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This is an update to a thread I started in DigDrive, but figured this may be a more appropriate place for it.
I wired up a 9V battery to a green LED and taped it into the disk well of my Rega Apollo so it would flood the laser reading area with green light. I had read about green and blue used in this manner in a few high end CDPs and with the Rega design it is sooo easy to rig up a little $3 test.
I did a number of ABAB comparisons with one of my reference recordings (Cowboy Junkies Trinity Session). I had let the system warm up for about an hour before I did this. To turn the LED on, I plugged in the battery and obviously disconnected it to turn it off.
What happened? There is a fairly distinct difference between on and off. With it on, there is much more clarity in the background like a fog has been cleared – very subtle kick drum, far more clarity in a male vocal harmony (which by the way I wasn’t sure was actually a human voice until earlier this year and I have had this CD for over 15 years). The decay of Margo Timmons voice in the catherdral’s acoustics is abbreviated without the green.
This is surprising as my preconception was that this would have no effect which is why I just taped the assembly in. Any ideas why this is working?
Follow Ups:
I have an old pioneer 25 disc changer that has a green LED inside it, I always wondered what it was for,,,,
Could you please tell me where I could purchase a green led so I can try this experiment?
I was just wondering........what color led would be best to try inside a sacd player and a blueraybdvd player?
nt
you can find them in bulk on-line or ebay.
would this be related to coloring the edges of CD's with a green permanent marker? They did a blind test on a news program back in the 90's and everybody picked the green CD's as the better sounding
Some comments from YBA's designer about why the blue laser might help.
Thank you for this information. But if I understand correctly what Yves is saying, he thinks there is more data on the CD other than 1 and 0. I am certainly no digital data capture engineer, but that is contrary to everything I understand about digital technology.
The laser read mechanism in a CD/DVD player is at the back end of a lens assembly, which focuses the laser beam bounced off the disc read layer (which is NOT the surface). Thus the reflected beam shines back through the lens and is focused onto a phototransistor.
This phototransistor has a certain range of sensitivity to light in terms of strength AND in terms of wavelength (or the color of the light).
Obviously, it will respond to the wavelength of laser light used for the laser beam sent by the CD player, but it is likely that it also will react to some extent to light that is at the right angle. Light shining inside the player that manages to bounce around and enter the lens assembly could also reach the phototransistor.
Any light other than the reflected laser light will possibly change the threshold of detection for the phototransistor, perhaps making it easier to trigger the phototransistor, or the circuitry following the phototransistor, so that the transitions that the laser reads off of the disc become more defined or clear.
Of course, the opposite could be true, additional light into the phototransistor might make things worse in some cases, due to taking the phototransistor out of it's optimum range of detection, etc.
My thoughts about this are that the laser beam strength is limited by FCC mandate, and the practicalities of a solid-state laser LED, thus, the phototransistor may not be getting an optimum amount of light to trigger as cleanly as it is possible. Thus, it is more likely that added light, a sort of "bias light" for the phototransistor, would be much more likely to help things than to hurt.
How the different wavelengths interact is not likely to be a factor, as a very precise wavelength would be needed to "cancel" or to do other things in terms of affecting the original laser light. I can see where the laser beam could interfere with itself, after all it IS coherent light, and could conceivably cancel itself (in fact, the way most of the read mechanisms work, the distance in depth between the CD pits and lands is 1/2 wavelength of the laser or very close, and so, the pits and lands represent a reinforcement or a cancellation of the reflected laser light). But I don't easily envision a mechanism where the laser beam could be affected directly by a different wavelength altogether.
Several things would fall out of the validity of this theory:
One, you would want the extra light source to be absolutely stable and steady, thus the use of a DC powered LED rather than an AC powered incandescent light bulb. This would help make sure the improved detection would remain stable and optimal. An AC powered light source would be going ON and OFF (or modulating it's strength) about 120 times a second.
