From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: using green marker on CDs
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Mon, 28 Sep 1992 08:03:32 GMT

In article <1992Sep24.134507.13987@odin.diku.dk> bengaard@diku.dk (Jens Martin Bengaard) writes:
>fnord@panix.com (Cliff Heller) writes:
>
>>In <1992Sep21.105438.22038@odin.diku.dk> bengaard@diku.dk (Jens Martin Bengaard) writes:
>
>>>The Danish high fidelity magazine, "High Fidelity" (surprise!), made a test
>>>of various methods to enhance the quality of CD output. They actually 
>>>found, on a listening test, that the quality of the sound improved with
>>>the green marker. 
>
>>Now you've done it.  
>
>>I guess this is clear evidence of the fall of the Danes in science.
>>What kind of "listening" test.  Howzabout some details.  If it wasn't
>>double bline, I'm not even interested.  "quality of the sound"  ?? How do
>>you measure that?
>
>There were no details in the magazine, they just sat down and listened.
>This is the _only_ way to measure quality of sound.

Wrong, this never works, because it's inherently unreliable and unrepeatable.
Listening tests don't measure anything but real-time subjective preferences.
When a person hears A and B, the room may have changed prevailing air
currents or something that made the listener perceive the listening
experience just a tad more/less enjoyable, and this gets erroneously
translated into a judgement call about the so-called quality of the sound.
Or maybe it's N seconds later since he started digesting his last meal and
the pressure on his stomach and intestines has just produced a subtle shift
in the ever-so-slight discomfort he is attempting to avoid while he is
concentrating on the music, yet is unduly influenced anyway.  Or even something
like the association subconsciously the music specifically has on the listener,
who without knowing it has been internally "setup" to have a preference for
hearing something either sooner or later, or the first time, or a second time
depending on what specific psychological factors are at work.  For example,
if someone often evaluates amplifiers with some favorite piece, it is
possible they have a bias towards favoring the second time they hear the
piece or the first time, or whatever.  There are just too many factors that
make such listening tests potentially bogus, even if there are no hidden
agendas.

>
>>>The alleged reason was that the green marker damped some laser reflections 
>>>from the edge of the CD, resulting in lower error rates. 
>
>>So what?  Error rates to not affect sound quality.  This whole green marker
>>thing comes from people who refuse to understand the basics of digital
>>audio.
>
>Error rates _do_ affect sound quality. You are in effect saying that it 
>doesn't matter what's recorded on the CD.

This indicates a total misunderstanding of how audio is digitally stored.
The short version is that the sound is created from a collection of bits
delivered to the part that matters, which we won't get into.  The information
itself has certain safeguards and redundancies so it can realize when certain
and most likely if occuring (which isn't actually likely at all) errors do
occur, and take corrective measures to eradicate the error.  There are sound
mathematical principles at work here, and this has nothing whatsoever to do
with the audio "level" of the situation.  What is happening here is simply
that it's possible for a digital system with redundancy to actually be
aware when its data is arriving corrupted, and that further corrective action
taking advantage of the redundancy can with reliability reproduce the exact
data intended despite the detected error.  Moreover, the likelihood of
an actual error going undetected is extremely small, and moreover highly
likely to creata a gross aberation of the sound easily recognizable and
beyond our discussion were that highly unlikely event to occur.

When the system detects an error that is beyond its capability to eradicate
by using the redundancy, it means that too much data was lost.  This also
occurs infrequently, and is so little a concern that most units no longer
even bother with an indicator of the problem, since it so rarely happens.

In any case, were this to happen, it would create a clearly audible aberation
of the signal that goes beyond our discussion.  So, for all the usual cases,
the data either arrives totally intact, or with enough of the redundant
error-detection and correction information intact to supply the proper
digital information in precisely the same form had there been no errors.
Thus, the same data is applied downstream in the rest of the player, and
therefore is no possibility of change. 

It was stated that devices that read the bits could tell if the pens were 
affecting anything.  This is absolutely true; no improvements will be found
because none are necessary.  The vast majority of reading of CD's is
really error-free, and the dominant majority of the tiny niche left falls into
the category of data corrected by the safeguards and redundancies in the 
digital recording format itself.  Virtually always the data provided to the
rest of the player and is totally identical in every way whether error free
or error-detected and corrected.  If there was anything to the pens, this
is the only place it can be detected.  Apparently it was shown that there
was nothing to detect, which is also consistent with the relevant laws of
physics that apply to how the thing works.

>
>>>They also found (perhaps not very surprising) that it is a good idea to 
>>>clean your CD when it's dirty.
>
>>This will affect tracking usually, but not sound quality.  It is digital.
>>Bits are bits.  I'll bet that they claim that a dirty cd sounds fuzzier or
>>more muddled, and that cleaning it gives a brighter crisper sound ("I can
>>see my reflection!")   This is not based in reality.
>
>The statement is at least consistent with the previous. Try painting the
>reflecting surface black.
>
>bengaard@diku.dk

cjl