The Advanced Access Content System was established by an eight-company consortium to prevent unauthorized copying of high definition formats (Blu-Ray Disk). It requires hardware manufacturers to bar high definition signals from being sent over analog connections. An AACS license is established that all device manufacturers have to sign.
High definition output should be only available over HDMI (HDCP), analogue connections should only output down-converted signals. The HD 1920x1080 signal will be down-converted to a 960 horizontal by 540 vertical line picture. For comparison, the standard DVD format is 720 horizontal by 480 vertical lines. The 960x540 format has still a 50% higher resolution than the standard DVD resolution, but it is only one-quarter of the full HD resolution.
The Blu-Ray DVD players are required to recognize and respond to a digital flag, called the Image Constraint Token (ICT). The ICT is a protocol flag set within the BlueRay's disc code. The decision to set the flag is made by the content owner. On the same disk one movie could have it enabled while another could have it disabled. Studios are required to label the discs on the outside of the box whether ICT is enabled or not.
If the flag is set to "on" the player down-converts all analog outputs of the device. Set to "off" the player is 'allowed' to output full HD signals over analogue connectors.
The image that is down-converted to 540 lines is permitted to be up-scaled again to 720 or 1080 lines. This way the output still works with a usable image quality but no real HD content is available through the analogue outputs.