UHF transmissions started in 1964 and closed in 2012. The launch channel was BBC 2. This would be joined by BBC 1, the ITV network, Channel 4 or S4C in Wales, Channel 5 as well as a network of local TV channels. Transmissions started using the System I standard, a 625-line monochrome picture at 25 frames/second (576i) and a 4:3 aspect ratio. Technical advancements included colour (1967), teletext (1974), and stereo sound (1991). The drive to switch viewers from analogue to digital transmissions was a process called the digital switchover.
