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DVD Copy Protection

Discussion in 'Electronic Design' started by Norm Dresner, Jan 25, 2004.

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  1. Norm Dresner

    Norm Dresner Guest

    In addition to being an engineer, I'm also a consumer and I've run into a
    situation that I'm curious about. A friend gave us a new DVD player as a
    Christmas present which had only audio/video outputs. Since our TV set is
    an older model (circa 1986) , it lacks these inputs but our VCR had them so
    I glibly plugged the DVD player into it and tried to watch a DVD. Of
    course, it doesn't work and the DVD player manual explicitly says that it
    won't because of copy-protection.

    I'm not looking to break the system -- I fully intend to buy a new TV set --
    but I'm technically curious. What are they doing to the signal that makes
    it acceptable to a TV set but unusable to a VCR? I'd assume they're mucking
    with the sync signals but how does one work and not the other?

  2. Jim Thompson

    Jim Thompson Guest

    What does "doesn't work" mean? Picture changing brightness and
    flopping in and out of sync? If that's the case, it's Macrovision.
    Just buy a Macrovision killer... they're cheap.

    ...Jim Thompson

  3. Same thing happens in satellite digital TV here in the UK, where certain
    "pay-per-view" movies are copy-protected and supposedly cannot be recorded
    on VCRs. I've tried recording these on my VCR and they recorded fine... It
    could be that modern VCR manufacturers have agreements with the movie
    companies and cable/satellite networks and have special circuitry that
    recognises certain signals hidden in the video signal and scrambles it. This
    is pure guessing though, I don't know how it's done. But as said above, I am
    able to successfully record the copy-protected satellite TV movies, so it
    seems to be something that depends on the type/brand of VCR.

  4. Ron G

    Ron G Guest

    Hi Jim---

    Where are they available at?

    Special Internet stores, or local electronics stores?

    Are there various updates?
    I had never heard of them, before reading your post.

    Thanks and Best to you---
  5. Jim Thompson

    Jim Thompson Guest


    Surf for "Sync Stabilizers". They're pretty easy to make, if you're
    fairly conversant in video formats. It's as simple as cleaning up
    garbage added in the VBI (vertical blanking interval).

    A few starting places...

    ...Jim Thompson
  6. Joel Kolstad

    Joel Kolstad Guest

    (Assuming it's Macrovision, as already described by Jim Thompson...)

    The automatic gain control on the video signal inputs on VCRs has a much
    slower response than that of a TV. This is due to the fact that VCRs have
    to deal with physical media -- tape -- that tends to suffer sometimes rapid
    signal level variations due to mechanical misalignment, aging tape, etc. so
    the slow AGC time-constant 'evens out' these variations.

    Macrovision, at its simplest level, places very high average level signals
    into the video lines during the vertical blanking interval of the frame,
    then slowly moves to placing very low average level signals there, and
    repeating. This causes the a VCR to first noticeably crank down its AGC,
    then crank it up again, etc. -- but with a time-constant on the order of
    several frames. Hence you see alternating dark and bright images on your

    When a macrovision tape is directly connected to a TV, the TV's faster AGC
    recovers quickly enough so that -- within a few _lines_ of being outside of
    the vertical blanking interval -- the AGC level is 'correct' and you view
    the picture as intended.

    It's not a perfect system. There used to be lists of VCRs that had faster
    AGCs that weren't particularly affected by Macrovision (as well as a few
    professional units that had manually set gain controls), but at some point
    various operating parameters of VCRs were 'harmonized' and most all units
    today are affected. Likewise, there were some TVs that had comparatively
    slow AGCs and wouldn't work with Macorvision either, but this too is now
    more of a historical curiosity.

    The cheap 'Macrovision killer' boxes out there just wait for the vertical
    blanking interval and then clamp the video level of each line to a fixed
    value, thereby not causing the AGC in the VCR to get thrown out of whack.

    There's tons of information about this on the Internet, and Macrovision
    themselves has a web site too. New versions of Macrovision are presumably
    more robust than the original (there have been many different versions over
    time), but there's still only so much they can do while keeping the signal
    compatible with a regular TV set. What I find amazing is that the
    Macrovision guys talked the DVD player guys into _adding_
    Macrovision-encoded outputs to the players -- hence everyone who purchases a
    DVD player is paying to have some of their rights to produce back-up copies
    of a DVD taken away. (Even though I would be the first to admit that about
    99% of people attempting to defeat Macrovision or any other copy protection
    scheme do so strictly with the intent to pirate the content.)

