The NTSC video standard (the broadcast standard used in North America and Japan) is defined with a 525-line vertical resolution. However, only 480 of those lines are used for transmitting video information. The extra 45 lines are used to carry control codes (such as interlace information), closed captions, and other similar non-video content. Macrovision copy protection works by adding certain codes to these control lines that are interpreted by an Automatic Gain Control chip in a VCR to scramble the video signal if the video is being recorded. Videocassettes that are copied from Macrovision-encoded source material will frequently exhibit color loss, image tearing, variable brightness, and picture instability. Since TVs and video switch boxes do not have Automatic Gain Control circuitry, the Macrovision signals are ignored when the DVD player is connected directly to the television, or indirectly through an A/V switching receiver or switchbox.
However, for this technology to work, the VCR that is used to make an unauthorized copy must contain the Automatic Gain Control circuitry, or no picture distortion will be witnessed. Macrovision estimates that 85% of the VCRs currently in use have automatic gain control circuitry in them, providing copy protection for nearly 550,000,000 videocassettes every year.
In addition to improperly designed VCRs, some TVs are also incapable of resolving the Macrovision signal. Some Macrovision signals resemble bad synchronization pulses (the signals that tell the television where the electron guns should be at what time), and some TVs confuse these signals with real synchronization pulses. This problem primarily manifests itself with rental tapes having unstable pictures; however, it may also affect DVD. Alarmingly, this problem isn't restricted to older televisions - in fact, televisions produced after the widespread use of Macrovision copy protection are more likely to suffer from this problem than older TVs.
Additionally, despite all the claims that DVD produces the best quality image available, it is well known in technical circles that the image quality from DVDs could be even better. Most TVs use a technique called interlacing: rather than drawing each of the 480 visible scan lines every frame, only 240 are drawn in an alternating pattern. Since DVD is a digital format, it is actually possible to output all 480 scan lines at once, and on high-quality TVs (which have Progressive inputs, in addition to composite and S-Video), such images are much more stable than interlaced images. DVDs implement Macrovision by attaching a small code to the video stream indicating whether or not to enable Macrovision protection. Then, when the DVD player outputs the frame, it adds Macrovision codes to the frame; however, due to technical limitations, it has proven extremely difficult to create a DVD player that can output 480 scan lines per frame in addition to Macrovision control codes. Because this could potentially allow unscrupulous users to connect their DVD players to computer-based recording devices and create perfect copies, the MPAA and DVD Copy Control Association (DVD CCA) stalled the release of progressive-scan DVD players for nearly 18 months.
Unfortunately, due to the DMCA, such devices are no longer legal. On April 26, 2002, under section 1201k of the Digital Millenium Copyright Act, no analog video recording devices may be manufactured that do not contain Automatic Gain Control circuitry. Additionally, on October 26 of this year, the sale, purchase, or manufacture of any device that disables Macrovision copy protection will be illegal under section 1201a of the Digital Millenium Copyright Act.