Results 1 to 10 of 10

Thread: Why isn't optical storage technology backwards compatible?

  1. #1
    Join Date
    May 2008
    Location
    Wisconsin
    Posts
    449

    Default Why isn't optical storage technology backwards compatible?

    "but xytrell, it IS backwards compatible! I burn CDs in my DVDRW drive all the time!"

    Yes, in the strictest sense. But you're not using a red diode to burn them, you're still using those old IR diodes. I understand why an IR diode can't burn DVDs (because the focal point is too large to fit many more pits on the same surface) but why can't red diodes burn CDs? Why can't Blu-ray diodes burn CDs or DVDs? And why can't a DVDs-worth of data be written on a CDR using a red laser instead of an IR laser?

  2. #2
    Join Date
    Jan 2009
    Posts
    1,622

    Default

    I would imagine it has to do with each type of disk medium being optimized for a specific wavelength. I can't say whether or not the materials in a blank recordable DVD would respond properly to 780nm or 405nm light rather than 660, but I doubt it.

  3. #3
    Join Date
    Jan 2008
    Location
    Stockholm, Sweden
    Posts
    799

    Default

    As a dye chemist I would say it's because the dye. You see, it is a photosensitive organic dye that is the actual data storage medium. These dyes are only active at certain wavelengths.

    Also, I suppose the gratings are designed for a certain wavelength.

  4. #4
    Join Date
    May 2008
    Location
    Wisconsin
    Posts
    449

    Default

    So why do game systems (ps3 for example) have three lasers? Factory stamped media doesn't use a dye (or does it?), and as far as the manufacturer is concerned, there is no legitimate use for DVDRs in a gaming system.

  5. #5
    Join Date
    Jan 2009
    Posts
    1,622

    Default

    DVDRs and commercial DVDs use the same wavelengths. A 780nm CD diode can't be used to read any old media just like it can't be used to write any old media. The physics just don't work. Each style of media needs to be read and written at the wavelength it is designed for. For example, if the data is written to a disc intended to be read at 405nm, the pits and flats will be too small to be read by any other wavelength. Also, optical media acts as a diffraction grating. Obviously if you use another wavelength, the light will be diffracted differently, perhaps enough so that the return stream doesn't end up centered on the right photodetector, thus interfering with the return stream.

    The reason a blu-ray disc holds much more data than a DVD, and the reason why a DVD holds much more info than a CD etc.. is due almost entirely to the wavelength of the light.

    These are interesting questions, and I'm curious why you ask?
    Last edited by ElektroFreak; 10-07-2009 at 08:08.

  6. #6
    Join Date
    Feb 2008
    Location
    East Sussex, England
    Posts
    5,248

    Default

    So, are you saying that my DVD burner actually has 2 lasers, one to handle the CD writing and another to handle the DVD writing? You learn something new every day!

  7. #7
    Join Date
    Jan 2009
    Posts
    1,622

    Default

    ^That's correct. Blu-ray drives have three. The optical setup that they manage to squeeze inside the confines of the reading/writing assembly is practically a work of art. They're pretty amazing when you really think about it.

  8. #8
    Join Date
    Dec 2006
    Location
    Netherlands
    Posts
    983

    Default

    Interesting question. It made me think that even if you can use a 405nm focussed to a larger dot to read and write IR media, that the development of the mechanics, optics, and electronics to do both will be far more expensive than just inserting an of the shelf available and reliable IR optical head next to the a dedicated 405 head.

  9. #9
    Join Date
    May 2008
    Location
    nerdtown, USA
    Posts
    1,165

    Default

    Another factor to bear in mind is the refractive index of the lens; it will focus closer at shorter wavelengths, and this is factored into the design of the media.

    Another another factor is the size of the spot on the disc- a small spot is not as good at reading large (maybe fuzzy) pits or spots.

    Games consoles have to contend with reading burnt media for the two reasons of software development and playing user media; while Sony may not be keen on you burning your own DVDs, they have to be compatible with DVDs burnt by camcorders, etc.

  10. #10
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,890

    Default

    And remember there are often 3 laser spots on the disk, left, right, and read. Left-Right handles tracking and focusing. Read is exactly what it means. Better drives with more computing power might use just two spots and a quadrant detector with 4 or more segments.

    The shorter the wavelength, the closer the spots and adjancent tracks have to be

    Steve
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •