2. Why Bother With High Definition DVDs?

The title of this section raises the central issue that surrounds both new HD DVD formats. The short answer to this question is "better picture, and possibly also better sound" but realizing those benefits comes with some interesting requirements - this can involve added cost for other pieces of gear in the output chain, from player to receiver and/or television set. Let's walk through the benefits of high definition DVDs, and explore those requirements as we dig into high definition DVD technology in general, and then take a look at the specific capabilities inherent in HD-DVD and Blu-ray.

The real secrets to high definition DVD content can be classified into four areas:

Resolution: Conventional DVDs support a maximum resolution of 720x480, which is roughly equivalent to what standard definition TV (SDTV) sets can handle. High-definition DVDs accommodate resolutions of up to 1920x1080, currently the highest digital video resolution commercially available, which only some HDTV sets can handle. Scan Type: DVDs and displays can support either interleaved or progressive scan at specific frame rates. On an interleaved scan, the display writes every other horizontal line to the screen each time the screen is refreshed; with progressive scan, the display writes every line each time the screen is refreshed. Most viewers report a slightly more detailed and sharper image when watching resolutions on progressive scans than on interleaved scans. Please note that differences in frame rates between source materials and display devices, as will often be the case with 24 fps motion picture video and 60 fps HDTV displays, are able to compensate for differences between interleaved and progressive scans to the point where they're virtually indistinguishable. Data Density: Both Blu-ray and HD-DVD use blue-violet lasers that operate at a wavelength of 405 nanometers (nm), whereas conventional DVDs use a red laser that operates at 650 nm; and audio CDs use an infrared laser that operates at 780 nm. The lower wavelength permits both Blu-ray and HD-DVD to address much smaller regions on the surface of the disc, and to use those regions to store individual bits more densely. Without going into too much technical detail - we'll see more on this later during our discussion of what differentiates HD-DVD from Blu-ray - the technical changes enable HD-DVDs to store up to 15 GB per layer and Blu-ray DVDs to store up to 25 GB per layer, whereas conventional DVDs store only up to 4.7 GB per layer. Given the higher storage requirements for accommodating high definition video data, this is as much a necessity as a virtue. Data Rates: Both Blu-ray and HD-DVD also support much higher maximum data transfer rates from the player's read head into the outbound video data stream. Whereas conventional DVD players support up to 11 megabits per second (Mbps), HD-DVD supports up to 36 Mbps for both data as well as audio/video, and Blu-ray up to 54 Mbps for audio/video and 36 Mbps for data. Here again, this higher capability is necessary to accommodate high definition video data, but also opens the doors for higher-bandwidth multi-channel sound formats on high definition DVDs as well (more on these in the following paragraphs).

Hopefully, these points help to explain our original short answer, and to shed a little more light on why a better picture is possible (more resolution and higher scan rates) and where the headroom necessary to accommodate higher bandwidth multi-channel surround sound encodings comes from. In fact, high definition video for both HD-DVD and Blu-ray Disc also benefits from improved video codecs as well: conventional DVDs support only MPEG-2 encoding, whereas both of these high-definition formats also add support for AVC MPEG-4 and VC-1 encodings.

AVC stands for Advanced Video Coding, which originates with the ITU-T Video Coding Experts Group (VCEG). It corresponds with the ITU-T H.264 standard, which currently remains synonymous with the ISO/IEC MPEG-4 Part 10 standard (ISO/IEC 14496-10). AVC's claim to fame is its ability to realize high data compression rates, so that good video quality may be delivered at substantially lower bit rates than is possible for MPEG-2, H.263, or even MPEG-4 Part 2. It also is known for its ability to accommodate high resolution video in forms suitable for broadcast, DVD storage, and even real-time streaming IP-based video formats.

VC-1 or Video Codec 1, is the street name for the SMPTE 421M video codec originally developed at Microsoft, based on the Windows Media Video 9 codec (aka WMV3). It includes coding tools for both interlaced and progressive video sequences, so interlaced content may be compressed without first converting it into progressive format. This makes it faster and more appealing for broadcast and video storage applications. Microsoft's involvement with HD-DVD also explains why VC-1 is the official video codec for the Xbox 360, and why it's supported within Windows Media Player 11. Both codecs offer great support for high-definition DVD content, with a slight bias toward VC-1 for HD-DVD content, and AVC for Blu-ray content.

Both HD-DVD and Blu-ray also offer support for more sophisticated multi-channel surround sound formats than you'll find on conventional DVDs. The older conventional DVD format supports only Dolby Digital and DTS 5.1 surround sound codecs. In contrast, both newer formats support various compressed (lossy and lossless) 7.1 surround sound formats, as well as Linear PCM, which supports up to 8 channels of uncompressed audio. These codecs include the following:

Dolby Digital Plus (DD+ or E-AC-3): Supports increased bitrates and 7.1 channel surround sound, and may be used to simulate/synthesize 7.1 surround sound from 5.1 sources on DVD (such as Dolby Digital, upon which this format is based). The DD+ specification supports up to 14 sound channels but both HD-DVD and Blu-ray limit the total number of sound channels to 8. This codec supports bit rates from 32 kbps to 6.144 Mbps, with sample rates at 32, 44.1, 48, or 96 KHz, and bit depths up to 24 bits per channel. Dolby TrueHD: Supports lossless audio encoding based on Meridian Lossless Packing (MLP) for up to 8 channels of audio information (often in 7.1 schemes). This codec is mandatory for HD-DVD and optional for Blu-ray, and supports 24-bit, 96 KHz audio channels at rates up to 18 Mbps. (By itself, this codec can handle up to 14 sound channels, but these high-definition DVD standards limit the total number of channels to 8). TrueHD output is only available through the HDMI interface, but most players can convert it to analog, DTS, or LPCM (which also only works over HDMI). Raw TrueHD streams may be transported via HDMI (1.3 or higher) to an AV receiver that can handle this format. DTS-HD High Resolution Audio: Supports up to 8 channels of 24-bit 96 KHz audio (usually in a 7.1 scheme) with bit rates of up to 6.0 Mbps for Blu-ray, and up to 3.0 Mbps for HD-DVD. This codec is optional for both HD-DVD and Blu-ray, and its effective bitrates extend from 769 kbps (typical conventional DVD encoding) to 1.5 Mbps (DTS audio sound tracks for movies) and all the way to 3.0 or 6.0 Mbps, depending on the high-definition DVD format used, giving it extraordinary sound reproduction capabilities. DTS-HD Master Audio: Also supports lossless audio encoding for high-definition DVDs, and bitrates of up to 24.5 Mbps for Blu-ray and 18.0 Mbps for HD-DVD. As with Dolby TrueHD, this means that this codec can deliver 7.1 audio channels at 24 bits and 96 KHz sampling that is bit-for-bit identical to the original studio master from where the original soundtrack comes.

By now we've certainly flogged the notion of "better picture, better sound" enough. Those interested in learning more of the relevant details will find a wealth of additional information online, where the outstanding eMedia Blu-ray FAQs make excellent jumping-off points, as do the official HD-DVD and Blu-ray sites.