HD DVD: Audio Mixing, and why you don’t need HDMI


There has been much confusion over multi-channel audio and HD DVD, so I thought I would try and explain how this works. As a side-effect you'll see why you don't need HDMI for the highest quality audio, and why you certainly don't need HDMI 1.3 for audio.


DVD: The Original Way


 


To start with, lets look at the audio path for legacy DVD. (To keep life simple I am assuming 5.1 channels in Dolby Digital or DTS: the codec type makes no difference to the discussion. Please also excuse my primitive abilities with Visio. Click the image to see a readable version). From the disc comes an encoded bitstream, containing 5.1 channels of data. It can go up to two places: one, directly to the S/PDIF output (optical or coax) to be decoded in your receiver, or (on some players) to an internal Dolby Digital decoder which turns the bitstream into 5.1 analog channels, and hence to your receiver. Quality is basically the same no matter which method you use to get the audio out.


Toshiba HD A1


 


Now lets look at the first-gen Toshiba HD DVD player. Right off the bat things are more complicated because HD DVD has three audio sources: main audio (used on the main movie), sub audio (used for the audio that goes along with the picture-in-picture) and effect audio (used for menu clicks and the like). The first two are multi-channel encoded bitstreams, effect audio is stereo WAV file only. The player has two audio decoders (both of which handle all the supported codecs) and an audio mixer. Once out of the mixer, those 5.1 channels can either be sent directly (via analog 5.1 or HDMI PCM) or be re-encoded to go over S/PDIF. The re-encoding may lose quality, in the Toshiba case it is re-encoded to Dolby Digital. If you receiver can take the 5.1 outputs then that is the preferred route (there is no quality difference between analog 5.1 and HDMI PCM), but if not the S/PDIF still gets you the 5.1 channels, but at potentially lower quality. (Some of the Toshiba second-gen players do not have the 5.1 analog outputs so your choices on those are S/PDIF or HDMI only).


Xbox 360 HD DVD


 


Now lets move on to the Xbox 360 with HD DVD add-on. The Elite's HDMI port cannot handle 5.1 channels due to hardware limitations, so multi-channel audio can only be sent via S/PDIF. At least you get a choice of which encoder is used to re-encode the audio: WMA Pro, DTS or Dolby Digital. Opinions vary on which is the best choice, but in theory at least the preference is the order I described, assuming your receiver can handle it.


HDMI 1.3?


 


But what about HDMI 1.3 you may say? Well what about it I answer. One of the theoretical advantages of 1.3 is that you can send a multi-channel audio bitstream direct to a compatible receiver. However, if you do this, you will lose some of the HD DVD audio, namely the sub- and effect-audio. This is probably why no player today offers this as an option. So you don't need HDMI 1.3, or advanced codec support in your receiver, and even if you did use it, you'd actually lose data. Dolby's own web site says that "it will no longer be possible (or necessary) to output raw audio bitstreams from the player".


Conclusions



  • Preferences for HD DVD multi-channel audio transports are HDMI PCM / Analog 5.1 / S/PDIF in that order: use the best one that your hardware supports. Choosing S/PDIF may lose you some audio quality if the disc is using the advanced audio codecs

  • You don't need HDMI 1.3 for audio

  • You don't need a receiver that supports the new audio codecs for HD DVD playback in order to play discs that use them

 

Comments (22)

  1. Adam Mazza says:

    Hi,

    The title is a bit misleading. You need HDMI or 5.1 Analog to take advantage of DTS MA and TrueHD (I realize the 360 drive doesn’t support this yet, but most other players do).

    Even with the newer receivers which can decode these formats, I’m not aware of any devices that will output either format in bitstreram format. They all decode internally and send as  LPCM which cannot be done over optical.

    Adam

  2. Andy-Pennell says:

    Adam: Why is the title misleading? “You don’t need HDMI” looks right to me. Isn’t that also what you are saying?

    Xbox does decode TrueHD: the spec says all HD DVD players have to. However due to the hardware limitations, they have to be re-encoded on output.

