XPost: alt.comp.os.windows-11, comp.os.linux.advocacy, comp.sys.mac.advocacy   
   From: G6JPG@255soft.uk   
      
   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_.)>   
   >> 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.   
   --    
   J. P. Gilliver. UMRA: 1960/<1985 MB++G()ALIS-Ch++(p)Ar++T+H+Sh0!:`)DNAf   
       
   Today, I dare say more people know who starred as /The Vicar of Dibley/   
   than know the name of the vicar of their local parish.   
   - Clive Anderson, Radio Times 15-21 January 2011.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)