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44 KHz (CD) not enough !? (Nyquist etc.), plethora of distortion frequencies?
zephirus
post May 11 2003, 17:40
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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?

zephirus

PS:
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
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zephirus
post May 20 2003, 21:48
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QUOTE (KikeG @ May 19 2003 - 06:48 AM)
If I could have access to some 24/96 music in wav format, I could set up one of those tests.

I would downsample the 24/96 wave to 16/44.1 using "realistic" filtering + resampling to 44.1 KHz in CEP (that should be equivalent to the filtering of a good ADC), and convert it to 16 bit with dither.

Then, I would play this wave with one of my cards, using its converters running at 44.1 KHz, and record the result with my other card at 48 KHz 16 bit. Ideally it should be recorded at 24/96, but I just have 1 good non-resampling card at 44.1 KHz that is used for playback, and happens to be the same one that also supports 24/96. My other card is non-resampling just at 48 KHz, but is limited to 16/48, although I think has pretty good quality. I think the the fact that the recording is done at 16/48 instead of 24/96 wouldn't have much influence in the test, specially if no differences are found at last. But this point should be analyzed with detail to see what problems it could have, and improved if found necessary. At first, I think that this 16/48 recording would capture all the "nastieties" (aliasing, smearing, etc) of the 16/44.1 DAC, but would introduce some of the 16/48 ADC, although at higher frequencies.

Then, I would upsample to 24/96 this 16/48 record, and people with good 24/96 cards could perform some blind listening tests comparing the original vs. the this other wave that has been downsampled, played, recorded and upsampled.

At the PCABX site there are some avalable clips to perform tests of this kind ( http://64.41.69.21/technical/sample_rates/index.htm ). They are interesting and show that AFAIR there is no difference in practice, but the clips available are not very representative of real-word conditions. They are very short on duration (and typical fans of hig-res formats won't consider them very representative), and in fact no real-world DAC has been used in their generation, just software processing.

The best procedure for such test seems to be to use one and the same output equipment (soundcard) with one and the same output sampling frequency (96 or 192 KHz, 20-24 bit). And to do anything else only in the digital domain.

Otherwise there will always be analog components that may interfere and that may distort the result of such listening test. The output sampling rate must be the same for all tests (96 or 192 KHz). And the test signal/soundtrack would need to have been been sampled with true high quality equipment at 96 or 192 KHz (20-24 bit).

The goal should be to prove that 44.1/16 is not enough for all signals, and that there is at least one signal (soundtrack) where the 44.1/16 resolution fails.

QUOTE
At the PCABX site there are some avalable clips to perform tests of this kind (...) and in fact no real-world DAC has been used in their generation, just software processing.

This probably is not good enough.

