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  1. #1

    Default How does it tell the difference?

    We have a DVD player built into our TV and we can also play CDs. The track number and time of the track comes up on the screen. My question is: What's the difference between a CD and a DVD and how does the player tell the difference?
    It is better to wear out than to rust out.
    - Richard Chamberlain, Tour of the Hebrides

    Us husbands are a sorry lot.

  2. #2
    Soaring like an Eagle gary67's Avatar
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    Default Re: How does it tell the difference?

    File formats

  3. #3
    Old guy
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    Default Re: How does it tell the difference?

    The same way an asprin knows if you have a headache or a sore toe.

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    Seasoned Member allblack's Avatar
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    Default Re: How does it tell the difference?

    Quote Originally Posted by CliveM View Post
    The same way an asprin knows if you have a headache or a sore toe.
    That's always fascinated me.

  5. #5
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    Default Re: How does it tell the difference?

    It's not file formats but file system. File formats is more to do with how files are encoded while the file system is how data is stored. Files are nothing more than data stored in a system and then software (like your operating system) turns it into something more human friendly, like giving it a file extension and a means to open it. These things are virtual however.

    CD-DA (CD Digital Audio) is the specification for music CDs. The whole collection of CD specifications can be found in the collection of Rainbow Books, music CDs are found in the Red Book. The DVD specifications are found in the DVD Books. This is just all the technical stuff but it is needed to create a standard that everyone can adhere to. You then need software to understand these specifications, which is what your DVD player would have and is why it can tell the difference between a CD and a DVD. It may not have all the implementations for every kind of CD/DVD but enough to do what it is mostly intended for.

    You will find this is common for software/hardware, there's a specification and then an implementation for it.

  6. #6

    Default Re: How does it tell the difference?

    Thank you for that, Kame. I was hoping for a sensible answer. Very well explained. Now I see how it works. Much appreciated.
    It is better to wear out than to rust out.
    - Richard Chamberlain, Tour of the Hebrides

    Us husbands are a sorry lot.

  7. #7
    VoidMaster
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    Default Re: How does it tell the difference?

    Ok, I don't know all there is to know about CD and DVD players, but I am sure that the two systems are completely separate at the interface to the disk and have separate lasers and lenses and guidance systems and only come together further down the chain to process the signal after the correct laser has sampled the disk surface.

    CD is based around 45 kHz sampling rate and DVD has yet another sample frequency for Video and another for sound, upon querying the disk on start up the system determines whether or not it is a CD or DVD and acts to activate the correct laser and lens and focus on the disk with a Phase Lock Loop system to read the disk and control the speed of rotation and dynamically control the focus to decode the noughts and ones coded as bumps and hollows on the disk surface, then puts the result through a Digital to Analogue Convertor then to the output to line audio.

    Presumably the frequency of the sample rate is a good indication of whether it is a CD or DVD on the player and it acts accordingly.

    At one time back in the past I am sure it was said that the CD player and then the VCR were the most sophisticated devices in the home.

    The action of a Phase Lock Loop is a beautiful concept like the action of an Operational Amplifier with a negative feedback loop around it which is my favourite lovely idea.
    Last edited by zqwerty; 15-08-2020 at 11:08 AM.
    It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche

  8. #8
    Senior Member 1101's Avatar
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    Default Re: How does it tell the difference?

    Philips came up with the format for music CD's

    When Music companies starting putting anti copying systems onto music CD's , they were no longer following the exact
    format for CD's .
    So Philips based CD players (Phillips, Marantz) & some other EU designed CD player would no longer recognise or play the Copy Protected Music Cd's as they didnt follow the CD format. So I had to make copies of some music CD's, removing the anticopying in order to play these CD's
    That was very annoying .

    Sony took things one step too far , with their copy protected Music CD's . I remember the issues this caused, PC CD drives stopped working properly
    from Wiki
    "The "Sony BMG copy-protection rootkit scandal" is a 2005 scandal regarding Sony BMG's implementation of copy protection measures on about 22 million CDs. When inserted into a computer, the CDs installed one of two pieces of software which provided a form of digital rights management (DRM) by modifying the operating system to interfere with CD copying. Neither program could easily be uninstalled, and they created vulnerabilities that were exploited by unrelated malware. One of the programs would install and "phone home" with reports on the user's private listening habits - even if the user refused its end-user license agreement (EULA), while the other was not mentioned in the EULA at all. Both programs contained code from several pieces of copylefted free software in an apparent infringement of copyright, and configured the operating system to hide the software's existence, leading to both programs being classified as rootkits. "

  9. #9
    VoidMaster
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    Default Re: How does it tell the difference?

    I remember the Sony rootkit saga very well, an interesting period. I had one of the disks that did that.
    It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche

  10. #10
    Senior Member
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    Default Re: How does it tell the difference?

    CDs and DVDs the technology remains the same in how they are created, it's just the newer they got, the data became smaller and more compact. This is why a DVD player can be backwards compatible with CDs. All you need to know is it's the same hardware used to read each disc but the lens needs to adjust focus before it can read the finer print (pits) properly. So no separate laser for each kind of disc. Now some DVD players had issues with reading CD-/+R(W) but again this comes down to how well they implemented the spec and how reliable the device that wrote the media was.

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