CD stoplight - some analysis

Don't worry, Martin, if I fail to do so, I have a backup:

http://www.belt.demon.co.uk/product/cream/cream.html

I merely smiled when I heard of the green pen; this made me fall off my chair laughing.

 

Thanks, Isaac, but how green is green in this case? Clearly the colour green is far from inconsequential as the colour of the absorbent, nothing else, determines what wavelengths of the visible spectrum will be absorbed. So, is it light, medium, dark?

 

The point is the CD laser's light is not visible, so judging whether the ink will absorb this by looking at it isn't particularly helpful.

 

Indeed it does tones, but the required absorbtion in this case is not in the visible spectrum, but it is in the infra-red.

Try an opaque light green acrylic paint. That is essentially what stoplight is. Marker pens on the CD, especially OHP markers, are likely not to be opaque.

 

But I thought that the laser was in the range 780-790nm, which AFAIK is most definitely in the red end of the visible spectrum.

P.S. I'm sure someone has asked this before but why not black, which by definition absorbs ALL wavelengths?

 

A search on Google groups reveals plenty on the green pen. Indeed, a number of threads do indicate that it was a hoax which actually started on the audio newsgroups.

See http://groups.google.com/groups?hl=...&selm=43vlj6%2443l%40access1.digex.net&rnum=3 (from 1995)

Second post quote: "I think it's time we all recall how "green pen" stuff got started.

It started, with me (among others) watching, when a techie type
net.audio reader said "well, if you're so worried about the
laser light leakage, why don't you paint the outsides of your
CD black to catch it", in a discussion where somebody was offering up an observation that "CD players are dangerous because they have a laser inside them".

A few weeks later, somebody took this sarcastic quote
seriously, and announced that indeed green permanant markers
improved the sound of the audio out of a CD player."

 

Actually, the product CD stoplight has been around for a very long time, and was reviewed in Stereophile in March 1990.

 

Ian,

I am no electronics expert, but I dont think that will be the case at all and I think you may be over simplifying things. I think there is a lot more to sound quality than frequency analysis plots. There are probably a whole host of other measurements such as harmonic distortion, IMD, etc. that should be measured too.

For example, remember the 1970s japanese mosfet amps that measured perfectly with no distortion, yet sounded awful? What about valve amps that measure terribly, yet sound wonderful? ISTM that we may well be measuring things that are not that significant when it comes to sound quality.

Graham,

sorry I dont measure up to your high standards. (There seems to be quite a few of us you have lumped into this category recently)

 

Further info at http://web.archive.org/web/20010617211358/emedialive.com/news/2000/07/news0700-02.html

and http://www.snopes2.com/music/info/greening.htm

 

The 780-790nm range is very much on the visible limit I believe, and basically near infra-red. A quick google gave me this link for instance:

http://en.wikipedia.org/wiki/Visible_spectrum

If you accept you can't see the laser then that covers the why not black ink question too I think.

 

Not everything that counts can be counted, and not everything that can be counted counts. - a certain albert einstein, you may have heard of him.
cheers


julian

 

You're right, Martin, I just checked - 780nm is indeed in the near-IR and just outside the visible spectrum. However, remember that black is also a very good absorber of IR - that's one reason why black surfaces absorb heat so well. It therefore more than ever seems to me that green is an odd colour to choose to absorb IR.

 

From http://www.snopes2.com/music/info/greening.htm

"Even if there were strong edge reflections, there is no guarantee that coating a CD's edges with a randomly selected paint or ink would reduce or eliminate the reflections. The initial color proposed for the treatment was green, on the mistaken assumption that since the wavelength of the laser light used in CD players is in the infrared region of the spectrum, a green pigment would absorb it. Maybe so, but substantial infrared absorption is not an inevitable consequence of something appearing green. That's why I used black paint in my tests, on the assumption that an opaque black pigment is far more likely to be infrared-absorbing than semi-transparent marker ink. It certainly disrupts the laser beam more when painted on the surface of the disc."

 

But we're not comparing two audio components in this case, merely a treated and an untreated CD being played back on a single CD player. If the frequency response of the treated and untreated CDs were to be the same when played on the same CD player, they must sound the same, surely? If they were to be played back on different CD players that's a different kettle of sprouts entirely.

-- Ian

 

I take your point. The why not black question I asked not very long ago too, and for the reasons discussed above decided there might be something specific about the ink in the Stoplight pen which made it a specifically good absorber at the wavelength in question, and that you couldn't necessarily tell for certain what would be a good absorber at this wavelength by looking at it.

 

I believe people have used black ink and claim to hear a difference. IIRC Steven Toy has.

 

It's certainly true that there are materials that are good absorbers of IR. I had a quick look and note that they are usually either dark brown or dark green dyes. However, these are specialist materials (could this account for why the pens are so dear?).

Nevertheless, it remains my understanding that black absorbs all wavelengths - this, after all, is the definition of black, the complete absorption of all colours and the reflection of none, so green remains a mystery.

In any case, I'll cancel my experiment, as I cannot assure a green colour that would match that of the stoplight pen and I'm too mean to buy the real thing!

 

Not if you get for arguments sake different levels of jitter between the two. A frequency analysis plot isnt going to show you that, is it? I am still not sure whether with the frequency analysis we are measuring the relevent thing when it comes to sound quality.

FWIW, when I use the pen I get a reduction in digital hash and glare and a more detailed, cleaner (not duller) treble which to me seems more consistent with a reduction of some form of distortion.

(the jitter thing may be a load of rubbish, i dont know. I'm just usung it as an example.)

 

The initial plot is simply the unprocessed analogue output of the CD player though - it's not a case of oversimplifying things as that's all the information there is. Nothing's simplified at all, that's everything the CD player is putting out The Fourier transform (frequency analysis) plots are basically beside the point until we've established whether or not there's a measurable difference larger than experimental error - if there is a difference the Fourier transforms will help in determining exactly what the difference is, but if all you want to know is whether there's an experimentally significant difference you may as well just compare the raw output. It seems there's some misunderstanding over what exactly is happening in the Fourier plots - it's just a mathematical transformation which can be used to resolve a wave into its harmonic constituents. This is categorically not the same as measuring the frequency response of the CD player or the treated disc - it's a way of processing the raw output data to try to get some idea of how any differences arise.

 

notaclue - that last link of yours was interesting, and more informative than many of those merely laughing at the silly idea of the green pen.

I wonder if it's worth debating the following but I'll give it a shot...

Can I ask about jitter? :duck: When trying to justify to myself how the Stoplight might work, I'd got the impression that the suggested mechanism was that the reflected laser light caused an increase in jitter, rather than causing misreads of ones and zeros. Can anyone explain roughly how the light signal received by the photodetector is coverted to the digital output as fed to the DAC (whether onboard or seperate)? I mean, if the light signal is 'noisier' how, if at all would this manifest itself? (I'd be very interested in this for considering all CD treatments, not just the green pen)

Martin