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Why is low DC-content desirable for data stored on a CD?

Discussion in 'Electronic Basics' started by c.j[dot]w, Nov 20, 2004.

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  1. c.j[dot]w

    c.j[dot]w Guest

    Hello,

    On a CD, information is stored in 14-bit words. Between those words are
    three merging bits. These contain no information, but are chosen,
    partially, "to keep signal DC content low", according to a text I'm
    reading. This is further explaned by "Low digital sum value (the number
    of one bits minus the number of zero bits) minimizes the low frequency
    and DC content of the data signal which permits optimal servo system
    operation."

    Why does low DC content permit "optimal servo system operation", and
    what is at all meant by "servo system operation"?

    Thanks in advance,
    Regards Carl
     
  2. Bob Masta

    Bob Masta Guest

    I'll take a guess at this. I assume the servo system they are
    talking about is the one responsible for tracking the signal
    on the disk. It tries to maximize signal strength by moving the
    sensor head radially as needed. Assuming it looks at the
    raw signal and is AC coupled, long stretches of 1s or 0s
    would give an erroneously low response that might interefere
    with tracking. There might also be issues if it is trying to
    derive a data clock based upon the frequency of edge
    transitions.

    Again, just a guess. Maybe somebody who actually knows
    what they are talking about will step forward... <g>

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  3. c.j[dot]w

    c.j[dot]w Guest

    Thanks for the answer.
    I don't really get it - could you explain further?
    This is true, and is one reason for wanting to avoid long streams of 1's
    and 0's. But as I understand the explanation I read, the other reason is
    more important.

    Anyone else is also welcome with other ideas, or support to the above
    answer.
     
  4. Active8

    Active8 Guest

    I think Bob's second answer is right. Normal information content is
    random and by keeping it balanced (low DC content) it appears more
    white noise like and is sure to have a frequency component at the
    data clock frequency or a multiple thereof say, bit rate * 14 bits,
    maybe. You reconstruct this clock frequency and compare it to a
    reference clock to get an error signal which "controls" the servo.

    That assumes the author considers the spindle speed as part of the
    servo system.

    As for radial tracking, I thought that was part of the focus system
    where the photo detectors keep the laser over the data track. At any
    rate, if it capacitively coupled, you don't want DC in the data, so
    3 zeros between all ones words might fix that.
     
  5. BobGardner

    BobGardner Guest

    CDs use CIRC and EFM.... Cross Interleave Reed-Solomon Coding and
    Eight-to-Fourteen Modulation. CIRC is an error correction scheme that adds
    about 50% redundancy to the other channel one frame behind, so hopefully, an
    error can be corrected with this addtional info (somehow?). But the EFM is a
    lookup table of 256 of the avilable 16 thou 14 bit codes carefully chosen so
    there is now run of 0s greater than 3. This allows the pll to stay synched even
    on quiet passeges with lots of 0s being transmitted. The analog data is 2 8 bit
    bytes... they feed each byte into the efm looker-upper. This is explained
    pretty well in Pohlman Digital Audio.
     
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