A Lesson In Audio Tone Signaling for Radio Control
Many pagers and other radio controlled equipment used in public safety utilize broadcasted audio tones to select specific devices that share a common radio frequency. In the industry, this is known as tone paging.
In the real world, many radio transmitters and receivers share a common frequency. In this arrangement, anything being transmitted on this frequency can be 'picked up' by any receiver tuned in to the same channel. However, there are times when a transmitted signal is intended for a specific receiver. To ensure that the target receiver is the only one to receive the message, tone signaling is often used.
In this basic method of encryption, tones are broadcast by a device called an encoder. When the tones are picked up by the receiver, they are analyzed by a decoder. If the transmitted tones match the tones stored in the decoder, then it successfully 'wakes up' the target receiver. If the tones do not match the decoder's programming, then the transmitted signal is ignored.
There are several different types of tone paging formats in use today. They include single tone, two-tone, group call, 5/6 tone, and DTMF. The rest of this discussion will concentrate on two-tone paging, as it is the most common choice for inexpensive, yet secure and reliable radio tone control. DTMF tone paging has gained popularity over the years, but caution is advised when selecting this type of paging format for a warning siren system, as it is sometimes easy to compromise the integrity of this format. DTMF paging uses the same tones as the ones built into a standard pushbutton telephone.
Valid two-tone pages are comprised of four basic elements:
A first tone frequency (also known as the 'A' tone) usually in a range of 300Hz to 3KHz;
A first tone time - the amount of time that a tone must be transmitted;
The second tone frequency (known as the 'B' tone), and;
The second tone time.
For a two-tone signal to be valid, the first tone must be transmitted at a pre-determined length of time, next, there may or may not be a gap of silence. Then, the second tone is transmitted at its predetermined time length. If any aspect of this paging sequence is not reproduced exactly, then the page is not successful. This lends to the inegrity of the system.
Of course, there are other methods of paging and signaling using digital encrypted technology, which is far more advanced than the audio tone signaling. Examples of devices that use higher levels of security include alphanumeric pagers, cell phones, satellite transmissions, and the like. This type of technology is also available for warning siren systems. It is known as SCADA telemetry. For most applications, this type of signaling is more technology than is needed. However, it is widely used in siren systems for nuclear power plants, some storm warning systems, and military operations, to name a few.
It isn't accurate to say that one type of radio signaling is superior to the other any more than it would be accurate to say that a car is superior to a truck. While the basic function is the same for both, each one has a specific application that it is best suited for.