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PostPosted: Wed Dec 28, 2005 6:26 pm 
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Nika Aldrich wrote:
The problem is realized in that while going through these digital gyrations and utilizing digital tools to amplify the signal as much as possible, both during mixing and during mastering, the ‘peak value’ of the sample points is closely watched to ensure that it does not get to full scale. Since, the peak meters in said DAW and digital mixing systems are inaccurate, and do not actually indicate the peak values of the resulting waveform, the result is that while the samples themselves do not exceed full scale, and are carefully monitored to insure this, the resulting waveforms represented by the samples may exceed full scale throughout any standard CD!

While the digital mixing system is not clipping the music or distorting the music, the digital to analog converters that have the task of recreating the audio through digital reconstruction filters are clipping repeatedly throughout most CDs on the market. The result is that most CDs and DVDs end up distorting with regularity when they are asked to reconstruct and play back audio that appears to be completely ‘legal’ because not a single sample actually clipped.

In a recent paper [Nielsen 2003], seven consumer CD players were subjected to tests designed to analyze their ability to reproduce and reconstruct signal levels above full scale (0dBFS). All of the players experienced difficultly dealing with signal levels this high, further showing that, while all of the samples can be legal, the level can still be hotter than is legal the result being that a CD player can be unable to reproduce the audio accurately.

It is nearly certain that this constant barrage of distortion that we, the consumers, are hearing on compressed and mastered CDs contributes to the ‘digital harshness’ still reported by the more sensitive audiophiles in the music industry. According to industry insiders, not a single off-the-shelf digital to analog converter chip made today can accurately pass a signal wherein the samples are under full scale but the waveform that they represent exceeds full scale. Only a few high end converters in the professional market can do this. This means that the preponderance of consumer (and professional audio) playback equipment is not designed to deal with these ‘hotter than full scale’ signal levels.


This makes sense, particularly if most everything in the file in question is compressed to near-zero. But if that is not the case, I tend to agree with Luke on this one.

I've taken the Henry Cow tracks, converted them to 32-bit floating point, took the gain all the way up to -0.1 dB peak on each track, then saved as 16-bit files, using POW-r 3 dither. I figured I'd put the number crunching and dithering through its paces.

How do they sound? They sound just fine to me; better than than the thin-sounding (and, yes, slightly compressed) remastered CD. I haven't noticed any artifacts. (That doesn't mean they are not there; I just have yet to notice anything amiss!) As I said, the levels on this CD are ridiculously low. In fact, using the "bit usage" meter in Peak on the original 16-bit rips shows that most tracks only utilize 14 or 15 bits to begin with.

If you guys are interested in a blind listening test, I could easily make WAV samples of these files in their various stages. I can also make samples using all three different POW-r settings (I want to investigate this further).

Anyone up for a listening test?

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PostPosted: Thu Dec 29, 2005 3:06 am 
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It's not *just* 'crunched' CDs that will have this problem though. Heavily compressed CDs will be the ones most likely to show the *extreme* version of digital overs.

Quote:
Studies have shown that waveforms can exceed full scale (considering the reconstruction filters on most digital to analog converters) by more than 6dB. This means that the peak amplitude of the actual waveform might be more than twice as high in amplitude as the highest sample value. This is only likely to happen when music is heavily compressed however, and most music will practically require less than 6dB of headroom above the highest sample value to ensure accurate reproduction.



But plenty of CDs are not heavily compressed, and so won't need 6 dB of headroom, yet are still subject to 'overs'. Hence Bob Katz et al's recommendation of -3 dB peak. Surely there are still lots of non-squashed CDs that yet have multiple peaks greater than -3 dB. The audible effect if any will also be player-dependent. What may be properly reconstructed on one player may not on another.

From the Nielsen and Lund paper:

5.2 Mastering against -3dBFS

Quote:
Discussing deteriorating sound quality with mastering engineers, we discovered that some of them had
started using a conservative level approach several years ago. Because of experiences from the real world,
their ears had told them to generally keep peak levels below -3dBFS. Bad experiences with on-air signals
or domestic reproduction equipment may be the reason why.
Before we get more precise level monitoring tools or new level guidelines for what is allowed before a
master tape is rejected, this conservative approach certainly seems appropriate in order to avoid the kind
of consumer equipment distortion we have described in this paper.

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Last edited by krabapple on Thu Dec 29, 2005 3:10 am, edited 3 times in total.

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PostPosted: Thu Dec 29, 2005 3:07 am 
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Btw, I'm up for a listening test. Flac 'em up.

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PostPosted: Thu Dec 29, 2005 5:19 am 
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Good. I'll get the samples ready over the next day or two.

I'll do two sets: one taken up to - 0.1 dB and one at - 3.0 dB.

It'll be interesting to see what the results will be like.

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