    ---Joel Kolstad
  7. As well as Macrovision copy protection as others have mentioned in this
    thread, there is CGMS. (Copy Protection Management System) It's encoded
    within the sync frame, and is only really useful against systems that
    specifically look for it. (Doesn't stop tape duplication)
    My PVR detects it, and as such, a "normal" DVD player won't work regardless
    of macrovision status.
  8. Iwo Mergler

    Iwo Mergler Guest

    Most modern recording devices don't have the AGC problems
    any more. In fact, the Macrovision signal is recognised
    by a specialised circuit and it's effect on old AGC circuitry
    is emulated to stop copying. These days, you can regard
    Macrovision as a do-not-copy flag.

    Kind regards,

  9. That is actually not correct.
    The modulation is FM, so the output signal is not depending
    on the signal strength from the tape.
    (but chroma is converted down and superimposed on the FM carrier).
    The AGC is in the INPUT chain.
    The AGC is only there to adjust the (can be any value almost) input
    video level so the FM carrier deviation is norm when recording.
    I published a macro vision remover with a PIC and 74HC4053 some month ago
    on (for the vertical interval pulses type).
    A build one is available from me for 40 Euro.

    Making a digital backup is no problem, most stuff is digital these days, DVD players
    are much cheaper then VHS now.
  10. Hmm. Strange. My VCR will pass a Macrovision protected video signal from
    video in to video out or the modulator. It just won't record properly.
    I'm assuming that this is what you are trying to do (with the VCR's RF
  11. Joel Kolstad

    Joel Kolstad Guest

    And do you happen to know why _this_ doesn't seriosuly mess up the PLL in
    the TV and thereby distort the colors?

    ---Joel Kolstad
  12. Luhan Monat

    Luhan Monat Guest

    Today's DVD players generate Macrovision II. There are two components.

    1) large data pulses a placed in the Video Blanking Interval and effect
    the AGC in most VCRs.

    2) 4 out of every 20 or so lines has their Chroma Burst signal phase

    Clamping down the data pulses in (1) along with simply removing the
    errant color burst on the offencing lines renders the signal clean
    enough for good recording.

    But hey, this is all just theoretical unless someone was really good
    with a PIC16F628.
  13. Roy Battell

    Roy Battell Guest

    Damn-it - on My 4 year old Toshiba it does, with direct SCART
    connection from a Pioneer DVD player with NTSC movies even
    using RGB.
    Forcing the player to output PAL solved the problem
    As it says in the logo 'Macrovision - quality protection' -
    i.e. you don't get any :-(
  14. FrAgFo0d

    FrAgFo0d Guest

    How about LG?
  15. Joel Kolstad

    Joel Kolstad Guest

    Yes, it occurred to me that my post didn't really answer the question of,
    'So why is there a problem when I just run video through the VCR?' -- thanks
    for the clarification. But clearly I was just plain wrong about where the
    AGC! Do you know if there is any reason for the AGC to have such a slow
    time constant other than to support Macrovision?
    For technically inclined people, yes. For the average consumer, we're not
    quite at the point where most of them can easily duplicate their DVDs (DVD
    burners have just become 'pretty darned cheap' in the past 3-6 months?), and
    the studios are doing everything in their power to keep it annoying to do
    Yep, fewer moving parts!

    ---Joel Kolstad
  16. Luhan Monat

    Luhan Monat Guest

    The TV uses the sync burst on each horizontal line to lock to the color
    signal. If no burst signal is present it simply goes on at its present
    prequency until the next one is found. VCR's, on the other hand, need
    to sync to each line before recording them.

    For a lot of information (public domain) look up Macrovision's patents
    at USPTO.GOV - they also list patents on ways to defeat their system.
    That way, they can go after companys that sell the decoding devices
    under patent law.
  17. This is interesting, what do you mean by that?
    Anyways in PAL the burst phase changes each line by 90 degrees,
    if you omit a burst, chances are the 9.8 kHz (derived from the burst swing)
    (h/2) is interrupted, and you get BW (color killer), although color killer
    seems hardly used anymore.
    Maybe NTSC world is simpler...
    There is no technical reason why the VCR should need a NTSC burst each line,
    the chroma is mixed with some frequency locked to H and then superimposed on the
    FM carrier to the heads.
    In playback that signal is taken from the heads before the FM limiter with a lowpass,
    and mixed up again using a oscillator locked to H.
    That way timing changes (as hor sync changes) are compensated in the final chroma.
    (and you get right color).
    So there must be an other reason for the color effect.
    I looked up macrovision on that patent site, but no technical text of significance...
    Maybe I missed it, can you be more specific about this?
    Perhaps the chroma AGC in the VCR turns up the color a lot if a burst is missing,
    burst is used for this gain control.
    That would perhaps be possible.
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