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[chrisj]

Yup! Thanks, Nika- much appreciated
Basically, I've just done some experimentation with waveforms with illegal content, removed the content by UPsampling a miniscule amount, and ABX double-blind tested them to see if the difference is audible.
By the time you're reducing the RMS loudness of the illegal content 4db or so, it does become an audible difference. Less than that, _I_ couldn't hear.
That also establishes that the potential exceeding of full scale is a lot more than half a db or so. Instruments like tambourines and shakers and hi-hats are capable of producing worst-case scenarios for this situation, and treble boosting and amplification complete the picture.

[Nika Aldrich]

chrisj wrote on Mon, 23 August 2004 16:54

Basically, I've just done some experimentation with waveforms with illegal content, removed the content by UPsampling a miniscule amount, and ABX double-blind tested them to see if the difference is audible. By the time you're reducing the RMS loudness of the illegal content 4db or so, it does become an audible difference. Less than that, _I_ couldn't hear.

I didn't get that.

Quote:

That also establishes that the potential exceeding of full scale is a lot more than half a db or so.

Yes, of course.  This is a probability issue, though.  What is the likelihood that a given waveform exceeds 0dBFS?  What is the likelihood that it exceeds by a given amount?  The probability curve is heavily tilted towards very low excursion over 0dBFS, with a very drastic curve such that after around .5dB or so over the chance on a given non-test signal is very low.  On specific test signals you can easiliy exceed FS by 6dB or more, depending on the reconstruction filter in the D/A conversion process.  I don't think anyone said you can't exceed FS by more than half a dB, just that the likelihood of doing so with samples at FS is very, very small.  

You might check out the paper I wrote at tllabs.com on the subject.

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Instruments like tambourines and shakers and hi-hats are capable of producing worst-case scenarios for this situation, and treble boosting and amplification complete the picture.

Yes.  The worst case is definitely test signals, but in real world instruments the problem is most noticable with high frequency transients that come from percussion instruments.

Nika.

[chrisj]

Oh, I quite agree. Anyone whose mastered output is not hotter than -4db or so (and that's REALLY HOT) won't ever have a problem with this at all.
That said- if you make stuff hot, bright, and if you use peak limiting or simple digital clipping, this becomes totally relevant.
And how many modern-day major label releases are hotter than -4db RMS more than 50% of the time? Maybe this shouldn't be a common problem, maybe it should be very hypothetical and nobody should ever do this, but seeing as they DO, it might be nice to stop that one kind of 'bad distortion'. The highs sound nicer when you do, even though it's still distorted as hell.
I know that I'm going to be remembering this trick for if I ever get something mix-mastered and damaged that heavily (or bright and clipped). Being able to excise all the illegal content and neatly leave all the legitimate overbright overdistorted stuff is nice and since it's UPsampling, if there's nothing illegal, you lose nothing (also established by ABX). Real win/win situation.
It would be great if I could run that routine 'converting' from 44.1 to 44.1. There seems no theoretical reason why you couldn't- and I think you'd get the same effect. That would be very interesting. I need to look at the first instants of the files to see if it's really having an effect when the interpolation could be essentially nothing...

[Erik]

Nika is talking -0.5 dBfs peak and Chris counters with -4.0dBfs RMS.  Back to Nika.

Nika, I think you need to clarify your whitepaper into a simple bullet point.  A 'mastering peak levels for dummies' if you will.  

What can the baboons do to avoid this problem, and, if they're truly neurotic and/or want to spend some money, how can they be sure they avoid this problem on their precious mixes?

Thanks,
--Erik

[chrisj]

No, I countered with nearly 12db RMS measured difference on a particularly heinous realworld clip-fest sample. Not a theory, a real file. Nika is of course also entirely correct here, he is just referring to something else. We're not contradicting each other at all. I'm not really sure why this is so hard to understand. Oh, right, this is the Erik show. I remember this from all those Bomb Factory threads. Never mind. (Is everybody enjoying the spectacle, or is it getting boring?)
4db is simply the point where it becomes obvious to the ear. Less than that, you won't really hear the distortion but it will still be there- and I am talking _RMS_ level of the change between source file and upsampled file. Nika refers to the amount that a peak is likely to exceed full scale on NORMAL music. Normal music doesn't have this problem, except imperceptibly, and I would defy anyone to ABX something that clipped on reconstruction but was say average 6db from full scale. You won't hear it.

