You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Sky_(United_Kingdom)

Context:
BSkyB's standard definition broadcasts are in DVB-compliant MPEG-2, with the Sky Movies and Sky Box Office channels including optional Dolby Digital soundtracks for recent films, although these are only accessible with a Sky+ box. Sky+ HD material is broadcast using MPEG-4 and most of the HD material uses the DVB-S2 standard. Interactive services and 7-day EPG use the proprietary OpenTV system, with set-top boxes including modems for a return path. Sky News, amongst other channels, provides a pseudo-video on demand interactive service by broadcasting looping video streams.

Question:
What is the proprietary system that Sky+HD uses?

OpenTV