A network clock or clock system is a set of synchronized clocks designed to always show exactly the same time by communicating with each other. Clock networks usually consist of 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.
Synchronization Sources
The master clock in a clock network can receive accurate time in a number of ways: through the United States 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 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.
Network Clock Function
A network clock in critical applications often includes 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 to which they are connected.
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-232, 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.
Network Clock Applications
One of the driving factors in developing clock networks was the broadcast industry. Television, 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.
