Wednesday, July 12, 2006

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

The first HD DVD players on the market are the Toshiba HD-A1 and HD-XA1, models whose audio and video capabilities are basically identical. They have four ways to output audio: two digital and two analog.

Digital audio can be output on HDMI and/or on the S/PDIF optical and coaxial digital connections that have long been provided by standard DVD players. HDMI carries both digital video and digital audio. S/PDIF, just audio.

Analog audio can be output on a group of six RCA jacks that permit up to 5.1-channel multichannel sound to be sent to an AV receiver capable of handling it. Or, analog audio can be output on a pair of stereo RCA jacks for just 2-channel sound. In both cases, the analog audio output is derived from linear PCM (LPCM) digital audio, which in turn has been derived from whatever format the digital audio is encoded in on the disc.

First I'll discuss how these players handle the audio from regular DVDs. Then I'll go into how they deal with the audio from HD DVDs.

The Toshiba HD-A1 and HD-XA1 behave like ordinary DVD players when playing standard DVDs, with their up-to-5.1-channel Dolby Digital (DD) and/or DTS audio tracks, through the players' S/PDIF optical and coaxial digital connections: the players can be configured to forward the raw bitstream from the disc to a multichannel surround-sound AV receiver for decoding by the receiver. Or the players can convert the audio from the disc into a 2-channel linear PCM format for their S/PDIF outputs, if the receiver/amplifier is limited to two channels.

S/PDIF stands for "Sony/Philips Digital Interface." Optical and coaxial digital-audio outputs on standard DVD players and on HD DVD and Blu-ray players use S/PDIF.

DD and DTS from standard DVDs can likewise be output as raw bitstreams on HDMI, to be decoded by an HDMI-capable multichannel surround-sound AV receiver. But in the case of HDMI audio output, there are not one but two forms of LPCM output which can be selected. One, called "downmixed PCM," is the 2-channel linear PCM format mentioned above. (Are there any HDMI receivers which are just 2-channel?) The other, called "PCM," is meant to be used when the HDMI receiver is multichannel.

The assumption here is that any HDMI-capable receiver to which the Toshiba player is connected will be able to deal with linear PCM audio input on HDMI.

The 5.1-channel and 2-channel analog outputs of the HD-A1 and HD-XA1 receive non-downmixed and downmixed analog versions, respectively, of the up-to-5.1-channel DD and DTS tracks that are present on standard DVDs.

When using the Toshiba HD-A1/XA1 players to play audio from discs in the new HD DVD format, things get more complex.

The new disc format theoretically supports several new, high-definition audio codecs (coders/decoders). Multichannel linear (i.e., non-compressed) PCM (pulse code modulated) audio is one new possibility. Two other new multichannel options are Dolby TrueHD and DTS HD Master, both capable in theory of supporting up to 7.1 channels of losslessly compressed sound: the player restores every bit that was discarded in the compression process. Then there are (again, in theory) up-to-7.1-channel Dolby Digital Plus and DTS HD (without the "Master" designation). They too are compressed, but using a lossy compression scheme in which the player can't restore every bit.

Finally, there is DTS HD "core" audio, which is like DTS HD except that no more than 5.1 channels are allowed, and the top data rate is restricted to 1.5 million bits per second (Mbps), not 3 Mbps.

The first Toshiba HD DVD players don't permit more than 5.1 channels to be present on discs in any of these encoding formats. Linear PCM audio tracks cannot include more than 5.1 channels. These players won't handle any variant of DTS HD or DTS HD Master except the 5.1-channel, 1.5-Mbps "core" version of DTS HD. Dolby Digital Plus is likewise limited to 5.1 channels. Dolby TrueHD 5.1 input is limited to 2-channel (!) output only; multichannel Dolby TrueHD output is not supported.

The codecs listed above that in theory allow up to 7.1 channels are likewise restricted in practice to a maximum of 5.1 channels on all actual HD DVD disc releases to date.

You might think that the encoding formats and quantities of channels which the Toshiba players can't process internally would be able to be transmitted as raw bitstreams to outboard AV receivers and processors where they could be decoded and used. But, alas, no.

The Toshiba players can under certain circumstances output an HD DVD disc's raw audio bitstreams on S/PDIF and HDMI — see below.

But this apparently is not a possibility if the bitstreams utilize unsupported codecs or more than 5.1 channels. This limitation is admittedly hard to confirm, based on the initial disc releases and the often unclear information given in the Toshiba owner's manuals.

One reason it's hard to confirm is so-called "advanced content." All of the supported audio codecs listed above are in and of themselves considered to be the "standard content" of HD DVD discs. But going beyond those encodings, all HD DVDs so far released in the U.S. are authored with "advanced content," which is a way of letting the player mix more than one standard-content audio (or video) stream together into a single output audio (or video) stream.

Advanced-content audio allows sounds to play when you select menu items from the screen, even as an in-progress movie, with its own soundtrack, is being shown (and heard) beneath the menu overlay. Advanced content is also called iHD, for "interactive HD." Apparently, advanced content or iHD is considered to be present on a disc even if it's not being used — i.e., if no disc menus are currently displayed on the screen.

The first Toshiba HD DVD players respond to advanced-content audio streams by decoding the audio input streams, whatever their original formats, into multichannel (up to 5.1-channel) linear PCM. The decoded streams are then mixed together as necessary and output in one of a handful of forms.

The analog 5.1-channel RCA-jack outputs get analog versions of the multichannel digital PCM being streamed internally by the player. The analog 2-channel RCA jacks get the same content, downmixed to two analog channels.

