Broadcasters have the ability to block Freesat receivers' component output for particular shows, it has emerged.
A flag can be transmitted with programme information which disables the connection. The function, a requirement of the Freesat equipment specification, is aimed at preventing piracy of high quality video.
Where the flag is used - likely to be on more valuable HD broadcasts such as sport, films and premium TV shows - viewers will only be able to watch in high definition via the HDMI connection. A relatively small number of people with HDTVs without HDMI inputs will not be able to watch the shows in HD at all.
The change will also prevent the use of some external devices to record the HD content. HDMI, unlike component, supports HDCP - high bandwidth digital content protection - a standard designed to make copying HD content more difficult.
Digital Spy forum members realised the block had been enabled in an over-the-air software update to Humax set-top boxes on Tuesday. Several BBC HD programmes have carried the preventative flag.
Humax representatives, who regularly provide support on the DS forums, confirmed the change was part of the Freesat specification.
Broadcasters also have the option of limiting output via the SCART connection to the lower-quality composite video format.
Sky began phasing out component output from its HD set-top boxes for anti-piracy reasons last month, though it is present and enabled on most of the boxes in circulation.
