Clock network
Encyclopedia
A clock network or clock system is a set of clocks designed to always show exactly the same time by communicating with each other. Clock networks usually include a central master clock
kept in sync with an official time source, and one or more slave clocks which receive and display the time from the master.
GPS satellite constellation, a Network Time Protocol
server, the CDMA cellular phone network, a modem
connection to a time source, or by listening to radio transmissions from WWV or WWVH
, or a special signal from an upstream broadcast network. Some master clocks don't determine the time automatically. Instead, they rely on an operator to manually set them.
Clock networks in critical applications often include a backup source to receive the time, or provisions to allow the master clock to maintain the time even if it loses access to its primary time source. For example, many master clocks can use the reliable frequency
of the alternating current
line they are connected to.
, a Network Time Protocol, or a Pulse Per Second
(PPS) contact. Others provide SMPTE time code outputs, which are often used in television settings to synchronize the video from multiple sources. Master Clocks often come equipped with programmable relay outputs to synchronize other devices such as lights, doors, etc.
, in particular, operates on a very strict schedule, where each second of airtime is planned ahead of time and must be executed precisely. A central clock system allows a television station's master control
and production
personnel to work within that schedule. A clock network synchronized to the standard UTC time also allows different television facilities to coordinate their activities without complicated out-of-band signaling. It also provides accurate timing to equipment in stations that are becoming increasingly automated.
While television broadcasters were some of the first users of clock networks, the equipment is becoming increasingly useful in other industries. For example, the National Emergency Number Association
issued directive NENA-04-002, offering standards in timekeeping for 911 dispatch centers throughout the United States. Other common clock network users include airports and schools.
outside the gate is still working.
In the period before the universal availability of A.C. mains or atomic clock
s, many clock networks were installed using a highly accurate pendulum clock
as a master clock. This clock resembled a longcase clock
, but had a very robust mechanism and a less ornate case. Electrical contacts attached to the mechanism generated minute, half minute and sometimes one second electrical pulses which were fed to the slave clocks on pairs of wires. The devices driven could be wall clocks, employee time clocks, tower clocks and occasionally clock chiming mechanisms. Some master clocks were set up to control the frequency of a generating authority's mains output; others, in the UK, were arranged to synchronise themselves with the Greenwich Time Signal
pips.
Reference clock
A reference clock may refer to the following:*A master clock used as a timekeeping standard to regulate or compare the accuracy of other clocks*In electronics and computing, the clock signal used to synchronise and schedule operations...
kept in sync with an official time source, and one or more slave clocks which receive and display the time from the master.
Synchronization sources
The master clock in a clock network can receive accurate time in a number of ways: through the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
GPS satellite constellation, a Network Time Protocol
Network Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
server, the CDMA cellular phone network, a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
connection to a time source, or by listening to radio transmissions from WWV or WWVH
WWVH
WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station in Kekaha, on the island of Kauai in the state of Hawaii....
, or a special signal from an upstream broadcast network. Some master clocks don't determine the time automatically. Instead, they rely on an operator to manually set them.
Clock networks in critical applications often include a backup source to receive the time, or provisions to allow the master clock to maintain the time even if it loses access to its primary time source. For example, many master clocks can use the reliable frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
of the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
line they are connected to.
Slave clocks
Slave clocks come in many shapes and sizes. They can connect to the master clock through either a cable or a short-range wireless signal. Some slave clocks will run independently if they lose the master signal, often with a warning light lit. Others will freeze until the connection is restored.Device synchronization
Many master clocks include the capability to synchronize devices like computers to the master clock signal. Common features include the transmission of the time via RS-232RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, a Network Time Protocol, or a Pulse Per Second
Pulse per second
A pulse per second is an electrical signal that very accurately repeats once per second . PPS signals are output by various types of precision clocks, including atomic clocks, radio clocks and some models of GPS receivers...
(PPS) contact. Others provide SMPTE time code outputs, which are often used in television settings to synchronize the video from multiple sources. Master Clocks often come equipped with programmable relay outputs to synchronize other devices such as lights, doors, etc.
Applications
One of the driving factors in developing clock networks was the broadcast industry. TelevisionTelevision
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
, in particular, operates on a very strict schedule, where each second of airtime is planned ahead of time and must be executed precisely. A central clock system allows a television station's master control
Master control
Master control is the technical hub of a broadcast operation common among most over-the-air television stations and television networks. It is distinct from a production control room in television studios where the activities such as switching from camera to camera are coordinated...
and production
Television producer
The primary role of a television Producer is to allow all aspects of video production, ranging from show idea development and cast hiring to shoot supervision and fact-checking...
personnel to work within that schedule. A clock network synchronized to the standard UTC time also allows different television facilities to coordinate their activities without complicated out-of-band signaling. It also provides accurate timing to equipment in stations that are becoming increasingly automated.
While television broadcasters were some of the first users of clock networks, the equipment is becoming increasingly useful in other industries. For example, the National Emergency Number Association
National Emergency Number Association
The National Emergency Number Association is an organization whose mission it is to foster the technological advancement, availability, and implementation of a universal emergency telephone number system in the United States. In carrying out its mission, NENA promotes research, planning, training...
issued directive NENA-04-002, offering standards in timekeeping for 911 dispatch centers throughout the United States. Other common clock network users include airports and schools.
History
One of the first clock networks was installed by Charles Shepherd for the Great Exhibition, held in London in 1851. Shepherd's technology was then installed at the Royal Greenwich Observatory, and his clockShepherd gate clock
The Shepherd Gate Clock is the clock mounted on the wall outside the gate of the Royal Greenwich Observatory building in Greenwich, London. The clock, an early example of an electric clock, was a slave mechanism controlled by electric pulses transmitted by a master clock inside the main building...
outside the gate is still working.
In the period before the universal availability of A.C. mains or atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
s, many clock networks were installed using a highly accurate pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...
as a master clock. This clock resembled a longcase clock
Longcase clock
A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...
, but had a very robust mechanism and a less ornate case. Electrical contacts attached to the mechanism generated minute, half minute and sometimes one second electrical pulses which were fed to the slave clocks on pairs of wires. The devices driven could be wall clocks, employee time clocks, tower clocks and occasionally clock chiming mechanisms. Some master clocks were set up to control the frequency of a generating authority's mains output; others, in the UK, were arranged to synchronise themselves with the Greenwich Time Signal
Greenwich Time Signal
The Greenwich Time Signal , popularly known as the pips, is a series of six short tones broadcast at one-second intervals by many BBC Radio stations to mark the precise start of each hour...
pips.