Science Fair Project Encyclopedia
Greenwich Time Signal
The Greenwich Time Signal or BBC pips is a time code heard on some BBC radio programs at the start of the hour, most notably on Radio 4 and the World Service.
There are six pips (short beeps) in total, which occur on the 5 seconds leading up to the hour and on the hour itself. Each pip is a 1 kHz tone which, for the five leading pips, lasts a tenth of a second, while the final pip lasts half a second. When a leap second occurs, it is indicated by a seventh pip. The leap second is also the explanation for the final pip being longer than the others. This is so that it is always clear which pip is on the hour, especially where there is an extra pip that some people might not be expecting. On Radio 4, at start of the 6 o'clock evening and midnight news the pips are replaced by the chimes of Big Ben, where the first chime represents the start of the hour. Since 1999, the pips have also been included into David Lowe's branding music and on-the-hour countdown of BBC News 24 and the resulting relaunch of BBC network news.
The pips have been broadcast since February 5, 1924, and were the idea of the Astronomer Royal Sir Frank Watson Dyson and head of the BBC John Reith. The pips were originally controlled by two mechanical clocks located in the Royal Greenwich Observatory that had electrical contacts attached to the pendulums. Two clocks were used in case of a breakdown. These sent a signal each second to the BBC, who converted them to the audible oscillatory signal that is broadcast.
Today the pips are timed relative to UTC, obtained from an atomic clock located in the basement of Broadcasting House that is synchronised with British Telecom's Rugby time signal and GPS. Although only normally broadcast on the hour, the signal is also generated at quarter-past, half-past and quarter-to every hour.
The BBC compensates for the time delay in both broadcasting and receiving equipment, as well as the time for the actual transmission. The pips are timed so that they are accurate for people living 100 miles from Broadcasting House. The pips are therefore most exact for people living in Leicester, Norwich, Swindon and Calais in France.
Newer digital broadcasting methods have introduced even greater problems for the accuracy of the pips: on platforms which use digital compression such as DAB, digital satellite and Freeview the pips are no longer exactly on the hour. The encoding and decoding of the digital signal can cause a slight delay. In the case of satellite broadcasting, the distance the signal has to travel to reach and return from the satellite causes the time signal to be delayed. The length of this delay depends on the orbital distance of the satellite, but is typically several seconds.
As a contribution to the 2005 Red Nose Day charity day the BBC have developed a "pips" ring-tone. See the external links for details.
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