CD-ROM Technology 1
  Single physical track recorded from the hub out to the rim. 
    22,188 revolutions.
  Tracks

  The term 'Track' when referring to a CD can imply one rotation around the
    disc or length of physical track that holds one song.

  Constant linear velocity (CLV) 120 cm/sec. 
    When used for audio data, provides constant data delivery.

  RPM 530 RPM at hub to 200 RPM at rim.
    Newer CD players may spin faster and buffer read data to provide skip 
      protection. 

    Newer drives, when used for data, spin at much higher constant rotational 
      rate (CAV - constant angular velocity).

    52x CD ROM spins 10.4 K RPM.
    
    CD spinning too fast
      https://www.wired.com/2015/03/happens-cd-spins-fast/

  The primary data substrate of a CD/DVD/Blu-Ray is a reflective track/layer. 

  Information is stored by altering the reflectivity along the track.

  A non-reflective spot is a 'pit' and a reflective spot is a 'land'.

  Information is read by noting the change in reflectivity not by
    the reflectivity itself. 

  Multiple contiguous lands or pits may exist in sequence as long 
    as sequence is not too long. 

  CD use eight to fourteen modulations which maps an 8 bit number on a 14 bit 
     number.

    en.wikipedia.org/wiki/Eight-to-fourteen_modulation

  This guarantees that there will be a minimum of 2 zeros and a max of
    10 zeros between any group of ones.

  This bit sequence is then recorded by altering the reflectivity of the 
    track referred to as NRZI (non-return to zero invert). 

    A change in reflectivity is a 'one' and a lack of change is a 'zero'.

    www.electroschematics.com/4997/compact-disc-how-it-works/

  The RLL/NZRI combination provide a way to accurately recognize and recover
    the bits correctly.


  Data actually exists on the top side of the CD right under the label.

  Read by laser. (see laser diode in wikipedia)
    Laser - light amplification through stimulated emission of radiation.

    Lasers produce coherent light - one wavelength (color), polarized, and
      waves in step.  ~ 785 nm  (infra red).

    The wavelength of the laser determines the minimum size of the pits or
      the density of the data.

    DVD uses a red 650 nm laser, smaller bits, more data per second.

    Blu-Ray uses a blue/violet 405 nm laser, smaller still.