What is DVB-SIPSI?

Answer 1

A DVB SIPSI is a unit installed between the multiplex output and the modulator for terrestrial television in the UK.

It takes in two feeds one containing audio and video encoded with mpeg2 and another containing PSI/SI data. (EPG data)

This data has come from a central collating source called the CSI which all broadcasters contribute to.

Without a SIPSI when you wouldn't be unable to see what an ITV channel was showing in the guide if you weren't watching an ITV channel at the time. Furthermore you wouldn't be able to get logical channel numbers etc.