Two, you would want to try different strengths of light, and different frequencies (wavelengths) would require different strengths due to the differing sensitivity of the phototransistor at different wavelengths.
You would want to shine the light into the laser read lens as stably as possible, this would mean that you would probably not want to bounce it off of the spinning disc if at all possible, and instead, shine it directly into the read lens assembly.
A quick check with several different CD's and CD-Rs showed me that blue and green LED light tends to shine THROUGH the data metallization layer.
This means that external light might be best shined through the disc, more directly into the read lens. Red didn't shine through as easily, and keeping in mind that the laser beam for CD's is an infra-red wavelength (just below, or shorter than red).
Of course, CD's with a painted label would mess with this angle of injecting light, so the best way would seem to be to bounce light off the entire inside area surrounding the disc area, hoping to get as stable an amount of injected light to the read lens as possible.
With this in mind, it is likely that multiple light sources of sightly different wavelengths might be a more optimal approach.
Anyway, this theory provides for a mechanism where added external light could provide a laser read lens phototransistor "bias light" to more cleanly sense the land to pit to land transitions, thus, reducing jitter.
Jon Risch
Jon - based on what you Al and unclestu discussed, I am going to make a small strip of 5-6 different blue and green LEDs and mount it on the bottom of the disk well next to the laser sled. I have already applied a strip of aluminum tape to act as a reflector. I have to go to RatShack for a couple blue LEDs, but I will try that and post again once I have had a chance to give it a listen.
:)
I think that's a good bet too Jon, well put.
I was thinking along the same line but the notion that it might be a way 'around' the laser power limits didn't occur to me and it seems like a good one.
If one really wanted to mess about with it I think looking at the output of the photodetector preamp, with a scope would probably allow the tweak to be tweaked. As you point out it's the product of detector sensitivity curve, the LED level and the path loss that would be the deal.
I've never messed around with CD readers and since they are becoming extinct as far as real-time playback goes it probably doesn't make much sense to do so now. Unclestu has told me several of the tweaks he used to do to them including ferrofluid in the focus coil gap and stuff to reduce the preamp noise so I guess they have always been 'tweakable' for those with guts and a steady hand.
Rick
You remembered! And yes I still use that tweak, although you have to be very careful not to drip any ferrofluid into the laser lens assembly. There is no way beyond destruction to clean off that silicone fluid base.
The use of the ferrofluid and the subsequent performance increase did indicate issues with the focus motor circuit. However,it seems to me no one ever spent much time on that portion of a CD/optical reader interface. A glance at the laser assembly while playing a disc certainly reveals the lens in almost continuous motion, and since the focus mechanism employs a feedback sensor and circuit, it is always an after the fact correction. That brings to mind a thought: perhaps those ultra high priced glass CDs sound better because the surfaces are also flatter than the plastic discs? Maybe there is simply less error correction.
Also the nature of the laser light involved has changed over the years. The emergence of the rerecordable CD means that the contrast of the data surface is significantly reduced in respect to the original stamped CDs. I do not know if sensitivity of the reader was simply increased or perhaps the laser output was increased.
Stu
Hi Stu, How could I forget!
But I have been thinking of you and am very pleased that you're posting again.
I've got an old pair of AR2's, at least that what I think they, under the house and if the mice haven't eaten them need to try the tape tweak you are advocating today on the tweeter. I quit using them because one had developed a cone breakup in the tweeter and the new tweeter I ordered from them had exactly the same problem. That's why I bought the Celestions, I figured that the Ti tweeter would be solid in-band. Wish I still had some of that old tape we used to use for laying out PCB's around, I bet that would be perfect. But I've got nothing to lose by trying to fix them.
Regards, Rick
...perhaps the green LED light is creating more contrast inside the player so the reading mechanism has an easier time seeing 1s and 0s.Light waves can cancel each other out, however this happens only with very specific wavelengths. I may be incorrect in this, but I believe that a laser has a very specific wavelength, whereas an LED broadcasts a more broad spectrum which would not interfere with or cancel other light sources easily. Therefore I believe the "cancelling" theory to be unlikely.
