This is a common frequency that can be derived from 625/50 PAL (and 525/60 NTSC) that runs through SD, HD and UHD digital television standards. It can be created from the analog black and burst signal. 2.25 MHz, or multiples thereof, runs through all the major digital television standards. These can be locked to black and burst. They include 1080-line HD at 25, 30 fps.
See also: 13.5, Tri-level sync