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

Australian HIFI - - EISA AWARDS – 2018-2019 -

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

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