Edits: 04/25/10
v.i.
Million miles from home.
The KPS-20 series (20t, 20i and 20i/l) had greed LEDs in the transport section.
a
nt
The blue or green light is absorbing stray scattered background light from the "red" CD laser that would otherwise be picked up by the photodetector as signal.
/
adding to geoffkait's post below.....
And BLACK absorbs all light. So with that in mind wouldn't it be better to have all interior surfaces of the CD/DVD black and all source of light sealed off?
I have seen the interior of a CDP with something that looked like thin black felt covering most of the interior cabinetry surface. I thought it was some type of damping thing, but that doesn't make sense, and scattered light absorption was probably the purpose. If you could get a similar effect with a couple LEDs, that would be a lot easier and cheaper to do. And possibly more effective if there are any shiny parts inside.
I think its more like light color X can cancel light color Y to null. Not any color works with another, its based on the light spectrum.
ET
Question "Authority", the mainstream media sucks - Go Independent and hold BOTH parties accountable instead of just the other guys!
?
Well, only a specific color (wavelength) light absorbs light of a certain color (wavelength). Blue-green is the "complement" of red so blue-green light will absorb red light (or near red).
Pop Quiz 1: what color light would be used for a Blu-ray player to absorb stray scattered light?
Pop Quiz 2: Why doesn't the blue-green light absorb the primary laser light as well, such that the data cannot be read (no pun intended)?
"Well, only a specific color (wavelength) light absorbs light of a certain color (wavelength)."
Earlier this morning I was enjoying the sunny, clear morning, the white-crowned sparrows hopping around under the Red Rhody blossoms and the green of the lawn that needs mowing. But I guess it was all a faith-based illusion. After reading your posts I looked again but there was only a murky darkness, not totally dark, vague areas of dimness appeared randomly and then dissipated. Sigh. It figures, a nation given to glorifying football and eschewing physics is doomed to live in darkness.
I'm hoping that towards dusk when the effective blackbody color temperature of the sun shifts more out of the eye's passband that there will be less cancellation and I'll be able to see at least dim outlines, meantime I'm stuck with darkness at noon, a veritable visual gulag... Thanks!
With apologies to Arthur Koestler,
Rick
nt
I understand , but it seems like a solid object would absorb, not other light. I could be wrong!
You're right. A solid object is not an active radiator, it simply reflects radiation that falls upon it and that it cannot absorb. A light source is an active radiator and can interfere/interact with light from another active radiator. This is "addition" of light. E.g. red, blue and green light added together give white (as in color TV).
However, white light filtereed through red, blue and green colored filters give no light at all = black. This is "subtraction" of light.
The color of a solid object we see is actually the color of the *light* the object reflects. An orange appears Orange to us because it reflects the color (wavelength) Orange and absorbs all other colors of light (wavelengths).
http://www.machinadynamica.com/machina34.htm
Philips machines had a green light which turned on upon playing. IIRC, there was a Sanyo player which had it's CD drawer painted or dyed green. Also Simply Physics offered a kit to add the green light for Philips based transports. Probably only old geezers like me will remember, though.
Stu
A humble NAD player. When I demonstrated to myself the effectiveness of "green paint" on CDs I painted the drawer on the NAD. Used a UNI Posca green poster paint marker. I could hear a decent improvement.
Regards,
Geoff
unclestu,
Did the green light that Philips used in those players or the green painted drawers have any sonic benefits as far as you could tell.
Do not recall ever seeing those mods put in any of your reference players.
Sonic differences were ranged from barely noticeable to of significant help, (subjective, of course). I brought in a couple of Simply Physics kits but eventually used a simple disc of green plastic cut to the size of a cd. It was cheaper and just as effective. Of course painting a drawer flat green was even cheaper, but much more difficult to make a comparison.
Stu
Stu
Rega Apollo is top loading and it has a green transparent lid.
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