Macrovision and Analog Copy Protection in VOB Streams

This article provides an overview of Macrovision analog copy protection and explains how DVD-Video streams communicate these protection triggers to playback hardware. You will learn the mechanical basis of the Macrovision scrambling process, the structure of MPEG-2 Video Object (VOB) files, and the specific data flags within navigation packets that instruct DVD players to activate analog protection on composite, S-Video, and component video outputs.

What is Macrovision?

Macrovision—now officially known as the Analog Protection System (APS)—is an analog copy protection technology developed to prevent consumers from copying commercial video media, such as VHS tapes and DVDs, onto consumer-grade videocassette recorders (VCRs).

Macrovision operates by exploiting the vulnerabilities of a VCR’s automatic gain control (AGC) circuitry. When an analog video signal is generated, Macrovision inserts deliberate voltage anomalies into the Vertical Blanking Interval (VBI)—the non-visible lines of video transmitted between display frames. These anomalies primarily include:

  1. AGC Pulses (Pseudo-sync pulses): Excessive positive and negative voltage spikes inserted into the vertical sync interval. Televisions ignore these lines, but a recording VCR interprets the high-voltage spikes as an excessively bright picture and lowers its recording gain. This results in recorded tapes with fluctuating brightness, severe picture darkening, or complete loss of video sync.
  2. Colorstripe: Rapid phase shifts applied to the colorburst reference signal on specific scanlines. This interferes with the VCR's color-processing circuitry, causing recorded video to display vertical color bands, loss of color, or rolling artifacts.

Because a DVD stores digital data, Macrovision is not recorded as an altered analog waveform on the disc. Instead, digital flags within the video container tell the playback hardware to generate the distorted analog waveform in real time via its digital-to-analog converter (DAC).

How Analog Copy Protection is Signaled in VOB Streams

A DVD’s video content is stored in VOB (Video Object) files, which are based on the MPEG-2 Program Stream format. Inside a VOB stream, the video data is divided into sequential chunks called Video Object Units (VOBUs). Each VOBU represents between 0.4 and 1.0 seconds of playback and begins with a specialized packet called the Navigation Packet (NV_PCK).

The signaling for analog copy protection occurs inside this NV_PCK rather than the elementary MPEG-2 video stream itself.

The NV_PCK Structure and Presentation Control Information

The NV_PCK contains two primary sub-structures:

Analog copy protection directives reside inside the PCI packet within the PCI General Information (PCI_GI) table.

APS Bits in the Stream

Within PCI_GI, specific bits are allocated for the Copy Generation Management System (CGMS) and the Analog Protection System (APS). The APS data is encoded using a 2-bit field:

These flags are defined dynamically at the VOBU level. This allows content authors to toggle Macrovision on or off between different sections of the disc (for example, keeping Macrovision active during the main feature while disabling it during interactive menus or bonus material).

Hardware Execution

When a DVD player reads the VOB stream:

  1. The demultiplexer isolates the NV_PCK from the incoming VOBU.
  2. The player’s system parser reads the APS bits inside the PCI_GI structure.
  3. The player passes these trigger bits to the internal video encoder/DAC chip.
  4. If an APS mode (01b, 10b, or 11b) is present, the DAC chip’s embedded Macrovision circuit actively modulates the VBI of the outgoing NTSC or PAL baseband analog signal.
  5. If the video is routed through a digital output (such as HDMI), the APS bits are ignored in favor of digital content protection schemes like HDCP, though the player may mirror the restriction across concurrent analog outputs.