An Important Factor in Choosing a Blu-ray Player
I recently took delivery of Classé Audio’s ambitious new $8000 SSP-800 controller for review. I was surprised, however, that the review sample did not decode the new audio formats, Dolby Digital Plus, Dolby TrueHD, DTS-HD, and DTS-HD Master Audio. Classé is working on an upgrade board—available free of charge to SSP-800 purchasers—that will add this capability. The upgrade will be offered next March. But is this lack of high-res audio decoding a shortcoming of the SSP-800? More to the point, does any surround-sound controller really need on-board decoding of the new high-res audio formats?
That might sound like a silly question—of course we want to hear soundtracks in high-res from Blu-ray discs. But the fact is that you don’t need a controller that “decodes” the new audio formats—but you do need a Blu-ray player that has one essential feature (keep reading).
The confusion arises from the term “decode.” Here’s how it works. The film soundtrack starts out as high-res PCM (Pulse Code Modulation) audio and is encoded into, for example, a Dolby TrueHD bitstream for storage on the Blu-ray disc. (I’ll use TrueHD as a stand-in for all the new audio formats.) You can think of this process as creating a Zip file on a PC.
The file must then be “unzipped” on playback; that is, the Dolby TrueHD bitstream must be decoded back into multichannel linear PCM audio for conversion to analog. This decoding can take place in the Blu-ray player or in the controller. If the decoding to PCM takes place in the player, the controller receives high-res PCM over the HDMI interface. The controller then converts the PCM to analog for listening. If the decoding of Dolby TrueHD to PCM takes place in the controller, the HDMI interface carries the TrueHD bitstream.
It really makes no difference sonically where the “decoding” takes place—in the player or in the controller. In fact, it may be advantageous to decode in the Blu-ray player rather than the controller because the format has the capability of mixing different audio sources on the fly during playback. An example of this is a director’s commentary posted on a movie studio’s website after the Blu-ray disc has been released; you can watch and listen to the movie from disc as well as hear the director’s commentary streamed from the web. There are many other examples of Blu-ray’s interactivity—features that are lost if the TrueHD-to-PCM decoding doesn’t take place in the Blu-ray player.
Not all Blu-ray players can perform this “unzipping” of TrueHD bitstreams to PCM. It requires a fair amount of DSP horsepower, making it an expensive (for now) feature for disc-player manufacturers. I expect, however, that all next-generation players will have this capability. For a full listing of Blu-ray player features (including whether the player can output decoded PCM), go to http://www.idoblu.co.uk/page2%20Blu-ray%20Players.html. Scroll down the page to see the player features. You’re looking for blue boxes in the True-HD and DTS-MA columns.
So, when you talk about whether a controller can “decode” the new audio formats, remember that there are two distinct functions—“unzipping” of the TrueHD bitstream to PCM, and the conversion of that PCM to analog. The former is best done in the player; the latter in the controller.
Unfortunately, the Sony BDP-S350 Blu-ray player I recently bought doesn’t decode the TrueHD bitstream to PCM. It will perform this decoding, however, with a software update. See http://esupport.sony.com/perl/news-item.pl?&news_id=274&mdl=BDPS350. I advise buying a player that has the ability to decode TrueHD bitstreams to PCM. Note that any Blu-ray player that has BDLive capability will decode TrueHD bitstream to PCM. That's because BD live involves mixing audio streams on the fly during playback, which requires decoding in the player.
Thanks to Classé Audio’s Dave Nauber for his discussion of this subject with me, and for his “unzipping” analogy.