Walter Knapp:
> This also means that recopying to MD is not something to be doing much.
Yes, that is the theory.
> It manages to hold on through a number of copies even then.
Yes. Depends on the material, though. Noise has high entropy, so it
doesn't compress well -- thus noisy recordings fall apart faster.
I ought to defer to Walt's practical experience with MD and these types of
recordings; my expertise is primarily as a former mastering engineer working
on music recordings from any number of sources.
> If you are one of the soundmangler's,
That would be me!
> you are probably on your own, and are probably not that interested in true
> reproduction either.
Just like I'm particular about how the guitar amp distorts, I'm particular
about what distortions I like in sample manipulation. This gets outside of
nature recording, of course...
> As one of the long time users I get drug into this far too often. Mostly
> because there are a whole lot of wrong ideas about it and I hate to see
> them passed around without at least a attempt at setting things straight.
Resist, Walt, resist. ;) I now let people make claims left and right about
differences in data-identical playback from CD without raising my voice in
protest, and I'm a happier guy for it. The world is filled with
unscientific ninnies...
Cheers,
-- Marvin Humphrey
CD design website - http://marvin.mrtoads.com
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>From Tue Mar 8 18:22:31 2005
Message: 7
Date: Wed, 10 Jul 2002 21:15:41 -0700
From: Marvin Humphrey <>
Subject: Re: ATRAC compression
KACastelein and DJLauten:
> While it might be true that ATRAC is not a problem in "our" field,
> is it really not a problem in any other field?
It is my understanding that it perceptually encoded material presents a
problem for broadcasters who perform perceptual encoding somewhere in the
broadcast chain, and that the broadcasters (the ones who care about quality)
complain. While one stage of perceptual encoding generally passes
unnoticed, especially something good like ATRAC or the AAC in MP4, cascading
codecs is a big no-no. That's all over the literature, it's not some crazy
theory.
The following passage is from Watkinson's Art of Digital Audio, Chapter 5.6.
It helps if you know some basic information theory, but here goes...
[begin Watkinson quote]
There are, of course, limits to all technologies. Eventually artifacts will
be heard as the amount of compression is increased which no amount of
detailed modeling will remove. The ear is only able to perceive a certain
proportion of the information in a given sound. This could be called the
perceptual entropy, and all additional sound is redundant or irrelevant.
Compression works by removing the redundancy, and clearly an ideal system
would remove all of it, leaving only the entropy. Once this has been done,
the masking capacity of the ear has been reached, and NMR [noise-masking
ratio] has reached zero over the whole band. Assuming an ideal masking
model, further reduction of the data rate must cause the level of distortion
products to rise above the masking level qually at all frequencies rendering
it audible.
The result is that the perceived quality of a codec suddenly falls at a
critical bit rate. Figure 5.7 shows this effect which is variously known as
a crash knee, graceless degradation or the cliff-edge effect....
P |
E |
R |
C |--------------------------- <-- Crash Knee
E | \
I | \
V | \
E | \
D | \
| \
Q | \
U | \
A | \
L | \
I | \
T | \
Y |_____________________________________________
COMPRESSSION FACTOR
Figure 5.7 It is a characteristic of compression systems that failure is
sudden.
[end Watkinson quote]
For nature recordings where there is likely to be minimal processing prior
to playback, you'll probably stay left of the crash knee, and everything
will be fine. For other recordists dealing with situations where additional
downstream processing is anticipated, perceptually encoded material may
present problems.
I agree with Walt that ATRAC gets more grief than it should. I have spent
far more time than I should have arguing with people over similar
controversies. :) I especially agree that ATRAC should not stop people
from getting started doing quality recording. Nevertheless, discussion of
codecs is not complete without a distinction being made between a
transparent single pass, and non-transparent multiple passes which are often
common practice in certain real-world situations, at least in other fields.
-- Marvin Humphrey
CD design website - http://marvin.mrtoads.com
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