An "Interval Signal" (IS) is a brief, easily - recognized sound which a station - especially an international station - transmits before it begins regular programming on any given frequency. OTL, the usage is limited to shortwave broadcasters; the purpose is to allow listeners to be sure they've tuned in the correct signal out of many possible signals.... the usual method is to find the frequency with the strongest IS for the broadcast they want to listen to. Stations also use the IS - OTL - when changing from one frequency to another - again, to allow the listener to adjust their radio for the new frequency. (It also allows the transmitter engineers to verify the transmitter is operating correctly on the new frequency).
NTL, VOA uses an Interval Signal pre-signon to give the operator a chance to verify the transmitter is working properly, and the listeners time to adjust their radios - especially the crystal sets, which may well drift off-frequency a bit, especially overnight. VOA will also be using the IS at power change - coming back up to 500 watts, 1kw, 2kw, after running at reduced output during the day. Again, this is to allow anyone trying to receive VOA Grantville time to fine-tune their radios.
A single broadcast of the IS (un-repeated) also occurs at the top of each hour - where it serves the same purpose for VOA as any other up-time radio station - a station ID; a reminder of what station you're listening to. While VOA is the only station on the air, this may seem trivial - but once VOL, and any other stations, start broadcasting, such as ACK ACK... (stupid snerk collar...)
Some examples of OTL interval signals:
The OTL Voice of America uses the opening bars of "Yankee Doodle", done by a fife and drum corps
LeSea Broadcasting uses "Onward, Christian Soldiers" (at least, the last time I heard W H R I)
Radio Paris used to use "The Marseilles" first few bars; again, I haven't caught them in a while.
The BBC used to use - probably still does ("stuffy Brits" :)) ) the Westminster Chimes - complete with Big Ben tolling the hour during operation.
Shortwave is an amazing part of the spectrum... all sorts of interesting stuff; spys, news, sports... Satellite TV has robbed Short Wave of some of its glory, maybe - but none of its magic.)
Oh, yeah... WHY is it called an "Interval Signal"?
Because it's the Signal a station broadcasts during the Interval before beginning operation, and between periods of operation, such as when changing frequencies, patterns, etc. (Shortwave pattern change is a bit more complicated than your local AM station - the entire array, which may consist of several different "wires" (actually copper-sheathed steel cables), needs to be re-tuned; your local AM station also "retunes", but they have it down to where they just switch between two presets, twice a day - sunrise and sunset (Critical Hours adds another pair of changes, and another preset - sometimes just a reduction in power - which is the exact opposite of OTL VOA's operating pattern - VOA Grantville actually runs maximum power during the Critical Hours... an hour before sunrise until about two hours after sunrise, and an hour before sunset until about two hours after sunset)
Well, to be fair, I should mention that there are a FEW AM stations that do NOT change anything - the "clear channel" stations - like 650 WSM, Nashville, TN; 700 WLW, Cincinnati, OH...which run 50,000 watts all the time, and in all directions (omnidirectional, or, as we say in the biz... 50 kw ND. most stations will be more like 5kw DA-N directional at night), or DA-2 (directional, but different day and night)... I know, more info than you ever thought you'd want...
so, as my Ham friends would say...
73
or as my reporter friends say -
- 30 -
Tom Spencer