Analog Protection System

The Analog Protection System (APS), also known as Copyguard or Macrovision,[1] is a VHS[2] and DVD copy protection system originally developed by Macrovision. Video tapes copied from DVDs encoded with APS become garbled and unwatchable. The process works by adding pulses to analog video signals to negatively impact the AGC circuit of a recording device. In digital devices, changes to the analog video signal are created by a chip that converts the digital video to analog within the device. In DVD players, trigger bits are created during DVD authoring to inform the APS that it should be applied to DVD players' analog outputs or analog video outputs on a PC while playing back a protected DVD-Video disc. In set top boxes trigger bits are incorporated into Conditional Access Entitlement Control Messages (ECM) in the stream delivered to the STB.

Image Lego Technic Advert PAL visually showing black and white macrovision blocks in the top and left hand corrner of the full composite signal frame.
Macrovision observed on a VHS tape decoded by VHS-Decode in 2023, black & white blocks on the top and left side of the video signal.

In VHS, alterations to the analog video signal are added in a Macrovision-provided “processor box” used by duplicators.

Bypassing & Defeating

Devices marketed as "Video Stabilizers",[3] along with time base correctors,[4] may be used to attempt removal of the Macrovision copy protection mechanism, with software defined decoding like with VHS-Decode[5] ignoring the disruption of signal entirely showing each frame in a stable state.

Digital recording devices (DVD recorders) often disallow the recording if they detect a protection signal on the input. The unit may display an error message about the program being copy protected.

APS can be also signaled digitally, in the CGMS-A bit field sent in the vertical blanking interval.

References
  1. "Rolling Old School With Copy Protection From The 1980s". May 28, 2018.
  2. "Macrovision Protection and Non Commercial VHS Tapes | NEPA Digital". www.nepadigital.com.
  3. "What to Know About Video Copy Protection and DVD Recording". Lifewire.
  4. Archived at Ghostarchive and the Wayback Machine: Macrovision vs a Time Base Corrector. YouTube.
  5. oyvindln (2023-03-28), VHS-Decode (It does more than VHS now!), retrieved 2023-03-29
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