Modern TV's have some form of digital TV built in but older sets need an add on box. There are a whole variety of boxes from cigarette box size to the all singing dancing with hard drives and DVD recorders.
There are two types of signal DVB-S and DVB-T the S uses a satellite dish and the T uses a Yargi Beam some have HD outputs some have SCART output and some use an RF output.
By using these converters and maybe a transponder if old 405 line set even sets built in the 1930's will still receive TV signals.
However there is a problem with the time signal sent out on analogue as it seems the converter boxes do not re-transmit the time signal.