High resolution

When Sony/Philips released the Compact Disk early 80’s , we were impressed.
650 MB on such a small platter!
Today the CD is very much like a 5¼ floppy disk, big and a low storage capacity.

DVD stores 4.7 GB and a tiny SD card 128 GB.


Given the technology at that time, Sony/Philips had to find a compromise between duration and sound quality.
In the end, they settled for 2-channel audio with 16-bit word length and a sample rate of 44.1 kHz.

16 bits allow for a dynamic range of 16*6= 96 dB and as Shannon/Nyquist tells us, a maximum frequency of 22.05 kHz (44.1/2).

 

Theoretically a sample rate of two times the highest frequency is sufficient to reconstruct the analogue signal (Nyquist frequency) .
Our audible range is 20 – 20.000 Hz. Sampling at 40 kHz is in principle sufficient but there needs to be some additional room for the filtering (transition band).
That’s why CD audio uses a sampling rate of 44.1 kHz.

 

Recordings are often made with a greater bit depth (24) and a higher sample rate.

Inherent to the audio CD is that regardless the bit depth and sample rate of the recording, it has to be re-sampled to 16/44 otherwise it won’t play.


The moment one switches to other media, one get rid of this constrain.

DVD often contains 24 bit/ 96 kHz audio.
Downloads can be in any sample rate.
The only requirement is that your media player/audio device can play it.

No need to cling to this 80's Redbook standard.

Benefits

A 24 bits recording has a dynamic range of 24*6= 144 dB
Sounds impressive compared with CD’s 96 dB.

In practice the noise floor of your gear e.g. -110 dBFS will be the limiting factor.
But probably listening to the decay of instruments at a high volume might reveal a subtle difference.

 

96 kHz has a Nyquist frequency of 48 kHz.
Sounds impressive too but our hearing stops somewhere at 20 kHz (when we are young).
It sounds like buying recordings containing information above the upper threshold of our hearing is like paying a premium for some emperor’s new clothes.

 

There are reasons why a hi-res recording can sound different.
Down sampling might introduce artifacts (filtering).

 

You get an alias regardless the sample rate. This alias starts at ½ fs.
In case of 44 kHz this is at 22 so pretty close to the upper limit of our hearing.
You need a very steep filter (brick wall) to preserve both the audible range and filter out the aliases.
In case of 192 the alias is at 96.
Again we need a filter but we might decide to use a less steep one and starts a bit earlier because there aint much musical live at 96 kHz.
This make the filtering less intrusive.

 

Our gear can sound different when modulated with signals > 20 kHz.

The IMD (inter modulation distortiion) generated by the content above 20 kHz might map into the audible range.

 

Obvious, high-resolution recordings are no snake oil.
They do offer a bigger dynamic and frequency range and are less demanding on filters .
The question is if all these technical benefits actually translate into a better sound quality.

Can we hear the difference?

Often no audible differences between CD audio and higher resolutions are reported on the internet. Just as no audible differences between CD audio and high bitrate MP3 is often reported.

As 1+1=2 some conclude that there isn’t a difference between MP3 and hi-res audio!

 

On audiophile forums, you will find enthusiastic claims that high-resolution sound better.  
The question of course is better than what?
Often hi-res recordings are recent recordings or recent remasters.
Recording and mastering technique has improved over the decades.
One might mistake improvements in recording technique for improvements in the format.


Recording a hi-res download with the same recording on CD sounds like a fair comparison but it can be a bit tricky.
There is no guarantee that the source is the same.
Even if it is, it might be different masters.

 

The best way to compare is to have a true high-resolution track and down-sampled it to redbook format (16/44).

Compare them in a unsighted test.

You can find a couple of them on the internet.

 

AVS/AIX High-Resolution Audio Test: Ready, Set, Go! - Scott Wilkinson

AVS/AIX High-Resolution Audio Test: Take 2 - Scott Wilkinson

24-Bit vs. 16-Bit Audio Test - Part II: RESULTS & CONCLUSIONS - Archimago's musings

 

Often the discussions on the fora turns in to a flare.

A nice example: Conclusive "Proof" that higher resolution audio sounds different.

 

Most of the time these listening test concludes that most of us can't hear the difference.

According to Pras and Guastavino trained listeners can hear the difference in a ABX test.

 

It is currently common practice for sound engineers to record digital music using high-resolution formats, and then down sample the files to 44.1kHz for commercial release. This study aims at investigating whether listeners can perceive differences between musical files recorded at 44.1kHz and 88.2kHz with the same analog chain and type of AD-converter. Sixteen expert listeners were asked to compare 3 versions (44.1kHz, 88.2kHz and the 88.2kHz version down-sampled to 44.1kHz) of 5 musical excerpts in a blind ABX task. Overall, participants were able to discriminate between files recorded at 88.2kHz and their 44.1kHz down-sampled version. Furthermore, for the orchestral excerpt, they were able to discriminate between files recorded at 88.2kHz and files recorded at 44.1kHz.

Scam

You can find reports like this one:

I am interested in everyone's experience with the quality of various HD Tracks labels in hi rez. I have had excellent results with Reference Recordings, Dorian, Chesky and 2L. But I was very disappointed with the BIS recording of Osmo Vanska's Beethoven symphonies 2 & 7. I had seen rave reviews of the SACD version, but when I downloaded it in 24/88.2 from HD Tracks, the sound quality was nothing special. My RBCD version by Gunther Wand on RCA has better SQ
http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=72939

Somebody listening to hi-res and is missing the hi-res sound.
Others chime in; ask for a sample and it turns out that this recording doesn’t contain any signal above 22 kHz.

One of those examples where you pay a premium for a hi-res track and get up sampled CD audio in return.
Unfortunately this is not a single incident.

http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=74993
http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=72939
http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=81291

 

As long as companies selling hi-res don’t tell you what the resolution is of the source used, you run the risk of being scammed.
These incidents also proof that some can hear the difference between CD and hi-res.

Listening with your eyes

There are a trick to check if a recording is true hi-res.

An obvious one is the frequency spectrum.

If the spectrum it is cut off at 21kHz the source is probably 44.1 (CD)

 

A nice overview of a couple of recordings with spectrograms by Fujak.

Reference
  1. Listening with your eyes - Bruce Brown
  2. Hires Audio - soundcheck's - audio@vise
  3. The HD music fft atlas reference thread - wgscott
  4. HD-Produktionen auf dem Prüfstand - Fujak
  5. The tradeoff of 192 kHz sampling - Mikael Vest and Peter Scheelke - Digital Audio Denmark
  6. Sampling Rate Discrimination: 44.1 kHz vs. 88.2 kHz - Pras, Amandine; Guastavino, Catherine. Affiliation: McGill University, Montreal, Quebec, Canada
  7. AVS/AIX High-Resolution Audio Test: Ready, Set, Go! - Scott Wilkinson
  8. AVS/AIX High-Resolution Audio Test: Take 2 - Scott Wilkinson
  9. Conclusive "Proof" that higher resolution audio sounds different - Amirm
  10. 24-Bit vs. 16-Bit Audio Test - Part II: RESULTS & CONCLUSIONS - Archimago's musings