Tuesday, July 11, 2006

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

I have yet to say much about the audio capabilities of HD DVD and Blu-ray, the two new high-definition video disc formats. In part that's because, with my lifelong hearing loss, I frankly don't have good enough ears to get the most out of even today's DVDs.

It's also because both formats apparently offer roughly the same somewhat confusing list of available audio options. These include modes that offer up to two more sound channels (7.1) than the 5.1-channel Dolby Digital and DTS many of us are now accustomed to.

Moreover, two of these new modes, Dolby TrueHD and DTS HD Master, will present us for the first time with the ability to hear digitally recorded 2.0-channel — and/or multichannel (up to 7.1-channel) — sound that has been compressed using "lossless" compression methods.

What does that mean? Well, bits are typically tossed aside during the encoding of audio for standard DVDs, and for certain of the new hi-def audio modes on HD DVD and Blu-ray discs, that the player cannot reconstitute at its end of the process; that's "lossy" compression. Such lossy forms of audio "data reduction" employ so-called "perceptual coding": throwing away bits the player can't reconstitute, on the theory that if the bits are well chosen, the ear won't notice.

The old-style Dolby Digital 5.1 and DTS 5.1 audio modes which are familiar from standard DVDs, and which the new hi-def disc formats continue to support, use lossy compression techniques. But with these new lossless audio encoding modes that are optionally going to be present on hi-def HD DVD and Blu-ray discs — Dolby TrueHD and DTS HD Master — no bits are thrown away that the player cannot reconstitute.

Two other new multichannel audio modes on HD DVD and Blu-ray discs are Dolby Digital Plus and DTS HD. They can offer the two extra channels, for a total of 7.1 — or they optionally can contain just 5.1-channel (or 6.1-channel) multichannel sound. Dolby Digital Plus and DTS HD, though, are lossy; they don't use a lossless compression methodology the way that Dolby TrueHD and DTS HD Master do.

Both camps' initial players also support wholly uncompressed, multichannel linear PCM ("pulse-code-modulated") audio input and output — up to 5.1 channels, for now. Linear PCM, or LPCM, digital audio is not compressed in any way, either lossy or lossless — that's why its "linear," I assume. The digital audio on CDs is LPCM. Optional audio tracks on some HD DVDs and some Blu-ray discs can also be LPCM.

Depending on the disc format, any given hi-def player, whether HD DVD or Blu-ray, may not be able to decode one or more of these new audio modes, if they are in fact present on a disc. For HD DVD players, apparently only

  • Dolby Digital
  • Dolby Digital Plus
  • Dolby TrueHD 2.0-channel (with 5.1- and 7.1-channel decoding optional)
modes are mandatory for players to support — so every disc must contain at least one of these formats.

I find that different discussions claim different mandatory modes are required in the HD DVD specification, so take this list with a grain salt. For example, some discussions say that another mandatory audio encoding mode that all HD DVD players must support is a version of DTS HD called "DTS HD core" audio (see below).

Note that not all mandtory decoding modes are necessarily supported by HD DVD players for every theoretically available number of channels defined by the modes' originators — say, Dolby Labs — for that mode of encoding. For example, the first HD DVD players apparently decode Dolby TrueHD audio on a disc only if it's 2.0-channel, not if it's multichannel.

HD DVD discs can include those optional audio formats its authors see fit to provide, of course, even if there's no guarantee that any given player will be able to use them.

Blu-ray discs, on the other hand, have to include at least one of:

  • linear PCM 5.1
  • Dolby Digital 5.1
  • DTS 5.1
These three modes seem to be the only ones Blu-ray players absolutely must be able to handle, and so at least one of them has to be present on every disc.

Notice again that neither format's discs, in tandem with the respective players, are guaranteed to offer audio modes that provide fully 7.1 channels of sound. In fact, it is my understanding that none of the original discs in either the HD DVD or the Blu-ray format have any 7.1-channel audio tracks.

Nor is the presence of lossless Dolby TrueHD/DTD HD Master and/or wholly uncompressed LPCM audio on either an HD DVD or a Blu-ray disc a given. I assume that all HD DVD and Blu-ray players will in general (immediately? eventually?) support the avoidance of lossy encodings in some way — if not in the first player models, in subsequent ones — but will all disc releases one day come to do so, too? Maybe, maybe not.

