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Topic: Does subtracting MP3 from WAV reveal artifacts? (Read 40303 times) previous topic - next topic
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Does subtracting MP3 from WAV reveal artifacts?

Reply #75
The very foundation of signal theory lies upon the presupposition that signals can be added and subtracted together.
Only in linear systems.

I look forward to your proof that the ear is a linear system, and hence that your subsequent analysis is relevant.


Hopefully you know this is nonsense. You don't really think the human ear is a linear device, do you?

Good. So the ear is non-linear, and the superposition principle doesn't hold. That might just give you a clue as to why your undergraduate mathematics don't say anything useful about the errors introduced by lossy coding (or anything else!) in as much as they relate to a human listening to the resulting audio.

If you have some use for mp3 other than "humans listening to audio", maybe you should have shared that at the start.

Cheers,
David.

Does subtracting MP3 from WAV reveal artifacts?

Reply #76
A very simple but clear example of why difference files are totally inadequate to evaluate psycho-acoustic compression.
An apparently serious attempt is being made by Soundexpert:
http://soundexpert.org/
It would be interesting to check if their results (codec ratings) are much different from those found by HA.

The last paper of the methodology "Difference level" dictates exactly how you can judge lossy quality by taking the difference signal and amplify it until it becomes audible. Higher the amplification, higher the quality. This is a direct application of how the difference signal can be used as a tool for quality assessment.
http://soundexpert.org/testing-methodology

Does subtracting MP3 from WAV reveal artifacts?

Reply #77
The last paper of the methodology "Difference level" dictates exactly how you can judge lossy quality by taking the difference signal and amplify it until it becomes audible. Higher the amplification, higher the quality. This is a direct application of how the difference signal can be used as a tool for quality assessment.
http://soundexpert.org/testing-methodology


Excerpts from the actual paper about difference level you have cited here:

Quote
Nevertheless, today we all are clever enough not to measure perceptual audio quality with any simple instrumental parameter or even a set of them.


Quote
Non-linear nature of human hearing and masking thresholds makes perception of gradually unmasked artifacts uneven.


Since the exploitation of masking is a fundamental aspect of modern psy-models, I wonder how you still not understand the ineptness of your proposal.

Does subtracting MP3 from WAV reveal artifacts?

Reply #78
Twice now (two different people) a direct challenge to the fundamentals of your argument have been posted, JapanAudio, pointing out that hearing is a non-linear system.

How about you refrain from picking minutia to respond to for a while and answer this challenge to the foundation of your argument.


EDIT: punctuation.
Creature of habit.

Does subtracting MP3 from WAV reveal artifacts?

Reply #79
A very simple but clear example of why difference files are totally inadequate to evaluate psycho-acoustic compression.
An apparently serious attempt is being made by Soundexpert:
http://soundexpert.org/
It would be interesting to check if their results (codec ratings) are much different from those found by HA.


I read the paper and  I think I see its point. The basic weakness of the paper is that it is somewhat based on a study of correlation.  There's an old saying "Correlation is not the same as causality". In this case we may find that  if we examine contemporary, competitive encoders, we may find that the ones with smaller errors are also the ones that get high scores in good subjective testing.  What should be inferred from this?

Should we infer that the encoder with the lower amount of error is always the one that will sound best? Should we infer that given two coders that have similar amounts of error, they can reasonably be expected to sound equally good? Should we infer that the coder with higher error is always one that sounds worse? Should we infer that since all coders produce finite error signals, their coded files all must be scenically distinguishable from the source .wav file? 

We've seen some or all of these assertions made in recent times.

My innate skepticism says that the evidence presented is not sufficient to promote any of the above assertions into global truths, even though they may be true for a reasonable-looking collection of good modern coders. We can think of enough counter-examples. We can think of enough possible reasons why counter examples may always exist.

Does subtracting MP3 from WAV reveal artifacts?

Reply #80
The last paper of the methodology "Difference level" dictates exactly how you can judge lossy quality by taking the difference signal and amplify it until it becomes audible. Higher the amplification, higher the quality. This is a direct application of how the difference signal can be used as a tool for quality assessment.
http://soundexpert.org/testing-methodology


Excerpts from the actual paper about difference level you have cited here:

Quote
Nevertheless, today we all are clever enough not to measure perceptual audio quality with any simple instrumental parameter or even a set of them.


