Thursday, July 13, 2006

HD DVD vs. Blu-ray (Pt. V)

In HD DVD vs. Blu-ray (Pt. IV), I discussed what forms of audio output the first Toshiba HD DVD players, the HD-A1 and HD-XA1, are able to deliver. Now I'd like to talk about the audio outputs of the initial Blu-ray players: Samsung's BD-P1000, which is the Blu-ray format's debut machine; and Sony's BDP-S1, the second Blu-ray player on the market, just now reaching consumers for the first time.

My comments are decidedly preliminary. They are based in part on Samsung's BD-P1000 user's manual, downloadable here. They are also partly based on this online discussion of the first HD DVD and Blu-ray players, which outlines the audio capabilities of Sony's BDP-S1 ... since I can't find a Sony manual online to speak more authoritatively about this player.

I must also disclose that I have yet to read any authoritative reviews of the first Blu-ray discs (BDs) now on the market, so I am in the dark as to which codecs are actually being used to encode the audio tracks on these discs.

That being said, I gather that the first Blu-ray players have the same roster of audio output connections as the first HD DVD players:

  • 1 HDMI digital audio/video output jack, version 1.1
  • 1 ea., S/PDIF optical and coaxial digital audio outputs
  • 1 set, 5.1-channel analog audio outputs (6 RCA jacks)
  • 1 set, 2.0-channel (left-and-right stereo) analog audio outputs (2 RCA jacks)
The Blu-ray players also echo the HD DVD machines in making a basic distinction between "bitstream" and "PCM" outputs on the two digital audio connections, HDMI and S/PDIF.

In principle, "bitstream" refers to a succession of 1's and 0's, output in the original form in which they are encoded on a disc — no matter what codec was used to do the encoding. Nominally, a bitstream audio output simply delivers the original data stream from the disc to the player's output port, where it will be transported to outboard gear such as an AV receiver. The receiver is then responsible for decoding it and using it to drive its speakers.

If the audio bitstream is mixed with video, as it is on HDMI, an HDMI-capable receiver must forward the video portion of the signal — or else pass through the entire original signal, audio included — to the TV.

A bitstream audio stream is characteristically compressed; for example, Dolby Digital, Dolby Digital Plus, DTS, and DTS HD audio streams are all digitally compressed bitstreams. Specifically, those four types are compressed using "lossy" compresion schemes.

Dolby TrueHD and DTS HD Master are two other forms of digitally compressed audio bitstreams. They differ in being compressed using "lossless" compression schemes.

"PCM" refers to digital audio in the form of linear (uncompressed) pulse-code-modulated digital audio, which may be either 2-channel or multichannel (typically, multichannel PCM uses a maximum of 5.1 channels, but it's theoretically able to carry fully 7.1 channels). Linear PCM (or LPCM) tracks may appear as such on HD DVD and Blu-ray discs ... or the respective players may derive LPCM from other types of digital audio tracks which are present on the disc, and output the derived LPCM on the players' digital audio output ports.

Converted-to-analog LPCM audio is also the basis for the non-digital audio outputs, both 2-channel and multichannel, of these players.

5.1-channel Dolby Digital Plus appears to be the workhorse audio codec of the first spate of releases on Blu-ray's competitor, HD DVD. It's a lossy multichannel encoding method that nevertheless offers noticeably better sound than the original Dolby Digital codec, also lossy. But the online discussion of the BDP-S1 mentioned earlier says the Sony Blu-ray player won't decode Dolby Digital Plus or even pass it through for external decoding (scroll down to the table at the end to see what I mean). That is apparently true also of the Samsung BD-P1000, judging by its manual.

Apparently, the Sony and Samsung players restrict themselves to the three mandatory Blu-ray codecs:

  • Dolby Digital 5.1 (as found on standard DVDs)
  • DTS 5.1 (also as found on standard DVDs)
  • 5.1-channel PCM (I'm not clear on whether standard DVDs ever have this)
All of the new, high-definition "bitstream" codecs are optional, however, and as such are (as far as I can tell) not included on the first Blu-ray players:

  • Dolby TrueHD (lossless, from 2.0-channels up to 7.1 channels)
  • DTS HD Master (lossless, up to 5.1 channels)
  • Dolby Digital Plus (lossy, 5.1- or 6.1-channel)
  • DTS HD (lossy, 5.1-, 6.1-, or 7.1-channel, up to 3.0Mbps)
  • DTS HD "core" audio (lossy, 1.5Mbps 5.1-channel, compatible with current DTS 5.1 decoders)

(See HD DVD vs. Blu-ray (Pt. III) for more discussion of these.)

Not only can't the first Blu-ray players decode these formats, they can't (again, as far as I can tell) pass them through as bitstreams for decoding by external gear.

The first HD DVD players from Toshiba have the quirk that they can't actually pass through compressed, non-LPCM bitstreams from a disc to their digital audio outputs when the disc is authored using "advanced content." Instead, they transcode all bitstreams to DTS 5.1, in order to be able to interactively mix (say) menu-click sounds from a disc with a movie's soundtrack. Such interactive abilities are also called iHD, for "interactive HD."

