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virduk
Mar 22, 2007, 09:52 PM
So I'm wondering about the lack of surround sound on the AppleTV.

Is there some hardware issue that'll stop from ever supporting Dolby Digital / DTS? Could a firmware upgrade solve the problem? Is the problem with the file format so that you couldn't include a a DD/DTS stream?



Avatar74
Mar 22, 2007, 10:49 PM
So I'm wondering about the lack of surround sound on the AppleTV.

Is there some hardware issue that'll stop from ever supporting Dolby Digital / DTS? Could a firmware upgrade solve the problem? Is the problem with the file format so that you couldn't include a a DD/DTS stream?

H.264 does support multichannel AAC but the problem in delivering it to a Dolby Digital decoder such as that in a surround receiver is that it must be transcoded to Dolby Digital AC-3.

This isn't tremendously difficult, but requires a transcoder such as Dolby Digital Live to do it. There are software and hardware considerations but I think AppleTV contains the hardware. The largest issue is multichannel AAC support in the AppleTV OS so it can recognize and read H.264 video with multichannel AAC and therefore push the multichannel AAC to either a software or hardware implementation of Dolby Digital Live inside AppleTV.

Since AAC was co-developed by Dolby Laboratories I'm not worried about Apple being unable to license the technology from Dolby or to integrate it without compatibility issues. Dolby Digital Live was designed to do exactly this sort of thing.

It's just a question now of if and when Apple will push such updates.

As for DTS, that's a more complicated issue because DTS bitrates on DVD are 768kbps. It's not a very efficient codec and despite what misinformed audiophiles like to imagine, it's not superior to Dolby Digital in terms of fidelity.

At 768kbps, DTS doesn't retain perceptual transparency... that is, it is not indistinguishable from the uncompressed multichannel equivalent. At 448 to 640kbps, however, Dolby Digital is perceptually transparent due to filters and parameters which reduce the bitstream requirements.

pilotError
Mar 22, 2007, 10:53 PM
The chip that handles the audio supports the Dolby Digital Live. I don't know if Apple crippled it or not.

We should know in a few days now that the disassembly has begun!

Diode
Mar 22, 2007, 10:57 PM
So I'm wondering about the lack of surround sound on the AppleTV.

Is there some hardware issue that'll stop from ever supporting Dolby Digital / DTS? Could a firmware upgrade solve the problem? Is the problem with the file format so that you couldn't include a a DD/DTS stream?


People on the handbrake forums having a working version of 5.1 I would say give them a few weeks for them to test and release a working version.

virduk
Mar 22, 2007, 11:03 PM
H.264 does support multichannel AAC but the problem in delivering it to a Dolby Digital decoder such as that in a surround receiver is that it must be transcoded to Dolby Digital AC-3.


Why not just include the Dolby stream rather than worry about transcoding from one lossy format to another? BluRay and HD-DVD manage to include h.264 video and actual Dolby (or DTS) streams together. Or is there an issue with the container Apple uses?

As for DTS, that's a more complicated issue because DTS bitrates on DVD are 768kbps. It's not a very efficient codec and despite what misinformed audiophiles like to imagine, it's not superior to Dolby Digital in terms of fidelity.

At 768kbps, DTS doesn't retain perceptual transparency... that is, it is not indistinguishable from the uncompressed multichannel equivalent. At 448 to 640kbps, however, Dolby Digital is perceptually transparent due to filters and parameters which reduce the bitstream requirements.

Well...it certainly sounds better to my ears. And DTS-HD Core sounds better again.

virduk
Mar 22, 2007, 11:04 PM
People on the handbrake forums having a working version of 5.1 I would say give them a few weeks for them to test and release a working version.

Not terribly helpful for me, being that I'm a Windows guy :)

Diode
Mar 22, 2007, 11:19 PM
Not terribly helpful for me, being that I'm a Windows guy :)

There is a windows version^^ When mac has 5.1 so will windows.

Avatar74
Mar 23, 2007, 10:51 AM
Why not just include the Dolby stream rather than worry about transcoding from one lossy format to another? BluRay and HD-DVD manage to include h.264 video and actual Dolby (or DTS) streams together. Or is there an issue with the container Apple uses?