That's the best answer. Don't smash levels, and don't intentionally digitally clip bright stuff to get level out of it. How's that for a bullet point?

For those who feel they must do all these things because the next guy is doing them- and that's too many people and might even include you, Erik, so far as I know- there is something that you can do to clean up the highs. (It works better than simply reducing overall volume by .01db to stop the pressing plant from reporting clipped samples)

[Nika Aldrich]

chrisj wrote on Mon, 23 August 2004 17:21

Oh, I quite agree. Anyone whose mastered output is not hotter than -4db or so (and that's REALLY HOT) won't ever have a problem with this at all. That said- if you make stuff hot, bright, and if you use peak limiting or simple digital clipping, this becomes totally relevant.

OK, Just to clarify, the maximum amount of excursion above FS that you can achieve is dependant upon the reconstruction filter in use and its coefficients.  Dan Lavry and I worked out a simple scenario that gave around 8dB of potential excess signal above FS.  TC Electronic came up with 15dBFS by using a different reconstruction filter design.  The important thing is to try to simulate the reconstruction filter in use in your D/As when trying to calculate how much of a problem you will have.  This is what we tried to do with the Trillium Lane plugin.

Quote:

And how many modern-day major label releases are hotter than -4db RMS more than 50% of the time? Maybe this shouldn't be a common problem, maybe it should be very hypothetical and nobody should ever do this, but seeing as they DO, it might be nice to stop that one kind of 'bad distortion'.

I'm not really comfortable with the 4dB line you are drawing in the sand.  I guess a rule of thumb is helpful for "the baboons," but I think it is more relavent to tell people that just because their samples don't exceed full scale does not mean that the signal won't when it is reconstructed rather than giving them a set amount not to exceed, especially when it is really more fluid than that.  I guess this is where I really wish people were more engineers than baboons about this stuff - that they understood what they were doing and why rather than just following overly-simple rules of thumb.

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The highs sound nicer when you do, even though it's still distorted as hell. I know that I'm going to be remembering this trick for if I ever get something mix-mastered and damaged that heavily (or bright and clipped). Being able to excise all the illegal content and neatly leave all the legitimate overbright overdistorted stuff is nice and since it's UPsampling, if there's nothing illegal, you lose nothing (also established by ABX). Real win/win situation.

I don't follow your solution to the problem, here.

Nika

[Nika Aldrich]

Erik wrote on Mon, 23 August 2004 17:49

Nika is talking -0.5 dBfs peak and Chris counters with -4.0dBfs RMS.  Back to Nika.

Do I hear -6?  How about -8.  Do I have any takers?  Will anyone go -12?  I'm taking -12.  Do I have a -14? ...

Quote:

Nika, I think you need to clarify your whitepaper into a simple bullet point.  A 'mastering peak levels for dummies' if you will.

Just because your samples peak at less than full scale does not mean that the reconstructed signal will stay under full scale, and since your converters can't pass anything above full scale, upon reconstruction your signal (which "looks" safe to you) may still clip when played back.

Quote:

What can the baboons do to avoid this problem, and, if they're truly neurotic and/or want to spend some money, how can they be sure they avoid this problem on their precious mixes?

They can do one of two things:

1.  Simply turn down their master outputs to allow a little bit of headroom so that their samples are not just below full scale, but below full scale by a little bit extra so that if the signal DOES exceed the level of the individual samples it has some room to reconstruct.

2.  Buy a peak level meter that is more sophisticated than one that just reads the level of the samples.  Buy one that tries to reconstruct the signal the way the converters do before it tells you your peaks.  Then, if your signal exceeds FS even though your samples don't the meter will tell you that.  This type of peak meter is called an "oversampled peak meter" and ones are available from TC Electronic in the System 6000 amongst others.  I personally endorse (and helped design) the plugin called "Mastermeter" sold by Trillium Lane.  It does as much as 8x oversampling for Protools users, at a very reasonable price.