Something needs to be said also about the resolution and quality at which these various encoding modes can or will be recorded on HD DVD and Blu-ray hi-def discs. In general, resolution and quality are a function of the number of samples per second at which the audio is digitally captured, plus the number of bits in each digital sample.

For example, linear PCM audio may be sampled using either 16-bit or 24-bit samples at rates (in kilohertz) of 48 kHz, 96 kHz, or 192 kHz. One kHz is 1,000 Hertz, and 1 Hz equals one cycle or sample per second. The faster the sampling rate, the smoother the sound. (CD sound is sampled at 44.1 kHz, I believe.) Also, the larger the number of bits per sample, the less "digital noise" creeps into the sound. (CDs use 16 bits, I understand. 24 bits per sample are said to be the norm for the new hi-def audio formats.)

You will also see references to the total bit rates or data rates which the various old and new audio encoding/decoding modes can sustain, expressed in Mbps or millions of bits per second, or in some cases in thousands of bits per second (kbps). For example, consider a statement from this online discussion of the first HD DVD players:

The mandatory decoders that must appear in all HD DVD players are losslessly compressed two-channel Dolby True HD up to 96kHz/24-bit quality; a core 5.1-channel DTS HD lossy stream that's compatible with existing 1.5Mbps DTS 5.1 decoders in A/V receivers; and lossy Dolby Digital Plus at data rates up to 3Mbps, including 5.1 and 6.1 variations. The players will convert Dolby Digital Plus to 5.1- and 6.1-channel Dolby Digital at a data rate up to 640Kbps for playback by existing Dolby Digital decoders.
The first clause talks about kilohertz and numbers of bits, as befits a lossless encoding method like Dolby TrueHD for which the output of the decoder in the disc player will be exactly the same as the input to the encoder in the HD DVD (or Blu-ray) authoring facility.

The part about a "core 5.1-channel DTS HD lossy stream" reflects the fact that what counts here is not so much the sampling rate and number of bits per sample, figures that apply to the digital audio stream prior to compression, as the overall data rate after compression. For HD DVD, DTS HD, a lossy format, is compressed so as to ensure that the overall data rate stays under 1.5 Mbps. (Note: this discussion does not apply to lossless DTS HD Master encoding.) Such a "core 5.1-channel DTS HD lossy stream," encoded at under 1.5 Mbps, is what other discussions is simply call "DTS HD core" audio.

Likewise, a similar lossy compression must be done to hold a 5.1-channel (or 6.1-channel) Dolby Digital Plus audio stream, when present on HD DVD, below a data rate of 3 Mbps.

The quote above also suggests that since existing Dolby Digital decoders in AV receivers can't deal with the new Dolby Digital Plus format, and they can't handle a 3-Mbps data rate, HD DVD players will have responsibility for "transcoding" 3-Mbps Dolby Digital Plus (and, by extension, other new hi-def formats) to old-style Dolby Digital at a data rate of up to 640 Kbps — 640 thousand bits per second.

I believe that transcoding-to-Dolby-Digital requirment may have changed since the article was written, since the first HD DVD players from Toshiba seem to transcode not to Dolby Digital but to 5.1-channel DTS. But the principle is the same: take the new, data-intensive audio formats that may appear on HD DVD and transcode them to an old, less-data-intensive format that consumers' existing gear can deal with.

So the quality of the lossy new audio formats depends mostly on the data rates at which they are compressed. But the lossless new compression formats — and also hi-def linear PCM, which is not compressed at all — are generally stated in terms of sampling rates and bit "levels": the number of bits per sample.

Yet, often the bit level of a hi-def audio encoding mode is not mentioned explicitly. When the bit level of one of these new hi-def formats is not mentioned, it can be assumed to be 24 bits. Accordingly, Dolby TrueHD (and also linear PCM) is defined as having three supported quality levels, stated solely in terms of sampling rates: 48 kHz, 96 kHz, and 192 kHz.

Those are the three quality levels that can be present on the disc itself. The player will often "downsample" these lossless/uncompressed formats for its internal processing — and also for output — at, say, just 48 kHz. It is relatively easy to downsample 192 kHz or 96 kHz to 48 kHz, because the first two numbers are multiples of the third.

That, then, is a rundown on many of the audio input options that can appear on HD DVD and Blu-ray discs and (possibly) be able to be decoded by players, either current or future. In my next installment, I'll take up the complex question of what audio output modes the first HD DVD players and discs actually support. In a subsequent installment, I'll ask the same question about the first Blu-ray players.

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