Because AC-3 can't be encapsulated in H.264 as far as I know. This could change at some point but it's not the case at present. In fact, I don't know of any multimedia video codecs in which AC-3 can be encapsulated. It isn't encapsulated on DVD's... The MPEG-2 and AC-3 streams are separate and synchronized.

Well...it certainly sounds better to my ears. And DTS-HD Core sounds better again.

That may be, but I would contend that in a blind test where you don't know which format you're listening to, you wouldn't pick DTS as the winner.

No harm no foul unless you made purchases predicated on the videophile misconception that DTS is better, which is furthered by asinine snob magazines with articles written by people who don't understand the fundamentals of perceptual coding.

When I really dig in and dissect what people like about DTS, they can only actually quantify one thing... it's louder.

Well, if you like louder, there's an easy answer... turn up the volume about 4dB, because that's the baseline difference in attenuation between Dolby Digital and DTS. The difference being that DTS doesn't have dynamic range control, dialogue normalization or low-pass filtering at 20kHz to eliminate aliased frequencies.

That last one is so critical it's the very first critical component of any digital audio encoding system that Ken Pohlmann stresses in Principles of Digital Audio, widely regarded as THE digital recording/engineering bible.

And I find it horrendously amusing that not one AV snob seems to understand it... They in fact believe in the exact opposite, sampling frequencies above the Nyquist limit which is patently absurd.

pengu
Mar 23, 2007, 11:14 AM
H.264 is the video codec, not the container. the mpeg4 container type will accept a hell of a lot of different content types for audio and video, and Quicktime container type accepts even more.

and just to provide some info on the whole 5.1 thing. if you have a 5.1 WAV file (i didnt think they existed either) which is effectively a WAV with AC3 or DTS content in it (rather than raw PCM audio) quicktime/itunes will send the stream straight out ur optical port to a decoder, IF you have EQ, sound effects etc off and itunes/qt volume at 100%.

further to this:

i saw something saying the ?tv uses a modified version of osx (no suprise really) and as such i imagine a handbrake update, and some hackery on the ?tv itself will result in true 5.1 out from the unit to a receiver.

Avatar74
Mar 23, 2007, 05:32 PM
H.264 is the video codec, not the container.

I know this. I meant the container in which H.264 and AAC are encapsulated.

Semantic quibbles aside...

the mpeg4 container type will accept a hell of a lot of different content types for audio and video, and Quicktime container type accepts even more.

That'd be great... Let me know when it happens. I was referring to the present. Currently, there's no media container I know of that encapsulates AC-3 bitstreams within.

and just to provide some info on the whole 5.1 thing. if you have a 5.1 WAV file (i didnt think they existed either) which is effectively a WAV with AC3 or DTS content in it (rather than raw PCM audio) quicktime/itunes will send the stream straight out ur optical port to a decoder, IF you have EQ, sound effects etc off and itunes/qt volume at 100%.

If I'm not mistaken, the format you speak of takes AC-3 and transcodes it into a stereo-matrixed surround format. Doing this is absurd since all the stereo AAC audio in every iTunes movie originally mastered in surround sound effectively carries Dolby Surround, which is a stereo matrixed surround format. You'd be replacing one wheel with a slightly crappier one.

Unless you have some hard documentation that 5.1 WAV is in fact a discrete multichannel format AND it preserves AC-3 proprietary metadata such as dialnorm, dynamic range control, etc... I'm not convinced that this is anything more than a mediocre stereo matrixed surround format.

The best solution to ensure compatibility with licensed Dolby Digital decoders is to implement multichannel AAC in the MPEG-4 containers and Dolby Digital Live in AppleTV to transcode to AAC on the fly.

further to this:

i saw something saying the ?tv uses a modified version of osx (no suprise really) and as such i imagine a handbrake update, and some hackery on the ?tv itself will result in true 5.1 out from the unit to a receiver.

If it can be hacked to do this then why are the hack-happy techies the ones doing the most complaining about the lack of out-of-the-box Dolby Digital support?