zephirus
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Posts in this topic
- zephirus   44 KHz (CD) not enough !? (Nyquist etc.)   May 11 2003, 17:40
- - tigre   44.1KHz is enough to represent everything below 22...   May 11 2003, 22:59
- - mrosscook   The most recent thread to flog this issue is here....   May 12 2003, 04:32
- - 2Bdecided   QUOTE (zephirus @ May 11 2003 - 04:40 PM)I ha...   May 12 2003, 12:08
- - zephirus   QUOTE (tigre @ May 11 2003 - 01:59 PM)The add...   May 12 2003, 13:07
- - zephirus   QUOTE (mrosscook @ May 11 2003 - 07:32 PM)The...   May 12 2003, 13:14
- - KikeG   QUOTE (zephirus @ May 12 2003 - 01:07 PM)Iīm ...   May 12 2003, 13:33
- - tigre   QUOTE (zephirus @ May 12 2003 - 04:07 AM)......   May 12 2003, 14:16
- - zephirus   QUOTE (KikeG @ May 12 2003 - 04:33 AM)QUOTE (...   May 12 2003, 14:23
- - zephirus   QUOTE (tigre @ May 12 2003 - 05:16 AM)OK. Fin...   May 12 2003, 17:12
- - zephirus   QUOTE (2Bdecided @ May 12 2003 - 03:08 AM)QUO...   May 12 2003, 18:22
- - zephirus   While my original dubious claims obviously had a v...   May 12 2003, 20:54
- - _Shorty   I thought that's what over-sampling DACs were ...   May 12 2003, 21:42
- - zephirus   QUOTE (_Shorty @ May 12 2003 - 12:42 PM)I tho...   May 12 2003, 22:39
- - Doctor   An oversampling DAC still needs to lowpass the sig...   May 12 2003, 22:54
- - Doctor   The distortions you believe in are probably either...   May 12 2003, 22:57
- - KikeG   QUOTE (zephirus @ May 12 2003 - 08:54 PM)The ...   May 13 2003, 07:54
- - Canar   QUOTE (KikeG @ May 12 2003 - 04:33 AM)If you ...   May 13 2003, 08:10
- - tigre   To approximate the waveform between two samples S[...   May 13 2003, 11:15
- - 2Bdecided   zephirus, You were right about the 1kHz tone in th...   May 13 2003, 11:24
- - tigre   Nice link you provided, 2Bdecided. So it seems li...   May 13 2003, 11:42
- - Canar   QUOTE (2Bdecided @ May 13 2003 - 02:24 AM)Can...   May 14 2003, 07:35
- - zephirus   Doctor, KikeG: Thanks for your explanations! ...   May 14 2003, 12:23
- - SikkeK   I think your snippet has alot of frequence compone...   May 14 2003, 12:36
- - DonP   QUOTE (zephirus @ May 14 2003 - 06:23 AM)The ...   May 14 2003, 13:00
- - morelli   it's all too complex for me to understand. but...   May 14 2003, 13:06
- - zephirus   QUOTE (SikkeK @ May 14 2003 - 03:36 AM)I thin...   May 14 2003, 13:39
- - 2Bdecided   Someone do a search. I'm too lazy. But I'v...   May 14 2003, 13:40
- - KikeG   From zephirus: QUOTE A continuous signal of 21800 ...   May 14 2003, 14:04
- - DonP   For an example of this frequency vs time resolutio...   May 14 2003, 14:26
- - zephirus   QUOTE (DonP @ May 14 2003 - 04:00 AM)A pure s...   May 14 2003, 14:40
- - mrosscook   Zephirus, Harry Nyquist formulated his sampling t...   May 14 2003, 15:07
- - DonP   QUOTE (zephirus @ May 14 2003 - 08:40 AM)...   May 14 2003, 15:21
- - budgie   QUOTE (mrosscook @ May 14 2003 - 06:07 AM)I...   May 14 2003, 16:07
- - zephirus   QUOTE (2Bdecided @ May 14 2003 - 04:40 AM)Aft...   May 14 2003, 18:07
- - ye110man   QUOTE (DonP @ May 14 2003 - 06:21 AM)The Nyqu...   May 14 2003, 21:46
- - DonP   QUOTE (ye110man @ May 14 2003 - 03:46 PM)i th...   May 14 2003, 22:22
- - KikeG   QUOTE (budgie @ May 14 2003 - 04:07 PM)20 bit...   May 15 2003, 00:03
- - budgie   QUOTE (KikeG @ May 14 2003 - 03:03 PM)I don...   May 15 2003, 08:51
- - dillee1   Prove of Nyquist theory always use stationary sign...   May 17 2003, 10:44
- - DonP   QUOTE (dillee1 @ May 17 2003 - 04:44 AM)Prove...   May 17 2003, 13:11
- - zephirus   QUOTE (KikeG @ May 14 2003 - 05:04 AM)This sn...   May 17 2003, 17:37
- - zephirus   QUOTE (mrosscook @ May 14 2003 - 06:07 AM)Har...   May 19 2003, 15:05
- - KikeG   If I could have access to some 24/96 music in wav ...   May 19 2003, 15:48
- - budgie   QUOTE (zephirus @ May 19 2003 - 06:05 AM)A ma...   May 19 2003, 15:58
- - 2Bdecided   QUOTE (zephirus @ May 17 2003 - 04:37 PM)QUOT...   May 19 2003, 16:02
- - KikeG   QUOTE (2Bdecided @ May 19 2003 - 04:02 PM)If ...   May 19 2003, 16:39
- - Pio2001   QUOTE (KikeG @ May 19 2003 - 06:39 PM)I have ...   May 19 2003, 20:00
- - Pio2001   QUOTE (zephirus @ May 19 2003 - 05:05 PM)The ...   May 19 2003, 20:16
- - budgie   QUOTE (2Bdecided @ May 19 2003 - 07:02 AM)P.S...   May 20 2003, 11:12
- - 2Bdecided   QUOTE (budgie @ May 20 2003 - 10:12 AM)You ca...   May 20 2003, 17:34
- - 2Bdecided   QUOTE (KikeG @ May 19 2003 - 03:39 PM)I don...   May 20 2003, 17:35
- - zephirus   QUOTE (2Bdecided @ May 19 2003 - 07:02 AM)QUO...   May 20 2003, 17:44
- - zephirus   QUOTE (KikeG @ May 19 2003 - 06:48 AM)If I co...   May 20 2003, 21:48
- - mrosscook   @2Bdecided -- You mentioned in your post that you ...   May 21 2003, 00:04
- - budgie   2Bdecided: Nice post, but it proves nothing, sad ...   May 21 2003, 08:33
- - KikeG   David (2Bdecided): I find your report interesting...   May 21 2003, 10:18
- - 2Bdecided   budgie, I agree that this has no relevance in the...   May 21 2003, 10:24
- - KikeG   QUOTE (zephirus @ May 20 2003 - 09:48 PM)The ...   May 21 2003, 10:25
- - 2Bdecided   QUOTE (mrosscook @ May 20 2003 - 11:04 PM)@2B...   May 21 2003, 10:34
- - 2Bdecided   QUOTE (KikeG @ May 21 2003 - 09:18 AM)David (...   May 21 2003, 11:02
- - KikeG   QUOTE (2Bdecided @ May 21 2003 - 10:34 AM)As ...   May 21 2003, 11:13
- - KikeG   QUOTE (2Bdecided @ May 21 2003 - 11:02 AM)Whi...   May 21 2003, 11:20
- - 2Bdecided   QUOTE (KikeG @ May 21 2003 - 10:13 AM)Then, a...   May 21 2003, 11:30
- - mrosscook   2Bdecided, Do you recall whether the setup of Demo...   May 21 2003, 15:15
- - DickD   With 24-bit there's enough thermal noise in th...   May 21 2003, 19:45
- - DonP   QUOTE (2Bdecided @ May 21 2003 - 05:30 AM)Now...   May 21 2003, 20:36
- - Pio2001   QUOTE (KikeG @ May 21 2003 - 01:13 PM)Also, I...   May 21 2003, 22:07
- - budgie   QUOTE (2Bdecided @ May 21 2003 - 01:24 AM)Tot...   May 22 2003, 09:09
- - towolf   QUOTE (Pio2001 @ May 21 2003 - 11:07 PM)There...   May 24 2003, 00:43
- - KikeG   QUOTE (Pio2001 @ May 21 2003 - 10:07 PM)There...   May 24 2003, 12:54


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