I gather that the Blu-ray camp has something called BD-Java which I assume does the same kind of thing. Java, of course, is a computer language familiar to Web users for its "applets" which occasionally respond to mouse clicks in a browser window by performing some complex task. I assume BD-Java consists of applets on a disc that give the disc its interactive capabilities.

Conceivably, BD-Java might, like iHD for HD DVD players, limit the ability of Blu-ray players to pass through bitstreams on the disc.

In the HD DVD world, "advanced content" or iHD is present on every disc released in the U.S. so far, or so I understand, and looks to be a given on future releases as well. Unless future players provide more options for bitstream pass-throughs than the original Toshiba players do, it is hard so see how (say) a Dolby Digital Plus audio track on a HD DVD disc could ever be routed to an AV receiver for decoding and use, instead of an inferior-quality, player-derived DTS track.

I simply don't yet know whether similar limitations might affect Blu-ray players, now or in the future. If all I say above is correct, the first two Blu-ray players can't pass through compressed bitstreams other than Dolby Digital and/or DTS, so the question is for now somewhat moot. Future players will surely be able to decode more advanced bitstreams such as Dolby Digital Plus and DTS HD Master, and might be able to pass them through to outboard gear as well.

Or, again, there might be some limitation associated with BD-Java which will circumscribe audio bitstream pass-through in the way "advanced content" does with iHD. Only time will tell.

It is worthwhile at this point to pause and get firmly in mind that both HD DVD and Blu-ray players, in their initial, out-of-the-starting-gate incarnations, have apparent limitations with respect to passing through compressed digital audio bitstreams from files encoded on discs to the players' HDMI and S/PDIF digital output ports.

New, more data-intensive codecs such as Dolby Digital Plus and DTS HD offer better options for home-theater sound than their long-in-the-tooth predecessors, Dolby Digital and DTS. But they, along with their truly lossless cousins, Dolby TrueHD and DTS HD Master, require more bandwidth than optical and coaxial S/PDIF connections can muster. In short, they need HDMI.

Specifically, they need HDMI 1.3, the first HDMI version that will support their ilk. But HDMI 1.3 isn't ready for prime time, and the first HD DVD and Blu-ray players content themselves with implementing HDMI 1.1. (By the way, why no HDMI 1.2?)

Once HD DVD and Blu-ray players have HDMI 1.3, as do the AV receivers to which they will be connected, the players may need to come up with ways of passing through superior-format audio bitstreams from disc to the outboard receiver for decoding and use.

Right now, when HD DVD discs are authored with "advanced content"/iHD — as are all HD DVDs released in the U.S. so far — that can't happen with the first Toshiba players. The players turn all bitstream output into plain old DTS 5.1. I suspect that similar limitations apply to the first Blu-ray players as well.

Is there a workaround? One possibility is to set the player to output PCM, rather than bitstream audio, on HDMI. The player will take whatever sound files it is playing from the disc and decode them into 5.1-channel linear PCM at high resolution — 96 kHz on the Toshiba HD DVD players — and it will then output that form of digital audio on HDMI.

This supposedly works with HDMI 1.1. However, I understand that not all AV receivers that are nominally 1.1-compatible can actually handle multichannel PCM input on HDMI. For such receivers, if they can accept multichannel analog audio inputs, those inputs can be connected to the equivalent outputs on an HD DVD or Blu-ray player for much the same effect.

The secret here is that multichannel PCM, when output as such, is not re-compressed by the player. Imagine what happens when an HD DVD player reads a Dolby Digital Plus audio track from a disc. If it is set to send bistream audio out over HDMI, and if the disc has "advanced content," the player will expand the Dolby Digital Plus bitstream into linear PCM, and then it will re-compress the LPCM into DTS for output on HDMI (and on S/PDIF, for that matter).

At the other end of the HDMI cable, a receiver will then expand and use the DTS stream.

Decompressing, re-compressing, and re-decompressing an audio stream introduces errors due to the lossiness of the original compression (if it was lossy, as with Dolby Digital Plus) compounded by the lossiness of the re-compression (as with DTS).

But the use of linear PCM at every stage of the proceedings after the intial decompression has been accomplished avoids compounding the lossiness problem, since linear PCM is not compressed at all.

Conceivably, future players might also offer options to transcode (say) Dolby Digital Plus, not to linear PCM, but to a lossless codec, such as Dolby TrueHD, for output on HDMI. That would very likely be as satisfactory as expanding to LPCM, since no more bits would be lost in the transcoding than had been lost in the original Dolby Digital Plus compression.

But even if future players don't transcode to lossless-compression output modes, picky consumers may find that they're perfectly happy with the expansion of on-disc audio bitstreams to multichannel LPCM and their output as such over HDMI 1.3-and-later audio/video connections to compatible outboard gear. It may be that "bitstream" digital audio output modes will one day become relics of a dimly remembered past.

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