Quote
Non-linear nature of human hearing and masking thresholds makes perception of gradually unmasked artifacts uneven.


Since the exploitation of masking is a fundamental aspect of modern psy-models, I wonder how you still not understand the ineptness of your proposal.

I think taking quotes out of context isn't really fair, anyway i am well aware that the author states that this method of "quality assessment" has to be further researched. Thankfully, my point doesn't have anything to do with quality assessment; I was just trying to emphasize on the title of the thread, precisely, that difference signals are the artifacts. Whether they be audible or not, good or bad sounding, and comparing them to judge quality is none of my business. Note that the author never counters the fact that difference signals reveal artifacts (however beautiful or inaudible they might be).

Does subtracting MP3 from WAV reveal artifacts?

Reply #81
I was just trying to emphasize on the title of the thread, precisely, that difference signals are the artifacts.


The point of lossy encoding is to save space by exploiting properties of the human auditory system, which allow us to hide the actual data reduction. Inaudible artifacts are a necessity in this endeavour. So what is the point of "emphasizing" anything about artifacts regardless of their audibility?

Don't make the backpedaling look even more pathetic than the failed argument.

Does subtracting MP3 from WAV reveal artifacts?

Reply #82
A very simple but clear example of why difference files are totally inadequate to evaluate psycho-acoustic compression.
An apparently serious attempt is being made by Soundexpert:
http://soundexpert.org/
It would be interesting to check if their results (codec ratings) are much different from those found by HA.


I read the paper and  I think I see its point. The basic weakness of the paper is that it is somewhat based on a study of correlation.  There's an old saying "Correlation is not the same as causality". In this case we may find that  if we examine contemporary, competitive encoders, we may find that the ones with smaller errors are also the ones that get high scores in good subjective testing.  What should be inferred from this?

Should we infer that the encoder with the lower amount of error is always the one that will sound best? Should we infer that given two coders that have similar amounts of error, they can reasonably be expected to sound equally good? Should we infer that the coder with higher error is always one that sounds worse? Should we infer that since all coders produce finite error signals, their coded files all must be scenically distinguishable from the source .wav file? 

We've seen some or all of these assertions made in recent times.

My innate skepticism says that the evidence presented is not sufficient to promote any of the above assertions into global truths, even though they may be true for a reasonable-looking collection of good modern coders. We can think of enough counter-examples. We can think of enough possible reasons why counter examples may always exist.

You're absolutely right. From the beginning i was never trying to talk about quality issues, because its a touchy subject that isn't fully regulated, and probably can't be.

Does subtracting MP3 from WAV reveal artifacts?

Reply #83
Thankfully, my point doesn't have anything to do with quality assessment; I was just trying to emphasize on the title of the thread, precisely, that difference signals are the artifacts.
So all you're trying to say is that if
c = a + b
then
b = c - a
?

 

Cheers,
David.



 

Does subtracting MP3 from WAV reveal artifacts?

Reply #84
An apparently serious attempt is being made by Soundexpert:
http://soundexpert.org/
It would be interesting to check if their results (codec ratings) are much different from those found by HA.

This has been discussed on the forum on more than one occasion.  While Serge may take his method seriously, HA does not.

Does subtracting MP3 from WAV reveal artifacts?

Reply #85
Ok, I believe now we can really close this thread.

To JapanAudio:

You've been told already the correct use of the term "masking".
Now I would like to add the correct use of the term "artifact".

An artifact is a difference, but not all differences are artifacts. Concretely, an artifact is an unwanted sound which is heard in the context of the decoded signal, that was not present in the original signal.

This is why there has been so much emphasis in this thread in disprooving your claim.  The difference signal may be helpful for codec developers to detect a problem in their codecs, but never as a way to determine its quality (except if the codec in question is a lossless codec, in which case we know the difference file has to be digital silence).


Edit: Also, there's another problem with the difference signal if we are going to take it as "what can be heard". The difference signal can have both, things that the lossy encoder added as well as things that it missed (the hush in your square wave example is an example of added sound, while the lowpass filter would be an example of removed sound).
This, coupled with masking makes the difference signal a lot less useful to know what artifacts can be heard on the decoded signal.

Does subtracting MP3 from WAV reveal artifacts?

Reply #86
I was waiting for you to chime in, JAZ.  Thanks, and thanks to everyone else.

Ok, I believe now we can really close this thread.

Agreed.