XPost: alt.comp.os.windows-11, comp.os.linux.advocacy, comp.sys.mac.advocacy   
   From: daniel47@nomail.afraid.org   
      
   On 19/11/2025 12:22 am, J. P. Gilliver wrote:   
   > On 2025/11/18 11:32:6, Daniel70 wrote:   
   >> 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.   
   >   
   > Well, I never came across a floppy drive where you had to turn over the   
   > floppy - in fact most (certainly for the 3½") had mechanisms to prevent   
   > you doing so. They _did_ have two heads. As for CDs and DVDs, I don't   
   > think I ever _saw_ a double-_sided_ one, though I believe they did   
   > exist. (Certainly never saw a two-sided - i. e. dual head - _player_.)>   
      
   Way, Way back when, I did have a 5.25 Drive where you actually had to   
   take the Disk out and flip it to get to Side B.   
      
   I had one or two Dual-sided DVD-RW's .... probably still do if I looked   
   hard enough. They were much thicker than your standard DVD in their   
   solid see-through, plastic cases, maybe getting on to 1cm thick.   
      
   >>> 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.   
   > You're welcome.   
   > I think virtually all movie DVDs were (are, for the small market that's   
   > still making and buying them!) dual-layer, except for very short movies.   
   >   
   So that would rely on the DVD drives being able to read these Dual-sided   
   disks. Can't say I've ever noticed that capability!!   
   --   
   Daniel70   
      
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