly) through the Left and Right Surround speakers
The Right Surround channel plays through the Right speaker, and (quietly) through the Left and Right Surround speakers

My reading of this is that the Apple TV is matrix-encoding the center channel into the stereo output when playing a 5.1 file, and my amp’s Dolby Pro Logic I setting is successfully decoding this as a separate Center channel. The Apple TV is mixing down the Left Surround signal into the Left channel, and likewise Right Surround into the Right channel, and my amp is playing these to the Left and Right speakers. The Dolby Pro Logic I setting on my amp is then also trying to extract a Rear channel, but is failing to find one, which is why a quiet version of Left / Right / Left Surround / Right Surround is playing through the Left / Right Surround speakers.

It’s a bit of a surprise to hear the separate center channel from a 5.1 AAC source, and it’s definitely more than I would have expected from Apple TV (and QuickTime). It’s interesting in itself that the Apple TV will ‘play’ 5.1 soundtracks at all (as a video iPod won’t play a 5.1 AAC soundtrack), and more so that it seems to be doing this matrix-encoding for the center channel. Most likely this is because behind the scenes it’s really a kind-of-Mac-Mini, running a kind-of-QuickTime, rather than a kind-of-iPod.

What’s disappointing is that the Apple TV doesn’t feature any way to play “true” 5.1 Dolby Digital AC-3 tracks – most likely because it is based on QuickTime, which can’t play them for the reasons described in my previous article. Even more disappointing is that it won’t encode 5.1 AAC tracks to AC-3 on the fly, most likely because this would involve licensing Dolby Digital Live from Dolby themselves, with an associated cost. And until the iTunes Store starts shipping movies encoded with Pro Logic II (rather than Pro Logic I), then the average user will be getting 4.0 at best from movies they buy from Apple to play on their Apple TV.

This seems to be indicative of the general approach taken by Apple with the Apple TV – i.e. make it good enough that most people will be happy with the playback it is capable of, whilst keeping it affordable enough that most people will want to buy it. For anyone wanting high-quality audio, it’s a shame that Apple didn’t make more of the opportunity, even if it meant paying Dolby some money. The same could be said of its video capablilities – 720p rather than 1080p – good enough for most people’s needs, whilst not satisfying the AV purists. Still, Apple’s success with the iPod suggests that they’re pretty clued up when it comes to knowing the right feature set to hit – and Apple’s “good enough” is usually still pretty good :-)

Of course, this could all change with a software update… here’s hoping that any extra features for the Apple TV will make it into QuickTime too.

Update: I worked out why the Apple TV (and QuickTime) are outputting a center channel to my Pro Logic amp. It’s not surprising at all, once you learn a bit about how Dolby Pro Logic works.

When QuickTime mixes down the 5.1 AAC to stereo, it splits the “center” input track equally between the left and right output tracks. After all, what else could it do with “center” but share it between “left” and “right”?

Here’s the twist. It turns out that this is precisely how Dolby Pro Logic I and II matrix-encode their center channel too. The Dolby Pro Logic decoder in your amp is looks for any audio which appears in both the left and right channels of the input, and extracting this as a center channel to be sent to your center speaker. So, QuickTime is inadvertently causing a Dolby Pro Logic-friendly 3.0 matrix-encoded mix when it mixes 5.1 down to stereo. Thanks, QuickTime!

Update: Good news – the latest version of HandBrake, version 0.8.5b1, features an option to convert 5.1 AC-3 tracks into a 5.0 matrix encoded Dolby Pro Logic II track. This is playable by the Apple TV (as described above), and looks like the best way (currently) to play surround sound on your Apple TV.

Handbrake 0.8.5b1 also now has the ability to convert a 5.1 AC-3 source to a 6-channel AAC, as mentioned above.

¹Strictly speaking it’s encoded with Dolby Surround, and decoded with Dolby Pro Logic. But this quickly gets very confusing, so I’ve used Pro Logic I / Pro Logic II to differentiate between the 4.0 and 5.1 versions of Dolby Surround / Pro Logic.

²Yes, I know – I’m British, and should be “centre”, not “center”. But I’ve been coding HTML since 1994, and these things stick.