how many grooves per inch on a CD?
Erich Burton
A coworker asked me if I knew how many grooves per inch there were on
a CD and I didn't know. He wants to use one as a diffraction grating,
and he wants to check his experimental results.
I did a rough back of the envelope calculation, and came up with
15,000 lines per inch (600 per mm). Does anyone know definitively
what the standard is?
Any help would be appreciated.
____________________________________
Erich Burton 3-2602 ebu...@bu.edu
Boston University Physics Department
Dan Miller
: ...and don't say 'one'.
There is no definitive 'standard'; manufacturers can vary the pitch
depending on how much data they hope to fit on a disk. There is probably
a maximum allowable by the spec. Your thumbnail sounds about right;
it's complicated by the fact that CD's are CLV (Constant Longitudinal
Velocity), so their RPM varies from (I think) about 300 to 1200 or so.
I think Philips distributes the specs, known as the 'Red Book' for audio,
'Yellow Book' for CD-ROM, etc. There is a digest of these specs I once
had, don't remember where I got it. Sorry if this isn't so helpful.
Dan Miller
w
Matt McIrvin
>...and don't say 'one'.
'One' is wrong anyway, isn't it? I didn't think CDs had a spiral groove.
But I could be wrong.
--
Matt 01234567 <-- Indent-o-Meter
McIrvin ^ Indentation will soon be too cheap to meter!
Yaakov Eisenberg
Matt McIrvin <mci...@scws4.harvard.edu> wrote:
>
>'One' is wrong anyway, isn't it? I didn't think CDs had a spiral groove.
>But I could be wrong.
CDs do have a spiral groove. (Is 'groove' the right word, anyway?)
Unlike records, it starts at the middle and works its way outwards.
If you have one of those portable CD players with a little window in it,
you can see that the CD spins faster in the beginning and slows down as
it gets to the end. This is because the data is read at a constant
linear velocity.
--
-Yaakov Eisenberg (yaa...@cc.gatech.edu)
me...@cars3.uchicago.edu
can take a laser (one of those lecture room pointers will do just
fine) and, using the disk as a grating, check the angular separation
of diffraction orders. Should work just fine.
Mati Meron | "When you argue with a fool,
me...@cars3.uchicago.edu | chances are he is doing just the same"
Matt McIrvin
Yaakov Eisenberg <yaa...@cc.gatech.edu> wrote:
>
> CDs do have a spiral groove. (Is 'groove' the right word, anyway?)
'Track,' I suppose, might do, but the *songs* are called 'tracks.'
>Unlike records, it starts at the middle and works its way outwards.
Thanks for the clarification. I was probably confusing them with
magnetic disks.
William H. Jefferys
#>
#>There is no definitive 'standard'; manufacturers can vary the pitch
#>depending on how much data they hope to fit on a disk. There is probably
#>a maximum allowable by the spec. Your thumbnail sounds about right;
#>it's complicated by the fact that CD's are CLV (Constant Longitudinal
#>Velocity), so their RPM varies from (I think) about 300 to 1200 or so.
#>I think Philips distributes the specs, known as the 'Red Book' for audio,
#>'Yellow Book' for CD-ROM, etc. There is a digest of these specs I once
#>had, don't remember where I got it. Sorry if this isn't so helpful.
#>
#Actually, it can be measured (I'll do it if I find a free hour). You
#can take a laser (one of those lecture room pointers will do just
#fine) and, using the disk as a grating, check the angular separation
#of diffraction orders. Should work just fine.
Tried it last night with my laser pointer.
The angle is about 30 degrees (varies from disk to disk, I tried
several) which, for a wavelength of ~650 nm gives a groove spacing
of ~2 micrometers.
This is a nice classroom demonstration, BTW.
Bill
--
Bill Jefferys/Department of Astronomy/University of Texas/Austin, TX 78712
E-mail: bi...@clyde.as.utexas.edu | URL: http://quasar.as.utexas.edu
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Erich Burton
:Tried it last night with my laser pointer.
:
:The angle is about 30 degrees (varies from disk to disk, I tried
:several) which, for a wavelength of ~650 nm gives a groove spacing
:of ~2 micrometers.
:
:This is a nice classroom demonstration, BTW.
Thanks to all those who followed up. Actually, this demo was what
prompted the question -- one of my coworkers set up the demo to use
a laser to determine the spacing of the grooves on the CD, but the
professor wanted to know how accurate the results actually were.
According to an article in _The Physics Teacher_, the actual groove
spacing of a CD is 1.6 um or 625 grooves per mm.