I recently bought a clock from you and the time is different to the pips on Radio4. Why?

When the time on BBC Radio is sounded there are five small pips lasting a tenth of a second and the final pips lasts half a second. On the final pip the time is exactly x.

The BBC receives its time from an atomic clock in the basement of television centre and is also cross synchronised with the time from the Rugby time signal. At this point the time is correct.

The issues lies when the radio signal is transmitted across the UK - the time taken for the signal to leave London and get to your radio makes the time different. When the signal is being sent to digital receivers such as DAB Radios, the signal is transmitted to satellites and back to earth, causing an even longer delay.

A more detailed answer..

The Greenwich Time Signal (abbreviated GTS) or BBC pips is a time code heard on some BBC radio stations 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. The actual moment when the hour changes is at the very beginning of the last 'long' pip.

When a leap second occurs, it is indicated by a seventh pip. In this case the first pip occurs at 23:59:55 (as usual) and there is a sixth short pip at 23:59:60 (the leap second) followed by the long pip at 00:00:00. 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. Prior to the invention of leap seconds, the final pip was the same length as the others.

Although only normally broadcast on the hour, the signal is also generated at quarter-past, half-past and quarter-to every hour. It has occasionally been broadcast at these times due to operator error.

Usage and homages

The pips are used on some BBC radio stations to mark the start of the hour. On Radio 4, at start of the 6 o'clock evening, 10pm and midnight news the pips are replaced by the chimes of Big Ben, where the first chime represents the start of the hour. In 1999, pip-like sounds were incorporated into BBC Television News by composer David Lowe. This included news on BBC ONE and later BBC NEWS 24 and BBC WORLD. The pips can be heard every hour on the BBC's worldwide radio station BBC World Service.

In the late 80s, BBC Radio 1 featured the pips played over a station jingle during Jacki Brambles early show and Simon Mayo's breakfast show. The pips are also currently featured on The Chris Moyles Show on BBC Radio 1 at 9am as part of the Tedious Link feature, 10am (at the end of the show), and sometimes before Newsbeat bulletins. They appear on BBC Radio 2 at 5pm and 8am, at which time Terry Wogan is renowned for 'crashing' them.

History

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.

The signal on the line was inverted; that is, the signal sent to the BBC was "on" when no pip was required, and was pulsed "off" when a pip should be sounded. This allowed a fault on the line to be detected easily.

Today the pips for national radio stations and some local radio stations 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. On other stations, the pips are generated locally from a GPS-synchronised clock.

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 as received on long wave 160 km (about 100 miles) from the Droitwich AM transmitter, which is the distance to central London.

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, Freeview as well as listening via the Internet; 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 travel to and from the satellite adds a further delay of around 0.25s.