The S/PDIF digital connections (optical and coaxial) can get (depending on which "Digital out S/PDIF" setting you choose in the player's on-screen menus) a bitstream that has been "transcoded" by the player to DTS — the kind of up-to-5.1-channel DTS long familiar on standard DVDs. Or they can get a digital, 2-channel linear PCM transcoding that, I believe, is downsampled as necessary to 48 kHz (48,000 digital samples per second).

The HDMI digital connection can, depending on the setting of another user-menu item, "Digital out HDMI," get either of those two, or it can get a digital, up-to-5.1-channel linear PCM transcoding at 96 kHz (96,000 digital samples per second). (Note that, unlike the S/PDIF digital connections, the HDMI output also carries a video bitstream, in addition to the audio stream.)

And that's it. Those are the only audio output options afforded by the Toshiba HD-A1 and HD-XA1 HD DVD players when playing an HD DVD disc that has been authored with advanced content.

If that's so, why did I say before that these Toshiba players can under certain circumstances output a HD DVD disc's raw audio bitstreams on S/PDIF and HDMI? The "circumstances" I had in mind occur when the players are playing discs without iHD/advanced content.

HD DVD discs don't necessarily have to be authored with advanced content. All U.S. HD DVD releases so far do have advanced content, so I hear, but rumor has it that Japanese HD DVD releases don't.

The Toshiba HD-A1 and HD-XA1 can ostensibly export raw audio bitstreams from non-advanced content HD DVD discs in their original form. Over S/PDIF, this ability is limited to Dolby Digital Plus and "DTS HD core" bitstreams. All other standard-content bitstreams are turned into 2-channel PCM, which is downsampled to 48 kHz, over S/PDIF. (You can force the latter behavior with Dolby Digital Plus and "DTS HD core" bitstreams as well, by setting "Digital out S/PDIF" to "PCM.")

Over HDMI, as opposed to S/PDIF, non-advanced content raw audio bitstreams from the disc can be exported as well. Again, this is just in the case of Dolby Digital Plus and "DTS HD core" encodings — as long as they are being sent to an HDMI-equipped receiver or processor that can decode them properly.

What is not entirely clear is whether these exported raw bitstreams can include, say, DTS HD Master, a codec that is not directly supported by the player. Also, can they include more than 5.1-channels? Probably the answer is no, in both cases.

In the absence of advanced content, the HDMI output of these Toshiba players can also carry 2-channel or multichannel linear PCM, depending on what's actually on the disc and how "Digital out HDMI" is set.

If what's on the disc is itself in the form of linear, multichannel PCM, it can be sent out over HDMI as such — except, apparently, if it's sampled at 192 kHz — not 48 kHz or 96 kHz — in which case it's reduced to 2-channel LPCM.

That emphasis on outputting linear, multichannel PCM in either digital or analog form is one reason why the Toshiba owner's manual states: "For Dolby Digital Plus and DTS HD (DTS Core only) multichannel audio playback [you need a] receiver or processor/amp combination with 5.1-channel analog input or HDMI multichannel LPCM-capable audio input."

Clearly, then, there are restrictions on what forms of digital audio can be sent out over S/PDIF and HDMI.

In the case of S/PDIF, the restrictions apparently come from limitations on the rates at which data can be transmitted over optical and coaxial digital connections.

With HDMI, higher data rates are nominally allowed, but limitations arise because the version of HDMI called HDMI 1.1 is the one being supported, not the new version, HDMI 1.3. Version 1.3 was not codified and implemented in chipsets in time for these Toshiba players.

According to Widescreen Review's coverage of the Toshiba HD-XA1, downloadable here, "Future Toshiba HD DVD players will support multichannel lossless TrueHD and/or DTS HD Master Audio via HDMI version 1.3 and [also] the multichannel analog outputs."

The restrictions on the S/PDIF outputs will apparently not go away in future players. It is not clear, furthermore, whether the multichannel analog outputs on future players will be enhanced to support 7.1 channels.

The initial HD DVD disc releases are from Warner and Universal. They all use Dolby Digital Plus 5.1-channel soundtracks. All of them as released in the U.S. are apparently authored with advanced content.

A handful of them also contain 5.1-channel DTS tracks — not DTS HD or DTS HD Master, just DTS, of the sort frequently found on standard DVDs. Clearly, it is permissible to carry over audio codecs from standard DVDs to HD DVD.

One and only one of the first spate of HD DVD titles, The Phantom of the Opera, contains a Dolby TrueHD track. It is just 2.0-channel. (Or, according to this article about the new audio codecs, it is 5.1-channel but is reduced by the player to 2.0-channel.)

There are as yet no releases with DTS HD soundtracks, "core" or otherwise, or with DTS HD Master audio tracks.

Nor are there any releases with linear PCM audio tracks, either multichannel or 2-channel.

Reviewers of the initial HD DVD releases are generally impressed with the improvement Dolby Digital Plus offers over plain Dolby Digital, as well as over plain DTS. The Dolby TrueHD track on The Phantom of the Opera also draws raves. Clearly, HD DVD audio using Dolby Digital Plus 5.1 offers the best sound yet heard in consumers' homes, with the possible exception of the Dolby TrueHD track on Phantom, which unfortunately plays back with just two channels.

But the potential of HD DVD to deliver high-quality sound hasn't really been exploited by today's players and discs. When HDMI 1.3 arrives in players and AV receivers, we can expect true multichannel Dolby TrueHD and DTS HD Master audio tracks, compressed losslessly, to appear on new HD DVD disc releases. We may also be able to look forward to multichannel linear PCM tracks that have not been compressed at all, losslessly or otherwise.

That's where we currently stand with respect to audio on HD DVD. In my next installment, I'll talk about Blu-ray audio. Hint: the players and discs we have now don't fully exploit the possibilities of the format, just as is true with HD DVD.

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