44 KHz (CD) not enough !? (Nyquist etc.), plethora of distortion frequencies?
44 KHz (CD) not enough !? (Nyquist etc.), plethora of distortion frequencies?
May 11 2003, 17:40
Joined: 11-May 03
Member No.: 6542
Remarks and conclusions added May 12 2003 - 1:55 PM, and edited May 14 2003 - 08:35 AM :
My dubious claims unfortunately had a very short life span due to the very successful enlightenment efforts of tigre, 2Bdecided, KikeG and mrosscook.
In short: I failed to come up with evidence that cd quality (I mean 44.1 KHz digital sampling) is somehow problematic. It basically was a story of using the wrong tools, jumping to the wrong conclusions, and not having enough of a clue about signal processing.
Nevertheless, I tried again to make less daunting claims that the 44.1 KHz digital sampling rate is not enough to represent all signals less than 22.05 KHz correctly.
And again my claims had a very short life span. This time due to further enlightenment efforts by DonP, 2Bdecided, KikeG, mrosscook and SikkeK.
The conclusion: Arguing against the technical specification of cd quality (44.1 KHz/16 bit) should not be tried by someone that severely lacks in signal processing clue (like me).
If the cd sound quality is perceived as suboptimal, it may have more to do with poor recording, poor mastering, and suboptimal reproduction equipment (i.e. cd-player and sound system/headphones).
What one still could try are listening tests:
Such tests would need to be done with one and the same high end hardware for all signals and all tests (preferably with 192 KHz resolution, with 20-24 bit, and with a DAC that is perfectly shielded and outside of any system that is rich of EM signals, like a computer, and has a near perfect analog circuitry). And when testing the 192 KHz signal against the 44.1 KHz signal, the latter would need to be a digitally downsampled version (to 44.1 KHz), which was upsampled to 192 KHz again. Using the best available algorithms (Cool Edit may do a resonable job here).
And still, asking the test persons for audible artifacts would most likely not work at all. It might be more rewarding letting them rate how the music "felt" (e.g.: more or less "relaxing" for music that should be "relaxing" but is rich in high frequency content nonetheless). This could be done in a way that is scientifically sound and statistically relevant.
My original post:
I have to admit: This 44.1 KHz topic more or less has been discussed to death already. It also seems likely that the following problem has been discussed on Hydrogenaudio several times as well (but I had no luck with the search function).
The 44.1 KHz sampling rate (CD quality) seems to create an infinite number of "mirrors" at its harmonics. These in turn create a complex set of distortion frequencies for every frequency in the analog source.
The strongest "mirror" is at at 22.05 KHz (44.1 KHz/2). But the problem can easily be demonstrated with the one at 11025 Hz (44.1 KHz/4) as well: if one creates a sine signal of 11025-1000 = 10025 Hz in a sound editor (e.g. Audacity, using a 44.1 KHz sampling rate) and plots the spectrum, then two additional frequencies are shown: one at 1000 Hz and one at 22050-1000 = 21050 Hz. More distortion signals can be seen if the FFT resolution is increased above 1024.
The general problem seems to be that a sampling frequency of 44.1 KHz does not guarantee that frequencies below 22.05 KHz are represented faithfully (as is mostly believed). Instead it probably more or less only guarantees that in the resulting complex signal the source frequency is significantly stronger than the numerous distortion signals.
Of course, the remaining question is if these distortions are audible (they resemble pretty much amplitude modulation). I cannot really test this with 44.1 KHz since I donīt have a 96 KHz soundcard. But the example with 11024 Hz surely looks rather disturbing (when looking at the waveform) and doesnīt sound very clean as well.
Did anyone do any respective (blind) listening tests?
The following example is very audible: When using a sampling frequency of only 2000 Hz (instead of 44100 Hz) and creating a sine frequency of 750 Hz (well below the Nyquist limit of 1000 Hz) then the result sounds pretty ugly (itīs some kind of mixed signal of 750 Hz, 250 Hz and 1250 Hz).
