Blu-R ay — Th e Sound of Sound on disc

This issue we’re looking at something a little different. Namely, we’re getting our bearings on what audio formats are actually available on various formats of disc. Why now? Because with the advent of the ultrahd Blu-ray disc, some confusing information has been put out, not least by those companies that distribute UHD Blu-ray discs.

4k Ultra HD blu-ray video

Amongst the benefits touted for UHD Blu-ray are higher resolution video, high dynamic range video, newer and better colour standards, improved video compression algorithms, and new, better, audio standards.

The first four of those are true. UHD Blu-ray can deliver resolutions of up 3840 by 2160 pixels (four times the resolution of standard Blu-ray). Less well known, it can also deliver up to 60 frames per second in progressive scan format (up from 24fps). Virtually all Ultra HD Blu-ray discs support HDR, which involves 10 bits of coding for each primary colour, up from the 8 bits used on DVD and standard Blu-ray. To put that into more intuitive terms, each of the three primary colours can be specified on a scale of 1024, up from the 256 (actually, 239 for technical reasons) of the older formats. That means smoother gradations, and more extreme boundaries.

Optionally, even higher standards can be used, such as Dolby Vision. That uses 12 bits (i.e. 4096 levels), and also a scene-by-scene sliding scale specified in metadata, effectively increasing that to even-higher resolution. Most UHD Blu-rays employ the Rec.2020 colour space. Regular Blu-rays use the Rec.709 colour space. Rec.709 covers about 36 per cent of the colour space determined to encompass all colours that humans can perceive. Rec.2020 bumps that up to about 76 per cent. (That increase sounds more impressive than it really is. Not all areas of the colour space are as important as others.) DVDs were encoded with the first-developed mainstream lossy compression system: MPEG2. Subsequently H.264—otherwise known as MPEG4 AVC—was developed and this allowed higher compression levels without additional loss of quality. Most Ultra HD Blu-ray discs use H.265, which is even higher in efficiency.

Ultra HD blu-ray audio

With Blu-ray came new, improved audio standards. With ultrahd Blu-ray the standards... remained the same. The reason is simple: the most recent audio codecs have been developed with backwards compatibility with regular Blu-ray in mind. Dolby Atmos, for example, is usually carried as an expanded version of Dolby truehd. Remember, Dolby Atmos contains not just some content defined for specific channels, but also audio ‘objects’ with a defined position (moment by moment). The decoder takes on a lot of responsibility for making sure the sound from each object is sent in the right proportions and timings to the right channels.

I have quizzed Dolby about how the extra data is packed into the audio track, but it has not been especially forthcoming. Regular Dolby TrueHD has an embedded Dolby Digital track. DTS-HD Master Audio has a core DTS track. In either case, if full decoding of TrueHD or MA is not available—say, when the audio is being bitstreamed out over optical—then the lesser quality audio is readily available. My guess is that with Dolby Atmos and DTS:X (a somewhat similar DTS system), the standard TrueHD and DTS-HD MA streams contain a flag which can be recognised by Atmos/DTS:X capable systems. If they see it, they use the audio from another stream. Incompatible gear would just ignore the flag. The good news from UltraHD Blu-ray, as far as audio goes, is that I have yet to see a single title in which the primary (English language) audio is encoded in a lossy format. All, so far, have been encoded in Dolby TrueHD, DTS-HD Master Audio, Dolby Atmos or DTS:X. For example, the full Harry Potter series uses DTS:X.

But do be aware, sometimes the very best audio track is not the default. The recent releases Tomb Raider and Ready Player One both default to DTS-HD Master Audio—most likely in 16 bits, if they conform to the standard Bluray inclusions—rather than to the Dolby Atmos both carry.

Stereo VS. MUlticHannel

There is a tendency to regard multichannel sound as more artificial than stereo. There’s no doubt that there’s more manipulation of a multichannel signal than stereo, since the multiple channels have to be decoded in highly technical ways. And most multichannel sound is movie sound. Movie sound is almost always assembled from components in a film mixing studio, not just captured and subsequently delivered to your home unprocessed. Almost the complete opposite of what can happen with some audiophile labels.

All that said, there is a strange misconception that stereo is natural. It isn’t. It’s equally artificial. As with multichannel sound, stereo strives to create an impression of the real world that actually works in a very different way. Consider this: you are in a room listening to a solo cellist. In real life, what you hear consists of sounds coming directly from the bow and strings and body of cello to your ears, mixed in with sounds reflected from various surfaces in the room. Your ears hear the direct sound first. It comes from exactly one place.

When you listen to a stereo recording of the same cellist, you are hearing two separate sources: the two loudspeakers. If you’re right on the centre axis between the two speakers, it can seem similar. But do remember that the reflected sounds you hear are those in the studio, which are picked up at different times by the two microphones, combined with the reflections in your own room. And those reflections are of sounds from two sources. And reflection of the reflections picked up by the microphones.

As we all know, at its best stereo sound can be astonishingly involving. But it’s not truly natural. So, if you do get a chance, give multichannel sound a go. It’s no less natural, and gives the sound producers a wider range of tools to produce the result they want.

Stephen Dawson