The art of noise: high-definition sound explained

Blu-ray offers more than just top pictures – it also delivers the best sound in home cinema, so you get to hear movies exactly the way Hollywood hears them

According to Randy Thom, one of Hollywood's greatest sound designers, "If you look closely at and listen to a dozen or so movies you consider to be great, you'll realise how important sound is in many if not most of them".

All the more reason to buy a Blu-ray player: it'll get you closer than ever to the technical prowess of the world's best sound designers.

In fact, Dolby sales manager Andy Dowell asserts: "You get to hear exactly, bit-for-bit, what the mixing engineer heard in the studio."

Blu-ray does it better
Warren Mansfield, director of consumer technology at THX, is equally effusive: "Blu-ray offers consumers a bunch of advantages on the audio side: for example, it's the only way of getting discrete 7.1 audio right now. Also, the higher fidelity on offer gives listeners a wider dynamic range, and that's refreshing: it's a great step forward."

And Anthony Wilkins, director of marketing at DTS, puts it this way: "When we talked about data compression for DVD sound, what we really should have said was data 'reduction', because elements of the original source PCM audio are permanently discarded during the encoding process. With Blu-ray's new lossless audio codecs, that doesn't happen: the result is identical to the original."

Here's how it works: Hollywood's master-quality movie sounds are engineered as uncompressed 24-bit/48kHz PCM audio (better than CD quality, which is 16-bit/44.1kHz PCM) before being mixed in the studio on a massive mixing desk like the one shown below (used at California's famous Skywalker Sound).

After that, the final soundtrack is usually heavily compressed to create final versions of the movie for distribution on film or, subsequently, for domestic use. This is similar to the way MP3 works on a CD-quality piece of music: indeed, Dolby Digital, the most common system, stores audio at transfer rates very similar to a good MP3 (384 to 448kbps).

This compromise is enforced by the technical difficulties involved in cramming surround sound on to a film reel, a limited-capacity data disc or, most recently, on to a DVD. But Blu-ray has up to 50GB of storage capacity, so the notion of 'cramming' is a bit redundant.

In fact, there's so much storage space on a 50GB disc that it can even accommodate a 7.1 channel 24-bit/48kHz PCM soundtrack, if desired by the studio.

Lossless can help
It's possible to fit uncompressed PCM audio onto a 25GB (single-layer) Blu-ray disc too, but it takes up a relatively large proportion of the available space.

To create room for extras, soundtrack options and video, film studios have adopted two approaches. Some prefer to downconvert the 24-bit PCM original into a 16-bit/48kHz version. This still sounds very good, because even downconverted, uncompressed PCM will deliver more dynamic range and detail than Dolby Digital.

An alternative is to use a 'lossless' packaging system, either DTS-HD Master Audio or Dolby TrueHD. These work rather like zip files in home computing: they repackage the 24-bit/48kHz PCM master into less space, rather than downconverting it.

All that's required is some way of 'unzipping' the data file to recover the PCM audio: it can be done inside your Blu-ray player, or inside most new surround amplifiers.

The best quality ever
The result is that a 24-bit/48kHz PCM original soundtrack packaged using Dolby TrueHD lossless uses about half the disc space of uncompressed audio. In theory, it'll be bit-for-bit identical to the 24-bit/48kHz PCM studio master: that's certainly what Dolby and DTS assert.

Equally, these losslessly packaged 24-bit soundtracks should sound better than a downconverted 16-bit/48kHz PCM alternative.

Whatever the theory, the key point is that most Blu-ray film soundtracks are far more dynamic and spacious than DVD equivalents. And if you're thinking, 'That's fine, but I won't hear any difference on my set-up at home', you'd be wrong.

Even through a TV set, Blu-ray sounds crisper, while through a proper home cinema system, the sonic advantages are instant – you'll be hearing more of the real-deal Hollywood experience than ever.

More speakers on the way?
And Blu-ray's higher-quality sound has helped to spur further advances in surround sound technology, too. Dolby and Audyssey Laboratories, the company behind the automatic set-up and EQ systems used by a wide range of highly-respected AV amps, are now advancing the idea of home cinema using additional front 'height' speakers, able to develop a wider and more enveloping soundstage than with regular 5.1.

That means you could, if you felt sufficiently motivated, be listening to your movies through a 9.1 speaker system. We can't imagine many UK households embracing that premise just yet - but who knows? With Blu-ray driving things, anything's possible...

Those logos explained

This is a Dolby Pro-Logic IIz set-up: what do the other logos and badges on your AV amplifier mean?

Space-efficient: up to 6mbps quality

Lossless: gives up to 18mbps quality

Processing mode: Up to 9.1 channels

Processing mode: Found in some amps; supports 7.1 sound

Processing mode: As with Pro-Logic IIz, offers the option of height channels, but also 'width' channels

Offers up to 6mbps quality sound

Lossless: Can give up to 25mbps quality

Andy Clough

Andy is Global Brand Director of What Hi-Fi? and has been a technology journalist for 30 years. During that time he has covered everything from VHS and Betamax, MiniDisc and DCC to CDi, Laserdisc and 3D TV, and any number of other formats that have come and gone. He loves nothing better than a good old format war. Andy edited several hi-fi and home cinema magazines before relaunching in 2008 and helping turn it into the global success it is today. When not listening to music or watching TV, he spends far too much of his time reading about cars he can't afford to buy.