This post has been edited by zephirus: May 19 2003, 15:49
May 21 2003, 10:24
Joined: 5-November 01
From: Yorkshire, UK
Member No.: 409
I agree that this has no relevance in the "real world" of average Joes and boomboxes. But then, anything better than a reasonable 128kbps mp3 is irrelevant to 99% of people too.
If the audible differences are only present on equipment costing more than most people earn in a year, then maybe we can forget it. However, there's a 95%+ mark up on audio equipment compared to the cost of the raw components. This means some talented audio amatures could get close to this quality, given a lot of time, and maybe a month's wages to construct the equipment. So let's not talk about $20k audio systems as though they're unattainable. There are people who could get close enough with $1k, a soldering iron, and lots of patience. If this hobby is your passion, I think that may be worth it. As someone who might try it, I'd (selfishly) like some nice recordings to play on it, even if you lot can't hear the benefit!
If there's any truth in the theory that, price for price, a DAC can sound nicer decoding 96k than 44.1k, even though an infinitely expensive DAC can do both perfectly, then 96k seems a good idea.
However, I'm worried that it might work in quite a different way. What if it's like, say, mp3 artefacts. To start with, for some people, they're hard to hear. Many people can't hear them. But some people do learn to hear them. You start with the most extreme examples, and, once you've learnt the "sound" of mp3, it's much easier to identify - even from quite difficult examples. Once your ear/brain learns the pattern, you can find it much more easily. Look at how the some of people involved with codec testing and tweaking are some of the most sensitive - Garf, Dibrom etc. It's probably practice.
So, I'm suggesting that, when it's more common place to experience CD vs something else (better?), more people will get a chance to notice the difference. The more you get to compare, the more you learn how to recognise it - and once you've learnt well, then the problems might annoy you. However, until something better becomes common, you have about as much chance of realising that CD isn't good enough as you would of realising that 160kbps mp3 wasn't good enough in a world where there was NOTHING better. You have live music, but if the recording doesn't sound like live music - well - that's just your stereo.
I'm not saying I actually think either of these things are true. I'm suggesting them as possibilities and speculation - time will tell, and I think I agree with your assessment. Whatever - stereo > multi-channel is a much greater leap than 44.1k>96k. If we can do both, great. If the multi-channel system is a version of ambisonics rather than 5.1 based, so much the better!
And you're right in one thing - I am deeply convinced the best audio engineers were in job between 1954 and 1965 (approx.). They played with mikes, they could perfectly record the space, their recordings have depth... One just can't believe what they could do with the relatively poor equipment. The recordings from that era are just unbelievable, marvellous, monstrous, incredible... I have a lot of them (many on vinyls, but they sound good also from good remastered CD) and they make me always joy when I listen to them!
Totally totally agree! Some of the early stereo recordings are so good, it makes you think "how did they do that?!" and why can't we do as well today!
You've reminded me: at the same AES, there was a workshop session where various producers talked about work that they'd done, and played examples. One had brought in the theme tune from Austin Powers - the original version (it's been remixed on subsequent films, and the track itself is around 40 years old), which had been recorded on a 3 track recorder. Brilliant! You could see people in the audience enjoying it. "How clever of you to anticipate the moves of Austin powers 40 years ago!" joked the interviewer. Another producer brought some Bolton song from a Disney movie - he'd chosen to bring it because it was the first production he'd worked on where he needed more than 100 tracks. It sucked! Not just because it was Michael Bolton singing Disney - but the recording was just so artificial and flat and BORING! No one was in the least bit interested. (Maybe i'm exagerating, but the poor comparison with the 1960s 3 track recording wsa quite embarasing!)
Now, you could say that it was the songs themselves that made a difference - true, they certainly did. But there was a magic in the actual recording from the 1960s that was completely absent from the others. I'm sure it was the primative technology, and the comparative simplicity of the mixing process that made the recording distinctive, good, and enjoyable.
|Lo-Fi Version||Time is now: 27th November 2014 - 00:35|