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5.1 Sound on Apple TV - Thoughts Scratchpad

J

JasonKerner

macrumors newbie
Original poster
Feb 4, 2008
19
0
Hi All,

Like many of you, I've been trying to work out how Apple are going to deliver 5.1 into the Apple TV. These are the things we know...

1. Its only for HD rentals.
2. HD rentals won't be made available from iTunes, only Apple TV
3. Its a pass-through solution, so the amp does the decoding rather than the Apple TV (exactly the same as taking optical out of your DVD player).
4. The MP4 container can't support an AC3 track as a soundtrack.

So what about these ideas about Apple is going to pull it off...

As the rentals are only for the Apple TV, and they're releasing a software update, they could mod the MP4 decoder onboard to allow it to play a non-standard, apple-hacked file with a AC3 file in it. The Apple TV has a hardware decoder for the H264 inside due its slow processor, so any change has to be able to cope with that.

It also has to work for people that have HD TVs, but not necessarily a 5.1 system. So something has to be able to convert that file to a normal stereo file. So they could use the Apple TV's sound card to do this - downmix the 5.1 to stereo while its playing. They definitely won't include a stereo mix of the soundtrack on the file as that will eat up more space and leave less for the video track.

Some people have had success adding an AC3 track to a Mp4 file and then saving it as a MOV file, but then we probably lose the ability to add posters, artwork, etc... and even then, normal speakers will just play static as they don't know how to decode the signal. So if we rip films for our machine, how are we to listen to the films when we watch them on computers as a lot of the time we only have stereo speakers and the only program that passes-through AC3 out the optical port is DVD Player.

Heres a thought - what if Apple include a seperate file for the Ac3? The MP4 is bundled with a stereo track (to keep in line with the spec and make it work for non-surround amps), but when the optical port is connected, the software on the Apple TV tells the currently playing track to use the corresponding AC3 file instead. Obviously, the problem here would be, if you're playing the film as you're downloading it - how do you stop the audio running out before the film does? AC3 is compressed at 384Kbps, so it'd be like downloading an high quality Mp3 off the internet, and they are a lot smaller compared to video. So if anything was to run out first, I'd assume it would be the video track, and you'd surely get a message saying its 're-buffering' or something anyway.

Let me know all your thoughts - I know we're all keen to get 5.1 into our own rips. Its all up the air until the update is released, but lets throw some ideas round while we're waiting.

Jason
 
There's two plausible approaches:

  1. Six channel AAC in an MPEG-4 container
  2. AC3 WAV in Quicktime container
The audio hardware in the AppleTV is capable of transcoding 6ch AAC into AC3 (the "Dolby Live" approach), but this capability is not offered by the current AppleTV software. It's also capable of transcoding 6ch AAC into ProLogic II, which it does today on the 1.x software.

The other possibility is that they avoid the Dolby Live licensing by delivering movies with an AC3 WAV encoded with Apple Lossless, and crammed into a Quicktime container. This works on an AppleTV today, but it's a dirty hack and most people don't want to encode their movies in that weird format.

Personally, I think the first approach is the most likely. Points in favour include Apple's aggressive MPEG-4 compliance and the ability to easily switch between analogue and digital outputs at the AppleTV.

Alternatively, something that suggests the other approach is the current refusal to offer these rentals to desktop iTunes users. While it's possible that this is merely to drive sales of the AppleTV, it's also possible Apple are trying to avoid the support nightmare of delivering AC3 soundtracks to desktop Macs. While the desktop version of Mac OS X can do an optical AC3 passthrough on compatible Macs, it can't do it in a way that's very friendly for normal users (specifically, adjusting the volume below maximum makes some horrible noises). In addition, very few desktop Macs are connected to AC3-capable amplification. It's unlikely Apple would ship that solution to normal users.

I really, really hope it's the first option. ;)
 
JasonKerner said:
Obviously, the problem here would be, if you're playing the film as you're downloading it - how do you stop the audio running out before the film does? AC3 is compressed at 384Kbps, so it'd be like downloading an high quality Mp3 off the internet, and they are a lot smaller compared to video. So if anything was to run out first, I'd assume it would be the video track, and you'd surely get a message saying its 're-buffering' or something anyway.
Click to expand...

Couple of things:

AC-3 is not compressed at 384 Kbps. Dolby Digital 5.1 channel surround (AC-3) is encoded at 448 Kbps for DVD and 320 Kbps for theatrical 35mm prints.

Desynchronization is easily avoided by clocking the signals using timecode. AC-3 encoded by a licensed Dolby Digital encoder typically has embedded timecode so that it can maintain frame synchronization with its corresponding program.
 