Cool?

Nika.

[chrisj]

Hmm-
Nika's meter should be an ideal preventative solution to the problem. 'Just don't distort', easy as that. I'll endorse that course of action, too.
Nika, I was pointing out that if you want to fix a file that's already damaged in this way, without attenuating anything in the normal range of hearing, you can cheat the metadata of the file so it claims to be 44.099K and then UPsample back to 44.1K using a really high quality SRC such as the Audacity program's sinc interpolation at a high quality level (actually, their low quality level is perhaps even subjectively better).
You shouldn't lose information in a high quality upsample- but you do lose all the illegal data that way. Nothing else is touched. In the absence of illegal data, this operation is imperceptible (from my ABX tests- anyone may try and better my results). If there's illegal data, the operation removes it, sometimes in an extremely obvious way. (from my ABX tests, 16/16)
Hence, a 'fix' for the problem that doesn't require turning the gain down on the rest of the audio. I thought that was relevant, because if someone making such a file was willing to turn the gain down, they wouldn't be having that problem in the first place, and so they can be assumed unwilling to not smash the levels and brightness. I think it's useful to be able to produce 'good' smash rather than just ugly smash. After all, it's fairly hard to hear clipped very high frequency transients unless they're doing something like aliasing or clipping the DAC. Excise the DAC-clipping stuff and you get simple digital clipping, which won't sound quite as nasty, just very loud.
Aside: isn't it fascinating how technology continues to provide uglier and uglier ways of clipping? First tube overdrive. Then transistors. Then TL071s driven beyond their power supply rails. And now, Gibb effect digital DAC-smashing. I guess it'll be DSD next. How much output can you get from hyperclipping SACD recorders?

[Nika Aldrich]

chrisj wrote on Mon, 23 August 2004 20:02

Nika, I was pointing out that if you want to fix a file that's already damaged in this way,

Hmm.  I'm a little leary, for this file isn't really "damaged" per se.  It is just too hot - it represents data that exceeds the bounds of the system (full scale).  The only solution is to reduce the signal level (by reducing the sample levels) until the signal is once again in the "legal" range.

Quote:

without attenuating anything in the normal range of hearing, you can cheat the metadata of the file so it claims to be 44.099K and then UPsample back to 44.1K using a really high quality SRC such as the Audacity program's sinc interpolation at a high quality level (actually, their low quality level is perhaps even subjectively better). You shouldn't lose information in a high quality upsample- but you do lose all the illegal data that way. Nothing else is touched. In the absence of illegal data, this operation is imperceptible (from my ABX tests- anyone may try and better my results). If there's illegal data, the operation removes it, sometimes in an extremely obvious way. (from my ABX tests, 16/16)

Ahh, I am suddenly understanding what you were going at.  Indeed a high quality SRC should be able to work around inter-sample overs, but only if it can deal with information in excess of full scale - I'm sure there are some that do so but I wouldn't count on a random, arbitrary SRC being able to do so.  But just because it can do so does not mean that it will adjust the signal level accordingly so that the file is once again legit.  It will more likely simply expose that you clipped (despite your meters) and remain a clipped file.  I wouldn't really call this a "solution."  I think it is better to simply drop the level accordingly so that you have enough headroom to play with upon reconstruction.  That seems to me to be the only solution to this problem.

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Hence, a 'fix' for the problem that doesn't require turning the gain down on the rest of the audio. I thought that was relevant, because if someone making such a file was willing to turn the gain down, they wouldn't be having that problem in the first place,

Hmm.  No, I think that people look at their digital non-oversampled peak meters and when they show no overs they assume that the file is clean, legit, and will be reproduced accurately.  I'll be that if people had a reconstruction, oversampling meter and saw that their signal was distorting and understood that, they would turn the signal down appropriately.  I don't think the problem here is people's stubbornness as much as a lack of the proper tools for the job and a lack of understanding.

