HI-RES AUDIO:
The science behind the numbers
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.