  3. Adam Mazza says:

    While it’s technically correct, I think it could mislead someone into thinking they were getting the latest and greatest in audio quality via the optical out.

    Sorry, I didn’t mean that it can’t decode them, I meant without re-encoding as DTS (I think that’s what the xbox does right?)

    Are there plans to eventually send LPCM via the HDMI interface?

    I owned the 360 HD-DVD drive. Price wise, it couldn’t be beat. I sold it when the HD-A2s came down in price, mainly because of the noise factor, but also because it sends out LPCM.

    Adam

  4. Andy-Pennell says:

    Adam: I improved the wording in the Conclusion section to make it clearer that S/PDIF is a lossy choice.

  5. Sweetmate says:

    Sorry Andy, but again – total nonsense. Saying "You don’t need a receiver that supports the new audio codecs for HD DVD playback in order to play discs that use them" is like saying you don’t need an HDTV to watch HD DVDs. Yes it is a totally correct statement but using it in an article called "why you don’t need hdmi" is very misleading.

    The reality is that you DO need hdmi to digitally transport DD plus, Dolby TrueHD, DTS-HD, DTS-HD MA and multichannel LPCM from an hd dvd player to a reciever. If you are correct that no player will ever allow the bitstreams to be sent to a receiver, the best solution is still to output the pcm mix from the player straight to the amp, which you state as your first conclusion.

    As for your diagrams, yet again you have ignored anything < 5.1. "Main" and "sub" audio tracks are very often 1 channel mono or 2 channel stereo sound, and this is my point in our other discussion – the 360’s decoder/mixer/encoder and seemingly Toshiba’s make no allowance for this.

    A 360 + HD DVD drive is an extremely unsatisfying solution for hd dvd audio due to its lack of hdmi 1.3, lack of analog outputs, and its very basic sound mixing and encoding options. You have to accept and acknowledge that, because the argument you’ve put forward in this blog post is totally wrong and quite insulting to audiophiles.

  6. Andy-Pennell says:

    Sweetmate: It is NOTHING like saying you don’t need an HDTV to watch HD DVDs. YOU DO NOT NEED A RECEIVER WITH NEW AUDIO CODEC SUPPORT. PERIOD. Please re-read the post.

    No player transports bitstreams for audio for the reasons I describe (and also the Dolby site: are you claiming they don’t know what they are talking about either?). Therefore you do not need HDMI: analog 5.1 is fine for that. There is a difference between analog and digital, but most ears and equipment will find it hard to tell. Not so the difference between DD and TrueHD, for example (or SD vs HD for that matter).

    I deliberately ignored non-5.1 scenarios to keep the diagrams simple. I’m not going to rehash your unique 2.0 claims again.

    Why is this insulting to audiophiles? Telling them they don’t need TrueHD decoding in their receivers, or HDMI 1.3 is insulting? I’d say the opposite: claiming you DO need those things is insulting.

    The Xbox HD DVD player is not perfect on the audio front: that’s not a secret. I regularly say this on AVSForum. It has hardware limitations that software cannot work-around. Its video performance kicks-ass though, but for an audiophile the Toshiba A2 is a better choice (actually the A1 is better still if you are using analog 5.1 connections thanks to the DACs).

  7. Adam Mazza says:

    I agree, if all you want is to watch movies with the latest codecs, an HDMI 1.3 receiver isn’t necessary. Unless you think your receiver can decode better then your player.

    There is a good post over in the AVS forums which covers this:

    http://www.avsforum.com/avs-vb/showthread.php?t=738511

    Adam

  8. Andy-Pennell says:

    Adam: true, I hadn’t factored in digital <-> analog conversions in my post: the fewer conversions, the higher quality, so in that regard HDMI (any version) slightly beats analog 5.1. That AVSForum post also talks about 7.1 support, and I don’t think we have any content or player support for that on HD DVD right now.