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After all, it's fairly hard to hear clipped very high frequency transients unless they're doing something like aliasing or clipping the DAC.

Which is what happens...

Quote:

Aside: isn't it fascinating how technology continues to provide uglier and uglier ways of clipping? First tube overdrive. Then transistors. Then TL071s driven beyond their power supply rails. And now, Gibb effect digital DAC-smashing. I guess it'll be DSD next. How much output can you get from hyperclipping SACD recorders?

Actually, that's one of the cool things about SACD - you can't clip it like that.  The more you compress the lower you have to turn down your signal or you get modulator overload, which the SACD format doesn't accept.  So people that try to compress their mixes find they have to lower the level consequentially, which kind of defeats the purpose of the compression!  It is for this reason specifically that a lot of SACD mixes (masters) have much less compression and sound so much better - the mastering engineers intentionally give them more dynamic range because reducing the dynamic range is self-defeating.

Having said this, I don't want to ignore that DSD is essentially always in a state of clipping, but this aside, what I said above is pretty significant.

Nika.

[Geetar]

Erik wrote on Mon, 23 August 2004 12:49

What can the baboons do to avoid this problem, and, if they're truly neurotic and/or want to spend some money, how can they be sure they avoid this problem on their precious mixes? --Erik

I doubt that I shall ever tire of your contributions, Erik. To paraphrase Sir John Gielgud in "Arthur":

"Usually one must go to a bowling alley to meet a man of your stature."

You may know quite a bit, but I doubt anyone has ever so successfully undermined their own standing in such spectacular acts of internet-based social hara-kiri; you've become quite the Zappa-esque "Posting Fool."

Long may you run.

[dcollins]

Erik wrote on Mon, 23 August 2004 09:49

What can the baboons do to avoid this problem, and, if they're truly neurotic and/or want to spend some money, how can they be sure they avoid this problem on their precious mixes? Thanks, --Erik

I think baboons everywhere will have to drop the level to "resamplers" if you don't want more than you started with.  

But how much?

You can create weird test signals, but for "music" -0.5dB should be plenty.

Won't -3.01dB be enough for full-scale white noise? Which could include todays "top" mastering jobs...

The real problem is post-conversion headroom in "your" D/A which you can't predict.  Except in a device where all you have is 2AA batteries.

Is "Gibbs effect" really from bandlimiting?

DC
 

[Erik]

This whole thread goes back to an argument on rec.audio.pro between Chris and Arny Krueger 2+ years ago.  

If you want more background, surf there; if you want more background on Chris, hit his website.  If the first page doesn't put it all in perspective, nothing will.

--Erik

[Nika Aldrich]

dcollins wrote on Tue, 24 August 2004 07:58

Won't -3.01dB be enough for full-scale white noise?

No.  I've gotten signals to exceed full scale by more than 6dB (8 or 9, I believe) and TC Electronic claimed 15dB.  It's rather easy to calculate once you come up with your coefficients for your filter.  The higher the summed absolute value of the coefficients the higher you can exceed full scale.

Quote:

The real problem is post-conversion headroom in "your" D/A which you can't predict.  Except in a device where all you have is 2AA batteries.

I don't think you'll find an off-the-shelf D/A conversion chip by Sony, AKM, Burr Brown, Crystal, or Analog Devices (did I miss one?) that has any headroom in the D/A digital reconstruction filter above FS.  I also don't think you'll find a custom designed converter that does this.  In other words, I think nothing will give you the headroom, including the device with 2 AA batteries.  I'm willing to be shown wrong, here.

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Is "Gibbs effect" really from bandlimiting?

No, not necessarily.

Nika.

[magicchord]

Erik wrote on Tue, 24 August 2004 00:36

...if you want more background on Chris, hit his website.  If the first page doesn't put it all in perspective, nothing will...

What are you implying?

[malice]

magicchord wrote on Tue, 24 August 2004 21:00

Erik wrote on Tue, 24 August 2004 00:36 ...if you want more background on Chris, hit his website.  If the first page doesn't put it all in perspective, nothing will... What are you implying?

YOu do know Erik don't you ?

malice