The sci­ence be­hind the num­bers

What Hi-Fi (UK) - - Better On Vinyl -

In the past few years, high-res­o­lu­tion au­dio has hit the main­stream, thanks to the re­lease of an in­creas­ing num­ber of hi-res-com­pat­i­ble de­vices and mu­sic ser­vices. This should be great news for con­sumers. Why? Down­load sites such as Ama­zon and itunes, and stream­ing ser­vices such as Ap­ple Mu­sic and Spo­tify, use com­pressed file for­mats with rel­a­tively low bi­trates, from 256kbps AAC files on Ap­ple Mu­sic to 320kbps Ogg Vor­bis streams on Spo­tify. This means that data is lost, res­o­lu­tion is sac­ri­ficed, and sound qual­ity is likely to be lower. You’re sim­ply not hear­ing the full pic­ture.

High-res­o­lu­tion mu­sic tends to re­fer to au­dio that has a higher sam­pling fre­quency and bit depth than CD, which is 16-bit/44.1khz. Un­for­tu­nately, the lack of an in­dus­try stan­dard means the term can be abused. Still, if we stick to ‘bet­ter than Cd-qual­ity’, then high-res­o­lu­tion au­dio files usu­ally use a sam­pling fre­quency of 96khz or 192khz at 24-bit, but you can also have 88.2khz and 176.4khz files too.

More data, bet­ter sound?

And this should, sim­ply, mean bet­ter sound, right? A 24-bit/192khz record­ing must sound bet­ter than a 16-bit/44.1 khz CD rip, yes? Not quite. Un­for­tu­nately – in­evitably –things aren’t as straight­for­ward as that.

Even be­fore you start lis­ten­ing there are a num­ber of fac­tors to con­sider.

What are the record­ing’s ori­gins? We’ve come across so-called ‘high­res­o­lu­tion’ record­ings that are touted as 24-bit/96khz or even 24-bit/192khz, but are lit­tle more than up-sam­pled CD mas­ters sold at rip-off prices. These are a con, pure and sim­ple.

Fur­ther­more, a high-qual­ity orig­i­nal mas­ter record­ing is a must. If the orig­i­nal mas­ter is en­gi­neered poorly, it doesn’t mat­ter how high a res­o­lu­tion the record­ing is, it just won’t sound good.

Then there is the play­back equip­ment used, of course. If that isn’t trans­par­ent enough to re­veal the dif­fer­ences, you’ve got no chance of hear­ing them.

Oh. And an open mind is use­ful, too.

Not all au­dio files are cre­ated equal

So, we know that stan­dard res­o­lu­tion files are pegged at 16-bit/44.1khz. This is the level of a CD. And any­thing higher than this in terms of bits or khz we con­sider a high-res­o­lu­tion record­ing.

What isn’t made clear from the ‘high-res­o­lu­tion’ tag is whether the mu­sic file is ex­actly the same as the orig­i­nal file. This is why some com­pa­nies

From desk­top to por­ta­ble, head­phones to high-end stream­ers, hi-res au­dio is now a re­al­ity. But why should you care? And will it al­ways sound bet­ter? We crunch the num­bers…

pre­fer to use the la­bel ‘Stu­dio Mas­ter’ in­stead (where it ap­plies, of course).

Mak­ing fair com­par­isons be­tween high res­o­lu­tion/stu­dio Mas­ter files and Cd-qual­ity al­ter­na­tives isn’t as easy as you might think.

It’s likely that the stu­dios take more care over high-res­o­lu­tion files, as they will tend to be heard by more dis­cern­ing users. The Cd-spec file will usu­ally be a down-sam­pled ver­sion of that file.

Not only are there losses in­volved in step­ping down the res­o­lu­tion, but it may well be en­gi­neered for dif­fer­ent uses such as com­mer­cial broad­cast or car play­back, and so sound dif­fer­ent too.

If we get past these is­sues (some­how), surely there’s a tech­ni­cal case for high-res­o­lu­tion record­ings be­ing bet­ter, right? Once again the an­swer isn’t as ob­vi­ous as we’d like.

A few bits and pieces

It’s best to split bit depth and sam­pling rate (the khz part), and talk about them in­di­vid­u­ally.

The more bits you have, the more ac­cu­rate the mea­sure­ment of the orig­i­nal wave­form, so 24-bit looks like a good thing com­pared with 16-bit. Con­sider 16-bit has a lit­tle over 65,500 steps to mea­sure a wave­form, while 24-bit takes that to more than 16 mil­lion.