  9. Adam Mazza says:

    Sweetmate, even if you have an HD-DVD player connected to a receiver via HDMI 1.3. If the disc you are viewing was authored using Advanced Mode (99% currently are) the spec says the sound *must* be decoded and mixed in the player.

    Basically having a receiver now that will support these advanced codecs is useless.

    Adam

  10. Gwyn Cole says:

    It’s great to see some technical detail on aspects such as these!

  11. MarcMAME says:

    There’s at least one good reason to use audio on HDMI 1.3 :

    Lip Sync support ! 😉

  12. MicK says:

    Andy, you have mentioned:

    “The Elite’s HDMI port cannot handle 5.1 channels due to hardware limitations, so multi-channel audio can only be sent via S/PDIF”

    So, do you mean I need to plug Elite’s audio adapter and extra cable for HD-DVD playback even I have Elite?

    I have Elite and have found games’ audio could output via HDMI with no problem. I am planning to buy XBOX 360’s HD-DVD drive but if HDMI cannot output movie’s audio, it could be a show-stopper.

  13. Andy-Pennell says:

    MicK: Sorry I should have said "cannot handle 5.1 channels UNCOMPRESSED" so no, you shouldn’t need to use the audio adaptor cable when using HDMI audio. Sorry for the confusion.

  14. pixelsword says:

    too bad Microsoft now thinks you need hdmi…

  15. David Boulet says:

    "I have a HDMI system with an Onkyo 805 and PS3. I have a pretty nice DefTech speaker system (BP10Bs, CLR2000, BP2xS, PM100MKIIs and a Velodyne CT100) I will be sure to do some comparisons tomorrow after I pick it up. Any timeframes I should look for where the differences are readily apparent?"

    That’s the problem. High-end audiophile applications should not be remixing in "gimmick" audio clicks and commentary on the fly. Sure, when those features are desired, compromising the primary audio signal is fine. But for high-end listening to a feature film, getting the bit-for-bit original waveform should be not only permitted, but encouraged. It’s one reason I WANT HDMI 1.3 on my gear. I want to bypass the compromise from re-processed audio data.

  16. Andy-Pennell says:

    David: if you have a decoded digital audio signal, and you mix it at 100% with another audio signal at 0% (e.g. main audio=100%, sub audio=0%), you will get the original signal bit for bit, so I don’t see the reason for your concern.

  17. Nexus Rho says:

    Could you fill me in on exactly what the hardware limitations are?  Is it possible to get DD+? I have the xbox 360 addon and all I want to get is DD+.  After listening to tranformers, I could care less about truehd.

  18. Andy-Pennell says:

    Nexus: The Xbox can play DD+ titles. However it cannot get that DD+ signal to you, either as a DD+ bitstream or as 5.1 analog channels, as there is no hardware to do that. So it does the next best thing: it re-encodes it as DD or DTS or WMAPro over S/PDIF.

  19. Now that I own a PS3 I was thinking about upgrading my receiver to one that supported the new audio formats

  20. Simon Goodwin says:

    OK, I’m coimb to this a bit late, but you still have some catching up to do.

    Unlike the ‘Elite’ 360 offering, HDMI 1.3 (as on PS3 and the newer PC AV cards) offers LPCM (uncompressed, 24 bit, up to 192 KHz) 7.1. If you play games this is much better than AC3, fixing the problem of holes i n the soundfield just where you can least afford them, to the sides and rear; and as I demonstrated at the AES Audio for Games conference in February it’s the only practical way to play existing PC or PS3 game audio in true 3D via loudspeakers.

    Roll on Xbox720 🙂

  21. Andy-Pennell says:

    Simon Goodwin: is this the same one that I worked with back in the Dark Ages on HiSoft BASIC? http://www.simon.mooli.org.uk/AF/article/commercial.html

  22. Simon Goodwin says:

    Andy wrote:

    "Simon Goodwin: is this the same one that I worked with back in the Dark Ages"

    Yep, it is indeed. Hi Andy! I’m currently at codemasters, email name simong.

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