XPost: alt.comp.os.windows-11, comp.os.linux.advocacy, comp.sys.mac.advocacy   
   From: daniel47@nomail.afraid.org   
      
   On 18/11/2025 12:57 am, J. P. Gilliver wrote:   
   > On 2025/11/17 12:44:21, 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.   
   >   
   > Yes, but the CD pits are obviously _not_ too small to be made out with   
   > the DVD laser, so _that_ still doesn't explain the need for two. It   
   > _may_ be something to do with refraction.>>   
   >>> 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??   
   > Two ways: one way, which I think was hardly ever used, was to use both   
   > sides, just like an audio record - basically two back-to-back. I think   
   > two reasons against: 1. you can't have a label (or, it has to be   
   > restricted to just the very centre); 2. the user has to turn it over (or   
   > you have to make a player with two readers, which is more expensive and   
   > makes it - the player - thicker).   
      
   So like 'they' did with 5.25 inch Floppies, 3.5 inch Floppies, CDs and,   
   I think, DVDs.   
      
   > The main way was/is "dual layer",   
   > where there are two layers of data: the innermost one is read _through_   
   > the outermost, which is thus out-of-focus. For commercial movies, the   
   > changeover point is obviously chosen to be a point in the movie where   
   > it's faded to black, or similar; one "half" (they don't have to be   
   > equal) plays as normal from the middle outwards, but to minimise the   
   > changeover time, the second half is played from the outside in, like an   
   > LP/single.   
   >   
   O.K., thanks to you and Paul.   
   --   
   Daniel70   
      
   --- SoupGate-Win32 v1.05   
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