EvilRob said:
There's two plausible approaches:

  1. Six channel AAC in an MPEG-4 container
  2. AC3 WAV in Quicktime container
Click to expand...

So, now that Take 2 is out, has anyone tested to see which way we should be going with 5.1 conversions? AAC or AC-3? (I can test both, but not until way later tonight.)
 
jbellanca said:
So, now that Take 2 is out, has anyone tested to see which way we should be going with 5.1 conversions? AAC or AC-3? (I can test both, but not until way later tonight.)
Click to expand...

It's been less than an hour!! Are you expecting miracles?! :eek:
 
jbellanca said:
LOL, I have two files ready to test, one with AC3, one with 6-channel AAC... I'm just at the office right now and can't test them. (Yes, I'm impatient.)
Click to expand...

I've tested an .mkv version of Serenity saved as a .mov which inspects properly (H.264 video, AC3 audio) and plays fine in Quicktime Player with Perian installed.


My AppleTV is connected to my Harmon Kardon receiver via HDMI and there is no audio detected - the audio channel inputs just flash, no sound on any configuration.

So for me a simple AC3 audio track is not passed thru.
 
jbellanca said:
LOL, I have two files ready to test, one with AC3, one with 6-channel AAC... I'm just at the office right now and can't test them. (Yes, I'm impatient.)
Click to expand...

Your AC3 might work, but your AAC won't. Straight from the ATV2 spec page:

Audio formats supported
AAC (16 to 320 Kbps); protected AAC (from iTunes Store); MP3 (16 to 320 Kbps); MP3 VBR; Apple Lossless; AIFF; WAV; Dolby Digital 5.1 surround sound pass-through

See the post 110 for a suggestion.
 
From the handbrake forums:

bitsflew wrote:
AC3 in a mov container works!

Here is how I did it:

Open an existing mp4 (I used a handbrake rip) file in QT player.
Open an ac3 soundtrack in QT player.
Paste the ac3 soundtrack into the mp4 file
Export the movie as a self contained Quicktime movie.
Import the movie in iTunes.

Looks like they're making some headway.
 
darkpixie08 said:
From the handbrake forums:

bitsflew wrote:
AC3 in a mov container works!

Here is how I did it:

Open an existing mp4 (I used a handbrake rip) file in QT player.
Open an ac3 soundtrack in QT player.
Paste the ac3 soundtrack into the mp4 file
Export the movie as a self contained Quicktime movie.
Import the movie in iTunes.

Looks like they're making some headway.
Click to expand...


Makes me wonder though - that file would have two audio tracks, the original and the AC3 - if the AC3 file is actually being played then why is my .mov file with just an AC3 file silent? Is the person sure that they aren't just hearing the Dolby Pro Logic II (or whatever) audio track and not the AC3?
 
Ac3 'pass through'

Maybe by AC3 passthrough what Apple mean is that its not discrete sound, as in six seperate cables.

What I think might by happening is that the six channel AC3 is being encoded on the fly to AC3 and sent out the optical jack. This would still make it work on normal stereo systems. The 'passthrough' is that the Apple TV is passing through the AC3 signal to the amp to decode, rather than the Apple TV decoding it.

I'll bet its six channel AAC and they're might be an option in Apple TV's audio menus now to turn Dolby Digital on or off

(haven't checked myself yet, update it still downloading)

Jason
 
JasonKerner said:
I'll bet its six channel AAC and they're might be an option in Apple TV's audio menus now to turn Dolby Digital on or off
Click to expand...

No just encoded a 6 discrete channel movie segment and it plays back as a 2 channel Pro Logic II just as its always done.

My fear now is that Apple has done something proprietary so that only their HD movies can playback DD5.1 :(
 
BobVB said:
Makes me wonder though - that file would have two audio tracks, the original and the AC3 - if the AC3 file is actually being played then why is my .mov file with just an AC3 file silent? Is the person sure that they aren't just hearing the Dolby Pro Logic II (or whatever) audio track and not the AC3?
Click to expand...

I am sure! The only problem is that audio is out of sync.
 
petvas said:
I am sure! The only problem is that audio is out of sync.
Click to expand...

Hmmm I wonder why the perian-saved QT movie doesn't work then? Especially puzzling considering the QT Movie Properties window easily recognizes it as an AC3 audio track. Do you remove the AAC track so that there is only the AC3?
 
Six channel AAC is not transcoded into AC3 in the AppleTV 2.0 software. It plays as Dolby ProLogic II. Presumably Apple choked on licensing Dolby Live.

