prk@charm.UUCP (Paul Kolodner) (06/08/85)
>Have you ever put your favourite CD up to a light source? Choose one >that doesn't have an opaque label. There are definitely defects there >that are more than 1 bit! Thank goodness for error correction. You need a lesson in optics. What you see is not what the laser sees. It has a one-micron depth of focus set below the level of what you're seeing. You don't.
rfg@hound.UUCP (R.GRANTGES) (06/09/85)
[] No, he was right. He said, "put it <up> to a light source." I.e., let light shine through it. Many CD7s pass this test with flying colors. Some others look like an astronomical print. Many of the holes thus revealed are obviously many bits long. I think there is some question, though, if the laser can't read at least some of the information even when the aluminum backing is missing. After all the "bits" are apparently deformations in the plastic <before> the aluminum is put on. They are not pits in the aluminum as I used to think. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg
ben@moncol.UUCP (Bennett Broder) (06/09/85)
>>Have you ever put your favourite CD up to a light source? Choose one >>that doesn't have an opaque label. There are definitely defects there >>that are more than 1 bit! Thank goodness for error correction. > >You need a lesson in optics. What you see is not what the laser sees. >It has a one-micron depth of focus set below the level of what you're >seeing. You don't. I don't see what that has to do with anything. When I hold a CD up to the light and see a large pinhole (a pretty common defect), it seems to me that there are no bits at *any* depth.
rfg@hound.UUCP (R.GRANTGES) (06/10/85)
[] "...no bits at any depth." The bits are much, much too small to see with the naked eye. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg
pchow@Shasta.ARPA (06/11/85)
> [] > I think there is some question, though, if the laser can't read at least > some of the information even when the aluminum backing is missing. After > all the "bits" are apparently deformations in the plastic <before> the > aluminum is put on. They are not pits in the aluminum as I used to think. Some more information further to the above... In IEEE Spectrum March 1984 there is a diagram showing the following layers. (Not to any scale). ______________ label ______________ protective film ____ ____ |____| aluminum reflection film ___ ___ |______| quarter wavelength pits transparent layer ______________ /\ /||\ light direction || This shows that the pits are indeed in the plastic and the aluminum layer is coated on top of them. The quarter wavelength pits cause destructive interference between the part of the beam hitting a pit and the part that doesn't. This is detected at the photodetector. If the aluminum is missing, then the amount of light reflected back should depend on the differences in the indices of refraction of the transparent layer and the protective film. Paul Chow decwrl!shasta!pchow pchow@shasta
reo@tikal.UUCP (R.E. Overby) (06/11/85)
> >>Have you ever put your favourite CD up to a light source? Choose one > >>that doesn't have an opaque label. There are definitely defects there > >>that are more than 1 bit! Thank goodness for error correction. > > > >You need a lesson in optics. What you see is not what the laser sees. > >It has a one-micron depth of focus set below the level of what you're > >seeing. You don't. > > I don't see what that has to do with anything. When I hold a CD up to > the light and see a large pinhole (a pretty common defect), it seems > to me that there are no bits at *any* depth. I agree with Ben. There were several recent articles on the regarding the need to _PROTECT_ the aluminized back since the manufacturers left the reflective coating essentially unprotected. What a clever way to insure that CDs do _NOT_ "last forever" 8-( (Denotes displeasure) There is a very effective error correction scheme, but I don't have the number of bits/sample available here at home. Any one remember out there? --- Robert E. Overby, Teltone Corp. 10801 120th. Ave. NE, Kirkland, WA 98033 Phone 206-827-9626 FAX (Group II) 206-827-6050 TWX 910-449-2862 .....uw-beaver!tikal!reo
dave@rocksvax.UUCP (06/11/85)
Most disks are somewhat transparent because the wholes are designed to be
1/4 wavelength of IR laser light which is lower in frequency than visible
light. This will let a small amount of visible light through, I think what
the original poster was noticing was those bright little spots thought the
normal "grey" haze. I do beleive these wipe out more than 1 bit of data
thus invoking the error corrector.
I looked through most of my CD's for this when I noticed the "swiss cheese"
quality of the CBS "Made in USA" CD I bought. All of the "Made in Japan"
CD's seemed to have at most 1 or 2 of these "bright" spots. The "Made in
Germany" ones had up to about 4. Now I can see why the US plant has a
90% or worse fall-out rate!!!
Despite any of these my Yamaha seems to play through it all without any
discernable noises.
Dave
arpa: Sewhuk.HENR@Xerox.ARPA
uucp: {allegra,ihnp4,rochester,amd,sunybcs}!rocksvax!dave