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
Anonymous
June 14, 2007
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. AdamAnonymous
June 14, 2007
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.Anonymous
June 14, 2007
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. AdamAnonymous
June 14, 2007
Adam: I improved the wording in the Conclusion section to make it clearer that S/PDIF is a lossy choice.Anonymous
June 14, 2007
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.Anonymous
June 15, 2007
The comment has been removedAnonymous
June 15, 2007
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 AdamAnonymous
June 15, 2007
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.Anonymous
June 15, 2007
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. AdamAnonymous
June 25, 2007
It's great to see some technical detail on aspects such as these!Anonymous
July 23, 2007
There's at least one good reason to use audio on HDMI 1.3 : Lip Sync support ! ;)Anonymous
August 21, 2007
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.Anonymous
August 22, 2007
The comment has been removedAnonymous
September 08, 2007
too bad Microsoft now thinks you need hdmi...Anonymous
October 29, 2007
The comment has been removedAnonymous
October 30, 2007
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.Anonymous
December 17, 2007
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.Anonymous
December 17, 2007
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.Anonymous
January 13, 2008
Now that I own a PS3 I was thinking about upgrading my receiver to one that supported the new audio formatsAnonymous
April 20, 2009
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 :-)Anonymous
April 20, 2009
The comment has been removedAnonymous
February 21, 2010
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.