The Apple-provided movies appear to have a stereo audio stream and a private AC3 stream as well, in an MPEG-4 container. The ID of these streams is used by the AppleTV to determine which to play.
 
My bet on how Apple is accomplishing this:
a) They are using an MPEG-2 Transport stream to mux the h264 and ac3 tracks together and deliver to the AppleTV. If the new aTV sw can now recognize a transport stream file and decode the h264 video and passthrough the ac3 to an external decoder, then this would be the most efficient way of going about this.
b) They are doing what has been described previously in that they have encoded an ac3 bitstream into a PCM compatible audio layer and muxed that with the h264 video. Note that this would be different than just taking an elementary stream ac3 file and opening it up in QT and pasting it into a h264 encoding .mov and then saving as a new file.
c) They are encoding to aac 5.1 and delivering to the box as aac 5.1, but when the file is played back the 5.1 channels are decoded and sent out as a compliant ac3 stream which can be decoded by a receiver. I really don't see this as being a likely solution however as this can easily introduce noticable syncing delays.

My company is focused on encoding needs for next generation systems, and we do lots of encoding and setup for IPTV and game consoles. For IPTV, the standard method of encoding for the set top boxes being deployed is similar to a). This method is also the preferred way to get the PS3 to use h264 video and multichannel audio. Since ac3 decoders are so prevalent in home theater setups, this method of encoding is far superior to encoding to a standard mp4 which cannot support ac3 (and there are very few multichannel aac dsp's available in the home theater market today that could support 5.1 aac digital decoding).

Option b) would sound less likely to me, but since its Apple, who knows? Maybe they just really like QT... But from everything that I heard recently, Apple really is focusing less on QT and the .mov format and more on the making QT a truly robust platform for .mp4 and h264 encoding/playback. I don't think that they are currently anywhere close to achieving this goal, and doing something like this would seem counterintuitive.
 
EvilRob said:
Six channel AAC is not transcoded into AC3 in the AppleTV 2.0 software. It plays as Dolby ProLogic II. Presumably Apple choked on licensing Dolby Live.

The Apple-provided movies appear to have a stereo audio stream and a private AC3 stream as well, in an MPEG-4 container. The ID of these streams is used by the AppleTV to determine which to play.
Click to expand...

On a technical note, I thought the whole problem was you CAN'T have an AC3 stream inside of a technically correct MPEG-4 container, i.e. there is no such animal.

And is the ID something consistent? Can I go in and rename my AC3 audio track to some standard name so it will be picked up by AppleTV?
 
BobVB said:
On a technical note, I thought the whole problem was you CAN'T have an AC3 stream inside of a technically correct MPEG-4 container, i.e. there is no such animal.
Click to expand...
Technically, you are correct in that an AC-3 elementary stream cannot be muxed into a MPEG-4 MP4 compliant file. However, there is a method for storing AC-3 inside of PCM audio (this method is pretty common for sending AC-3 audio in a SDI embedded stream). If Apple is using this method to store the AC-3 in a private PCM audio track inside a MP4 muxed file, then this could possibly be considered legal.
 
csimon2 said:
Technically, you are correct in that an AC-3 elementary stream cannot be muxed into a MPEG-4 MP4 compliant file. However, there is a method for storing AC-3 inside of PCM audio (this method is pretty common for sending AC-3 audio in a SDI embedded stream). If Apple is using this method to store the AC-3 in a private PCM audio track inside a MP4 muxed file, then this could possibly be considered legal.
Click to expand...

But if so, why would there be reports of 'cut and paste' AC3 audio tracks into a .mov container working?

I haven't gone to the effort of doing the quicktime 'cut and paste' yet, but I do know that a Perian-enabled QT save of an .mkv format file into .mov does NOT work (someone else has reported that in the HandBrake forums).

There is the new AppleTV audio setting of 'Dolby Digital Out' of off or on - is it that the track just has to be the right name? The default in the Perian save is the standard 'Audio Track' for the AC3. When you do a cut and pastes is that somehow different?
 
Handbrake 0.7.1 converted 5.1 videos work!

5.1 audio is coming out on Handbrake Version 0.7.1 (2006022400) converted videos where the AC3 track was encoded with the video. I didn't have to do any copy/paste. It just worked. I have a Logitech Z-680 hooked up to my AppleTV.
 
Last I checked Handbrake was free, and VisualHub costs extra money. I found with the demo I couldn't control the aspect ratio of the video and it was harder to set settings just right for AppleTV.
 
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