The TD-4 Select ive Call ing Module is an economy touch-tone decoder with one latching output. This versatile mod-ule is primarily designed to mute the speaker of a receiver or transceiver until someone calls by sending a 4-digit dtmf signal, thus making it unnecessary to listen to all the activity on a channel just so someone can call you once. The Kaar units looked much like a Sonar with no channel selector and were fitted with an aftermarket Speedcall IIRC LC filter based two single tone decoder that operated either group or individual call that was fitted after CB user incursion began to occur sometime after the 60's and wasn't a factory product. A single selective call consists of a combination of four pre-selected audio tones whose transmission requires approximately 2 seconds. The tones are generated in the aeronautical station coder and are received by a decoder connected to the audio output of the airborne receiver. Jet Call Selcal Decoder. Ground-to-air Selective Calling Decoder. External thumbwheel coding. No wire jumpers or remote switches. Two or five decoder channels are available to handle VHF and/or 2 HF transceivers. 16 available code letter tones for all four codewheels provide 43680 code letter combinations and 10920 tones.
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Two-Tone Paging Signals
Many public safety pagers and control systems use audio tones to activate. The most common type are two tones that are sent in sequence. Hence it’s names, Two-Tone Paging or Two-Tone Sequential.
A valid two-tone page is comprised of four elements:
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- First tone frequency (“A” Tone) – usually between 300 Hz and 3000 Hz.
- First tone time – minimum time that the tone must be present.
- Second tone frequency (“B” Tone) – usually between 300 Hz and 3000 Hz.
- Second tone time – minimum time that the tone must be present.
If all these items match what the decoder is expecting, it processes the signal and takes action such as beeping, engaging relays, starting times, etc.
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If any of these items are incorrect, the page is ignored, and the decoder waits to analyze the next paging signal.
We also take this normal structure and extend it much, much further with non-predictive, Free Format Control.
Listen to an actual page. 600.9 Hz / 788.5 Hz with a 1 second / 3 second timing. This is the Motorola Quick Call 2, Reed Group 2, Code 16.
Listen to an actual page. 788.5 Hz / 600.9 Hz with a 1 second / 3 second timing. This is the Motorola Quick Call 2, Reed Group 2, Code 61. You may have noticed this two-tone page uses the same tones as the page above it, but the tones are reversed. This is quite acceptable. The decoder is watching for the correct tones and the correct tone timing so it treats both the first audio page and the second audio page as completely different events.
Listen to an actual page. 378.6 Hz / 422.1 Hz with a 1 second / 3 second timing. This is the Motorola Quick Call 2, Reed Group 4, Code 35.
These pages are all sent over the same radio channel for every radio, pager and remote decoder in the area to hear, and decide if the signal is intended for them to initiate some action.
- In the above actual radio pages, the first page activates firemen’s pagers as well as an RXC-2000 which turns on the lights at the station, unlocks the entrance door and turns on the PA system inside of the building so everyone can hear the dispatcher’s message.
- The second page is to alert an Ambulance crew’s personal pagers.
- The third page activates an RXC-2000 which is attached to an outdoor storm warning siren. The RXC turns on the siren for three minutes to alert the local community.
Example
Consider a paging signal that has the following characteristics …
- 1 second duration of first tone.
- 3 second duration of second tone.
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In the communications industry, this would be referred to 321.7 / 339.6 at 1 second / 3 second
When this signal is transmitted over the two-way radio, any pagers or control equipment such as sirens, pumps, etc. would be activated if this paged matched what they were looking for.
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If any part of the page did not match the decoder in the field, then the radio decoder or pager would ignore the page and would not activate.
If the tone frequencies were both correct, but the decoder required 3 seconds of the first tone, then it would ignore the signal when it only received 1 second of the first tone.
Single-Tone Paging
Single-Tone paging is many times referred to as “All-Call” Paging. Where Two-tone pages are normally employed to contact an individual, or a group of decoders or pagers, the All-Call paging signals are normally used to activate all groups. An Single-tone page is only one tone, and it lasts at least double the time of a normal Two-tone page. A common Motorola format has an All-Call tone length of eight seconds.
Common formats are:
- European
- GE
- Motorola
Other Signaling Codes