XPost: alt.comp.os.windows-11, comp.os.linux.advocacy, comp.sys.mac.advocacy   
   From: daniel47@nomail.afraid.org   
      
   On 18/11/2025 12:25 am, Paul wrote:   
   > On Mon, 11/17/2025 7:44 AM, Daniel70 wrote:   
   >> On 15/11/2025 10:29 am, Lawrence D’Oliveiro wrote:   
   >>> On Fri, 14 Nov 2025 23:17:19 +0000, David B. wrote:   
   >>>   
   >>>> Can you explain WHY there is a need for different lasers?   
   >>>   
   >>> The DVD pits are too small and too close together to be made out   
   >>> with the CD laser.   
   >>>   
   >>> Similarly, Blu-Ray (and its late competitor HD-DVD) required   
   >>> moving to a blue laser (hence the name) to read its even finer   
   >>> pits.   
   >>>   
   >> I can recall seeing DVD's that had 9.4GB capacity.   
   >>   
   >> I was told that, somehow, they had two Data levels, rather than   
   >> using 'half size' bits all on the one level.   
   >>   
   >> How did these High Density Disks work .... and how could a normal   
   >> DVD Player read them??   
   >   
   > Those are DL. (DVD+R DL)   
   >   
   > In typical Wiki production, the first article is missing any view of   
   > the layer stackup.   
   >   
   > A second article has to pick up the slack (which can lead to a   
   > non-specific description).   
   >   
   > https://en.wikipedia.org/wiki/DVD%2BR_DL   
   >   
   > https://en.wikipedia.org/wiki/Optical_disc   
   >   
   > "In DVDs, the same processes as in CDs are carried out, but in a   
   > thinner disc. The thinner disc is then bonded to a second, equally   
   > thin but blank, disc using UV-curable Liquid optically clear   
   > adhesive, forming a DVD disc.[48][5][49][50]   
   >   
   > This leaves the data in the middle of the disc, which is necessary   
   > for DVDs to achieve their storage capacity. In multi layer discs,   
   > semi reflective instead of reflective coatings are used for all   
   > layers except the last layer, which is the deepest one and uses a   
   > traditional reflective coating.[51][52][53]"   
   >   
   > Bluray can have more layers than that.   
   >   
   > http://www.repairfaq.org/sam/Blu-ray/site1/optics.html   
   >   
   > "After some serious patent digging, it is apparent that the objective   
   > lens sits less than 600 μm away from the disc when reading Blu-ray   
   > discs. Since there are Blu-ray discs that are multilayer, you would   
   > probably end up with the objective considerably closer to, if not   
   > embedded in the disc, when reading deeper layers! The collimating   
   > lens on the servo is therefore variable so as to extend the range of   
   > focal lengths available at the objective.   
   >   
   > Paul   
   >   
   Ah!! O.K., thank you, Paul.   
   --   
   Daniel70   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)