One sort of public time signal is, of course, a striking clock. These clocks, however, are only as good as the clockwork that activates them; they have improved substantially since the first surviving clocks from the fourteenth century. For many members of the general public, a public clock such as Big Ben was the only time standard they needed.
When more accurate public time signals were desired for use in navigation, a number of traditional audible or visible time signals were established for the purpose of allowing navigators to check their marine chronometers. These public time signals were formerly established in many seaport cities.
As an example of such a signal, in Vancouver, British Columbia, a "9 O'Clock Gun" is still shot every night at 9 p.m.. This gun was brought to Stanley Park by the Department of Fisheries in 1894 to warn fishermen of the 6:00 p.m. Sunday closing of fishing. The 9:00 p.m. firing was later established as a time signal for the general population. The Brockton Point lighthouse keeper, William D. Jones, originally detonated a stick of dynamite until the Time Gun was installed. A similar "Noon Gun" is still shot every noon at Cape Town, South Africa. A cannon was shot at one o'clock every weekday at Liverpool, England, at the Castle in Edinburgh, Scotland, and also at Perth in Australia to establish the time. The Edinburgh "One O'Clock Gun" is still in operation. A cannon located at the top of Santa Lucia Hill, in Santiago, Chile, is shot every noon. In places where a cannon is used for a time signal, locals often joke that they can spot tourists because they jump in surprise while locals check their watches. In many Midwestern US cities where tornados are a common hazard, the emergency sirens are tested regularly at a specified time (such as, say, noon each Saturday); while this is not primarily intended as a time signal, locals often do check their watches when they hear it.
The ceremony of "dropping the ball" at New Year's Eve in Times Square in New York City is a vestige of a visual indication of time. The first such time ball was installed in 1833 on the roof of the Royal Observatory in Greenwich, England. The ball was set to drop at 1 P.M. to enable the astronomers to establish the correct moment it should drop by observing the sun at noon. Liverpool, Edinburgh, and Perth also had balls that were dropped concurrently with the firing of the time cannons. Because the speed of light is much faster than the speed of sound, visible signals enabled greater precision than audible ones; however, the audible signals could operate under conditions of reduced visibility. In 1861 and 1862, the Post Office Directory had time gun maps published that related the number of seconds it took for the report of the time gun to reach various locations in Edinburgh.
Sandford Fleming proposed a single 24-hour clock for the entire world. At a meeting of the Royal Canadian Institute on February 8, 1879 he linked it to the anti-meridian of Greenwich (now 180°). He suggested that standard time zones could be used locally, but they were subordinate to his single world time.
Standard time came into existence in the United States on November 18, 1883. Earlier, on October 11, 1883, the General Time Convention, forerunner to the American Railway Association, approved a plan that divided the United States into several time zones. On that November day, the United States Naval Observatory telegraphed a signal that coordinated noon at Eastern standard time with 11 a.m. Central, 10 a.m. Mountain, and 9 a.m. Pacific standard time.
A March, 1905 issue of The Technical World describes the role of the United States Naval Observatory as a source of time signals:
The audio portions of the WWV and WWVH broadcasts can also be heard by telephone. The time announcements are normally delayed by less than 30 ms when using land lines from within the continental United States, and the stability (delay variation) is generally < 1 ms. When mobile phones are used, the delays are often more than 100 ms due to the multiple access methods used to share cell channels. In rare instances when the telephone connection is made by satellite, the time is delayed by 250 to 500 ms. To hear these broadcasts, dial (303) 499-7111 for WWV (Colorado), and (808) 335-4363 for WWVH (Hawaii). Callers are disconnected after 2 minutes. These are not toll-free numbers; callers outside the local calling area are charged for the call at regular long distance rates.
Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.
WCBS and many other CBS Radio Network affiliates air a "bong" (sounding A=440Hz) that immediately precedes each top-of-the-hour network newscast. (The same bong could be heard on CBS Television Network affiliates in the 1970s.) An automated "chirp" at one second before the hour signals a switch to the network broadcast.
In Los Angeles, KFWB broadcasts a tone precisely on the hour and half-hour during regular news broadcasts (although not during special programming such as baseball games.) KNX also broadcasts a tone on the hour and half-hour.
In Canada, the national non-commercial CBC Radio network broadcasts the daily National Research Council Time Signal at noon (Eastern Time, GMT-5) on French language Radio-Canada and one hour later on the English language CBC network.
Australia lacks an easily available source of accurate time. The VNG User's Consortium is campaigning for the availability of accurate time in Australia, while developing ways around this problem. The Global Positioning System can also be used as a time reference for radio clocks, but require an accurate 1PPS output to be reliably used for time signals. A device is being developed by the VNG User's Consortium that will convert GPS signals into an accurate time signal, to replace the now-defunct VNG service.
Semiconductor options for real-time signal processing: designers of real-time-signal-processing systems face lots of options and an ever-changing technology landscape.(design feature)
Nov 25, 2004; IF A UNIVERSAL SEMICONDUCTOR component existed that could allow engineers to realize every real-time-signal-processing system...