What bits buys you is dy­namic range – the dif­fer­ence be­tween the qui­etest and loud­est sounds on the record­ing: 24-bit gives a 144db range, 16-bit drops things down to 96db.

It should be noted that these are the­o­ret­i­cal fig­ures, com­pro­mised to a cer­tain de­gree de­pend­ing on the hard­ware used and the other sig­nal pro­cess­ing the file goes through. It’s pos­si­ble to lose a hefty chunk of dy­namic range in this process.

The very best clas­si­cal record­ings have a dy­namic range of around 60db, while it’s not un­usual to have pop record­ings hov­er­ing around the 15db mark.

That means, for play­back pur­poses, old fash­ioned 16-bit has enough ca­pac­ity to more than cope with any record­ing we’re likely to play.

Any is­sues with the greater mea­sure­ment er­rors suf­fered by 16-bit over 24-bit are cer­tainly re­duced by us­ing dither (in­ten­tion­ally added ran­dom noise) dur­ing the dig­i­tal pro­cess­ing. Yes, adding the right kind of noise is a good thing.

How can hi-res make a dif­fer­ence?

The ar­gu­ment for 24-bit makes more sense in the record­ing process, as there are so many ‘lossy’ pro­cesses in­volved.

While a sin­gle track of 24-bit record­ing has a large dy­namic range, it re­duces no­tably when mul­ti­ple tracks are used. A 48-track record­ing could lose as much as 36db of dy­namic range – that’s around 5 6 bits of in­for­ma­tion, even be­fore losses in­volved in other sig­nal ma­nip­u­la­tion come into play.

Sys­tem noise and other fac­tors such as the need to pre­vent over­load eat away at the dy­namic range of a record­ing sig­nif­i­cantly. Us­ing 24 bits gives the ex­cess ca­pac­ity to al­low this while main­tain­ing de­cent sound

The case for in­creased sam­pling rates is stronger still. 44.1khz was cho­sen for CD be­cause it al­lowed an up­per fre­quency limit of just over 20khz – the up­per limit of what hu­mans can hear. You’ve got to be pretty young and have pris­tine ears to do it though.

The way dig­i­tal works means that there are an aw­ful lot of un­wanted sig­nals gen­er­ated above that up­per fre­quency limit. These have to be fil­tered ag­gres­sively; other­wise they’ll re­sult in more dis­tor­tion in the au­di­ble range.

That fil­ter­ing in­tro­duces its own dis­tor­tion which folds back into the au­di­ble range. Rais­ing the sam­pling rates ever higher means that the fil­ters can be set to work at far higher fre­quen­cies, tak­ing them and their un­wanted ef­fects fur­ther away from the au­di­ble fre­quen­cies.

The raised up­per-fre­quency limit also means that the up­per har­mon­ics of in­stru­ments can be rep­re­sented bet­ter, even if sci­ence strongly sug­gests we can’t ac­tu­ally hear them.

Hear­ing is be­liev­ing

There have been plenty of threads on the

What Hi-fi? fo­rums, and the wider in­ter­net, sug­gest­ing high-res­o­lu­tion record­ings are lit­tle more than a con. We cer­tainly don’t agree.

The case for higher sam­pling rates cer­tainly looks stronger on a tech­ni­cal level than the ar­gu­ment for 24-bits (we’re talk­ing about for play­back rather than for the record­ing process).

And, cru­cially, many high-res­o­lu­tion files we’ve heard sound gor­geous, mak­ing con­ven­tional Cd-spec­i­fi­ca­tion ver­sions of the same mu­sic sound crude in com­par­i­son.

Whether that’s due to the in­creased bit depth, higher sam­pling rate or some out­side fac­tor such as the care taken in the mas­ter­ing we’re not sure. It’ll be fun try­ing to find out though.

Thanks to sup­port from mu­si­cians, stu­dios, record la­bels, mu­sic ser­vices and hard­ware man­u­fac­tur­ers, it’s eas­ier than ever to lis­ten to high-res­o­lu­tion au­dio.

If you en­joy mu­sic, we highly rec­om­mend that you try it for your­self and draw your own con­clu­sions.

To get the full hi-res ex­pe­ri­ence of Tidal, you need to lis­ten on its desk­top ap­pli­ca­tion

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