HD DVD and Blu-ray aren't just about high definition pictures. Along with these formats comes the possibility for much improved sound. The multichannel digital audio that we've become used to on DVD continues with the new disc formats but it can be expanded to more speaker channels and delivered at much higher quality.
Given that it's early days for both formats, it's no surprise that the hardware is playing catch-up with what's possible through super-efficient audio recording and compression. The result can be confusing, with different standards, settings and sockets to consider. Here we explain how to get the best sound from HD discs, whether your surround sound system is the latest state-of-the-art or more 'classic'.
What are the new audio formats?
Blu-ray Disc (BD) and HD DVD (high definition DVD) are both designed as the next generation of DVD, able to carry more data and to provide high resolution images plus enhanced sound. International hardware and software giants lined up behind either side, with some supporting both.
There are three formats which take advantage of the higher capacities of Blu-ray or HD DVD, although many discs may use conventional Dolby Digital or DTS options and, in some cases, uncompressed Linear PCM audio. LPCM is 'raw' digital sound as used in the original mastering (typically sampled at 96kHz/24bits for a modern movie) and compatible with practically any AV receiver. However, LPCM is not space efficient on discs and cannot hold 'metadata' to help decoders work in the optimum way for a particular soundtrack.
Dolby Digital Plus is backwards compatible with Dolby Digital equipment by using a standard 5.1-channel 'core' element, though in its broader form it can carry up to 7.1 channels sampled at 96kHz/24-bits and encoded at bitrates of up to 1.7Mbps for Blu-ray and 3Mbps for HD DVD. This compares to a maximum sampling frequency of 48kHz and typical bitrates of 384-448Kbps for normal Dolby Digital. At lower bitrates, Dolby Digital Plus is also suitable for TV or internet distribution.
At the high-end, Dolby TrueHD is a compressed but lossless system offering faithful reproduction of the studio master at up to 192kHz/24-bit sampling across six channels (or 96kHz/24-bit for eight channels) and a maximum bitrate of 18Mbps. For this reason it uses more disc space than Dolby Digital but two to four times less than the data-heavy LPCM.
DTS has its own lossless DTS-HD Master Audio equivalent. Basically, they are like DVD-Audio sound with HD pictures too, though they will not be carried on all high-def discs, especially older movies where – even if the picture is remastered for HD – sometimes it's not practical or possible to do the same with sound.
These new formats can be decoded by the disc players, providing they have the necessary technology, however some lack the circuits, software or connections to make them do this fully or at all. There is usually a decent next-best option, though, as explained below.
SPDIF (Sony-Philips digital interface format)
This is the digital output used for years with DVD, MiniDisc, DAB radio and digital TV boxes, usually as an optical or coaxial bitstream connection. It also features in Blu-ray and HD DVD players to deliver basic digital sound but it cannot stream Dolby Digital Plus, TrueHD or DTS-HD at full quality. Due to backwards compatibility, in most cases you should hear 'something'. With Dolby Digital Plus, for instance, the core 5.1-channel bitrate can be up to 640Kbps, so you may get a slightly better sound than conventional Dolby Digital.
Some players use SPDIF to deliver a re-encoded version of a high-end soundtrack. For example, Toshiba's HD-E1 HD DVD player deals with such soundtracks by restricting sampling to 48kHz and encoding into a high-bitrate (1.5Mbps) DTS 5.1 stream output via SPDIF. Even older AV receivers should accept them, however, they do not reproduce the original at full resolution and the real-time encoding can introduce lossy compression flaws, so it's inferior to the two options below.
Multichannel analogue outputs
Most AV receivers have a set of 'external in' phono sockets. These use analogue audio and can take very high resolution signals, as should be familiar anyone who has played DVD-Audio or SACD. Most next-gen players also provide multichannel analogue outputs, so you don't necessarily need to upgrade your AV receiver. Some early players do impose limits however. The Samsung BD-P1000 only has 5.1 channel outputs (so to get 6.1 or 7.1 configurations you must have an HDMI-equipped receiver), while Toshiba's HD-E1 has no multichannel analogue outputs at all, compelling you to use either HDMI or (the poorer) SPDIF.
HDMI (high definition multimedia interface)
HDMI can carry LPCM (converted by the player or raw from disc if provided) or a bitstream output, though the quality and performance from the latter depends on what kind of HDMI connection is available in both player and AV receiver.
Only recent AV receivers have HDMI and many use older v1.1 or v1.2 systems, as do early Blu-ray and HD DVD players (eg Panasonic's DMP-BD10, 
Samsung's BD-P1000 and Toshiba's HD-E1). This means that the full quality of high-end soundtracks cannot be streamed but if the player can decode the high-end formats and convert to LPCM, then it can output that audio via any HDMI port, including up to 7.1 channels and interactive audio features, such as commentary tracks accessed from the internet.
If both player and receiver use the latest v1.3 protocol, then you will be able to stream the audio data directly from the player – like SPDIF only better and with video alongside, minimising cabling. However, one downside is that you may lose interactive audio features, because this decoding and mixing must be done by the player rather than the AV receiver. For these, outputting LPCM via HDMI or using the multichannel analogue phonos are the best ways to connect.
Conclusion
To sum up, HD DVD and Blu-ray players are amazingly powerful but they are complex and evolving technologies, using connections and AV standards that are also developing. If you are upgrading to one of these players, get one with all the right decoding built in, as well as an HDMI v1.3 output. And get a new surround sound receiver with HDMI v1.3 so it will match. Ideally, wait for a wider choice of compatible hardware later in 2007, especially if you want to hear DTS-HD.
This will give you a more user-friendly, flexible and reasonably futureproofed system. As long as your surround sound speakers are well matched and up to the job, you should be ready to get the most out of either of the new disc formats.
Writer: Ian Calcutt
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