I'd heard about the green marker tweak for CD's a couple years ago and initially dismissed it as someones overactive imagination. I even saw one company marketing a green pen for something like $40, and thought man, audiophiles, we're just sitting ducks for all kinds of quacks. Then one evening while listening to a CD that seemed a bit muffled sounding and being a bit frustrated, I thought what did I have to lose by trying it? I had a permanent black marker handy so I used that, putting it around the outer edge only. Much to my surprise, the sound indeed became more open and transparent, the bass was deeper and tighter, and there were more details coming through. I was nearly in shock as I sat there listening to the difference it made. The CD I was considering trashing was now sounding pretty danged good. Thinking it was a fluke, I found another CD that was a bit muffled from my collection and tried it again, and this one cleared up remarkably well too, with improvements to the bass, and more details being noticed on this one as well. The difference that I was hearing was not at all subtle... no, there was a dramatic improvement in the clarity, the openness, the detail, and in the tightness of the bass... as much of a difference as going from some cheapie low-end IC's and changing over to reference quality IC's...
the improvement was so significant that I tried it on another 3 or 4 CD's in my enthusiasm for what I'd found, and all with the same results to varying degrees. Well, I had been listening for quite a long session that night and realized my ears were already pretty fatigued, so I thought I'd give them a rest before I went too far with the black marker only to find something wasn't right with the sound when my ears were fresh.
On the way home from work the next day, I stopped and got a green highlighter so I could continue this experiment that evening when I got home. When I began my listening for the evening, I put a few of the CD's on that I'd treated with the Black permanent marker the night before, and as I had suspected, my ears had been tired... yes, they were clearer and more open sounding and had tighter bass, and more detail... but to my dismay they also had a pretty obvious metallic sounding midrange glare that made my teeth hurt, kinda like chewing on aluminum foil, something like fingernails scraping across a blackboard in it's effect. Well, the most important thing at that point was to find some kind of solvent to remove the permanent marker... I found that "goof-off" did the job well, and sure enough after I'd removed the black marker the CD's no longer had the metallic sound when they played. It was quite a relief to find I had not ruined all those CD's at that point.
So, with things once again restored, I then tried the green highlighter on one of the CD's that I had used in the previous experiment, applying it around the outer edge only. I found the green marker also brought greater clarity, more detail, and tighter bass, but with the green marker there was no unpleasant glare in the midrange at all as there had been with the black one. Successive days of listening confirmed there were no ill effects with the green marker at all.
I tried a dark green permanent marker afterward as well, but there didn't seem to be any significant difference between the green highlighter or the permanent marker on most recordings, though the darker permanent marker did have a bit more pronounced effect on a couple CD's. For the most part I decided to stick with the highlighter, as it's easier to remove should I not like the effect on one of the CD's I've treated. (I also tried a permanent blue marker around that time, but I found it was close in effect to the black marker, though not as pronounced in the "metallic" sound, but still, green was the best by far).
I should mention that I do not treat all CD's with the green highlighter, since there's no need on CD's that are already very open, transparent, and have accurate bass and nice detail. On CD's that are already very good sounding there is little difference, if any at all, to be heard by treating them this way. But inevitably, if I put a CD on that's new to my collection and the instruments are not crisp and the soundstage is not very open, I'll try the green marker around the outer edge, and there's inevitably some degree of improvement to be had with this.
I've experimented with coloring the inner ring as well, but the greatest improvement has always been on the outer edge alone, so this is the only place I put the marker when treating a CD now.
As far as why this works, and why green in particular...
I did a bit of searching right around that time, after I'd heard the difference for myself, and I came across some technical documentation that was written for engineers, I believe, which explained that CD's are not a digital medium as most people think of them as, they are an optical-digital medium, with the laser and lens being a very sensitive optical tracking system which converts the data stream from one that is composed of light into one that is electrical. It seems that green just had the right filtering or absorbing properties to allow the lens to more accurately track the bits stored as microscopic pits which may have been otherwise distorted through the plastic surface which covers them. It was a pretty good explanation at the time, and settled my curiousity pretty well on the matter, although I certainly can't recall all the details well enough to relate them for your benefit at this time.
All I can tell you is this isn't rocket science. Use your ears. If you haven't tried it don't knock it, and if you have tried it on a CD that was already very open and detailed, what did you think you were going to hear...? Find a CD that you haven't been satisfied with in the past, with muffled bass, no definition, and not much depth to the soundstage... one that you "know" there's something there, but your system just doesn't bring out... then put a marker around the outer edge, and go ahead and use black if it's all you've got... after all, this is just an experiment...right? then put the CD back in your transport and give it a listen. Don't be surprised when you hear the difference this simple tweak provides... I've demonstrated this for a few friends in the last couple years, and I've yet to find anyone who has not been able to easily hear the difference, or been less than amazed at the improvement in sound that this simple tweak can make... I even have a couple of duplicate CD's I keep handy, one treated and one untreated in each set, so I can easily demonstrate the difference to people (and don't have to search for a poor quality CD when I'm wanting one... you know how that is... ) so, go ahead and give it a try... and then go out and get yourself a new green highlighter at the office supply store... it may just become your favorite item that you keep handy when you begin to realize the $$$ it will save you from needlessly upgrading IC's trying to get the same result... (of course, I'm assuming you have reference quality, or at least high-quality, IC's to begin with...
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