Decoding, yes, but in which box?

The other day I emailed Dolby a question concerning where it thinks Dolby TrueHD and Dolby Digital Plus will end up being decoded. I think their views also related to DTS-HD Master Audio and DTS-HD High Resolution, although of course they wouldn’t be interested in commenting on those formats. For what it’s worth, I’m agnostic about the merits of Dolby TrueHD vs DTS-HD Master Audio (both are lossless compression systems and, if they work properly, should provide identical results), and Dolby Digital Plus vs DTS-HD High Resolution (both are high bitrate lossy systems, so there is more potential for difference here, but DD+ is being used extensively in HD DVD, while DTS-HD High Resolution is rarely used).

So this is what Dolby Laboratories says (editing to remove some unnecessary words):

To get the full interactivity of next-generation formats, consumers should look for players with the decoders built-in. So look for players with Dolby Digital Plus and Dolby TrueHD logos on.Next-generation disc standards have been designed to be extremely interactive, with menus sounds (and pop-up menus that can be accessed while the film is playing), as well as secondary audio that can be streamed off the internet (imagine a director’s commentary that wasn’t finished until two months after the disc release). To bring all this together, the player has to decode all these audio sources internally, and mix them together accordingly. So you’ll only get this level of interactivity if the player can decode all the signals, to then mix them together.

Both HD DVD and Blu-ray (in the latter case, with players that conform to the Final Standard Profile) provide for mixing audio streams. They could, for example, have the director’s commentary in one audio track, and the sound of the movie in another, and overlay the former over the latter. To do this it is necessary for the player to decode both tracks and then mix them together. Having decoded and mixed them, the next obvious step is for the player to output the signal as multichannel PCM.

In theory, I suppose the player could then re-encode the whole lot as, say, Dolby TrueHD and send it out as a bitstream. But in practice, I doubt it. The audio codecs used on DVDs and their successors tend to be asymmetrical: they take longer to encode than decode. They require more processing grunt to do the former than the latter. I think it’ll be a long time before a Blu-ray or HD DVD player will have sufficient power to re-encode in real time without, at the least, introducing significant latency (ie. delays).

In any case, what would be the point? The Dolby response goes on to say:

From an audio point of view, people always think that you are best decoding in the AV receiver, because the components and circuitry are better quality. This was perhaps true for DVD, but next-generation disc players, when decoding, need to support hi-res sampling frequencies up to 192kHz, and so have high quality DACs in them (digital to analogue converters). The chips and components in next-generation disc players can be of higher quality than some AVRs.However, some people will still believe that their AV receiver is king and are not concerned with interactivity. That is why, for these AVR devotees, Dolby has worked hard to make sure Dolby Digital Plus and Dolby TrueHD can be passed over HDMI 1.3, and that next-generation AVRs have been developed to decode both codecs. This way, the AVR aficionados can still stream the main soundtrack straight off the disc, and pass it to the AVR over a single HDMI 1.3 connection.

I think Dolby is right and ‘some people’ are wrong. Where I expect a home theatre receiver to excel is not in decoding some compressed digital audio format into PCM, but in performing the next step: the digital to analogue conversion. Certainly, with Dolby TrueHD and DTS-HD Master Audio, if the decoder is minimally competent it will provide absolutely bit perfect decoding. Remember, these are lossless systems so an exact replica of the original PCM signal should be restored after decoding. One or other of the Dolby or DTS systems may provide better levels of error correction and so forth. That I wouldn’t know. But they should produce the same results if all is working adequately.

Once again, since both Dolby Digital Plus and DTS-HD High Resolution are lossy systems, the possibilities for quality variances are higher. But since I presume that the decoding algorithems are tightly defined and controlled by Dolby and DTS respectively, I would be surprised if there was any difference in the decoding quality, whether performed in the player or the receiver.

Initially I was a believer in the player piping the raw bitstream (via HDMI) to the home theatre receiver. But now I think it might be better to let the player do the decoding, and send the six or eight channels in PCM format to the receiver.

NOTE: I wrote this on Saturday. It is now Tuesday. My ISP Bigpond was down, at least as far as my computer goes, for most of that time. It’s normally very reliable. It is a very strange feeling not to have Internet access. Anyway, that is the reason for the delay.

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