High-res audio: the science behind the numbers

Mon, 3 Feb 2014, 5:31pm

It should all be so simple. 'The higher the number the better the sound' is an easy message to communicate. So, a 24-bit/192kHz recording must sound better than a 16-bit/44.1 kHz CD rip, right?  Not quite, unfortunately – things aren’t as straightforward as that.

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

1) What are the recording’s origins? We’ve come across so-called ‘high-resolution' 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.

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

3) Much depends on the playback equipment used. If that isn’t transparent enough to reveal the differences, you’ve got no chance of hearing them.

4) An open mind is useful, too.

Standard resolution files are pegged at 16-bit/44.1kHz. This is the level of CD. Anything higher than this in terms of bits or kHz is considered 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. This is why some companies 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. I've talked to a number of people in the recording industry, and it looks like the two types of files aren’t necessarily treated the same.

It’s likely that the studios take more care over high-resolution files as they will tend to be heard and bought 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 less discriminating uses such as commercial broadcast or car use, 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. Impressive.

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 96dB.

It should be noted that these are theoretical figures, compromised to a certain degree depending on the noise generated inside the hardware used and the other signal processing the file goes through. It’s possible to lose as much as 10-20dB of dynamic range due to these reasons. 

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 (technically referred to as quantisation) 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 high-res make a difference?


The argument for 24-bit makes much 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 5-6bits 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. 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

I’ve noticed the odd thread on our forums suggesting high-resolution recordings are little more than a con. But, having heard a fair few examples, I can’t agree.

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

Many high-resolution files I’ve heard sound gorgeous, making conventional CD-spec 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 I’m not sure. It’ll be fun trying to find out though.

MORE: High-resolution audio: everything you need to know

 

by Ketan Bharadia

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Comments

It is almost funny how this article is being presented as something 'new', when there is nothing new here. The audiophile hi-rez SACD community have been enjoying hi-rez studio master SACDs (in stereo and/or multichannel) for years now, with more than 8000 SACDs on market. Most of the hi-res download material is just ripped from these SACDs. And unlike downloads, the SACDs are collectible and seem to keep their value (or even increase in value), and if you don't like them you can sell them ... whereas the download version is just a sunk cost. Some say the age of the download is already over, with Nielsen stats showing downloads already heading into decline, being replaced by streaming services.

The lesson we learn from the LP is that physical product will always be around, ... I am sure downloads will be too (in some form or other - mostly for free), but they feel soulless and will never have any value.

We're not saying hi-res audio is new, just (hopefully) giving those who may not know much about the subject some useful background information.

Some of the classic stuff, e.g. Dark Side of the Moon, is mastered on analogue tape anyway. So a Hi Res digital copy can be made directly without any worries about digital intermediaries.It can also be a genuine 24 bit. 

The early digital recordings (Tusk?) , cannot be improved- unless you consider upsampling.

Having said all that, it is debatable whether it is possible to improve on those archive recordings anyway as they would have had limits on dynamic range and other parameters.

 

A good article, thanks. For me the crucial statement was:

 

"Many high-resolution files I’ve heard sound gorgeous, making conventional CD-spec versions of the same music sound crude in comparison."

 

I'm presuming you're comparing the CD to the hi res download offering. From what we know now it's unlikely that the same master was used for both, so we're making a leap when attributing the improved sound quality to the sampling rate of the file. It's been said many times, but downsampling a hi res file to 16/44 produces a file no different in audable quality. Maybe for the purposes of the debate you'd like to carry out this for yourself? 

My understanding is that translating a high-res master to CD spec does indeed influence quality. Unless someone very clever proves to me otherwise, I'm willing to accept truncation errors in shortening the word length from 24-bit to 16-bit and the errors introduced in converting the sampling rate from say, 96kHz to 44.1kHz, introduce degredation. From my experience no piece of hardware or software (including the kit used in studios) does a perfect job. The question is whether this is audible.

There are no truncation errors if dither is used during bit depth downconversion, which any competent engineer will use.  Ditto sample rate conversion, as there are extremely high quality resamplers available at low or no cost to anyone (e.g r8brain, PPHS,SoX).  No cleverness is required, just knowing what proper practices are.  Perfection is not required.  The imperfections merely have to be kept below audibility in the final product.   You can test this yourself, by taking a hi-rez file and downconverting it, with dither, using a resampler, then do comparative listening using ABX software.

 

Audible differences between hi-rez and CD are almost always simply mastering differences -- use of difference sources, or different EQ, compression, noise reduction, or other standard mastering moves.    

 

 

 

 

Hi,

How does SACD/DVDA compare with HD audio?

DVD-A is merely a storage medium for HD Audio, and is capable of holding multichannel high res mixes. SACD is HD audio, but using DSD rather than PCM.

Hello Ketan,

enjoyed reading your post, thanks for the write up.

my question is, in CDs, I personally feel sometimes one of the variables of good natural sound quality from a CD, is the quality of the pressings... Whether it's pressed by EDC in Germany or US, or by EMI etc...

now, is there a variable like that that you have discovered with high resolution music purchased from different sites?

Also, what is your opinion on Mastered for iTunes albums which in many cases I've tried, sound much superior to even their CD counterparts... Mylo Xyloto for instance...  Bob Ludwig  has also expressed his support for Mastered for iTunes which honestly surprises me... 

The way the market is right now, Mastered for iTunes sound much better than even their CD counterparts, and if that is the basis of the mastering these days for a lot of rock/pop,  why would spending in high res music make sense? Because they fundamentally are recorded in a lower quality...

I haven't come across differences when buying the same recording from different sites.  Logically, I can't see why there would be any. I haven't made many comparisons between 'Mastered for iTunes' and their CD counterparts, so can't comment on that. I do think your playback system would have a big influence on preference. 

I have yet to realise any improvement from hi res due to not knowing how to play them back effectively. I expect the majority of Joe Public would have the same problem. I am being a bit old school as I have noticed that some vinyl LPs can sound better than CDs but also vice versa. Hi res is still too geekish for the mass market (unfortunately!)