Two Tone Sequential Decoding Systems

Tw0-tone decoding systems (known in the commercial world as Motorola Quik-Call II, et al) is a paging, alerting and equipment control signaling format for use on VHF and UHF that makes use of two sequential audio tones. This signaling format has been in use for over 50 years and is still in use today, primarily to call out volunteer firefighters and emergency medical technicians. Although this system has been in use for decades in the commercial paging and two-way radio market, as of 2021 we are starting to see this capability included in some transceivers marketed to amateur radio operators. As of April 2021 some products from Alinco, Anytone and TYT include two tone decode (as well as encode) capability. Unfortunately, most of these amateur radio products do not work in a way that’s consistent with commercial operation in North America. This article describes the basics of two tone signaling, and how I think two tone radio transceivers should operate.

Some Details

The tones used for this signaling format are those from a table of audio frequency values that have been selected to minimize false positive responses and they range from 288.5 Hz to 2468.2 Hz. US Alert has a good paper that describes how to select particular frequencies for a two tone decoder. See: https://www.usalertllc.com/Attachment/DownloadFile?downloadId=156

In the Two Tone system the first audio tone is transmitted for 1.0 seconds followed by the second audio tone for 3.0 seconds. Motorola documentation specifies a silent period of at least 400 ms after the transmission of the second tone. In the case of our local county fire dispatch system (as of 2021) that trailing silent period consists of active carrier with no modulation, followed by discontinuance of carrier for approximately 400 ms to 2 seconds. After that first tone sequence has been transmitted (1st tone, 2nd tone, carrier with no modulation, followed by no carrier), additional tone sequences may be transmitted using the same format and timing (to alert additional stations). Note for our county Fire/EMS two tone transmissions that the carrier *always* drops after each two tone transmission has been sent. After all paging sequences have been completed the carrier will resume and the dispatcher will make a voice announcement to the group of alerted stations. In some systems the two tone sequence is used to remotely control some sort of apparatus (one sequence to engage, a different sequence to disengage) and for those control transmissions it’s unlikely that a voice announcement will follow the transmission of the tone sequence.

Desired Functionality

A two tone decoder in a handheld or mobile transceiver should keep the radio silent until the selected two tone sequence is received *OR* the user disables the two tone decoder. To help with the discussion I’ll define the term Two Tone Mute (or 2TMute) to describe if the radio is in normal mode (2TMute disabled: radio functions as it normally would with speaker output controlled by a noise squelch (or CTCSS squelch or DCS squelch) or is being muted by a Two Tone decoder (2TMute enabled so that the radio will remain silent until 2TMute is disabled or the Two Tone decoder hears the tones it has been programmed to detect). Think of 2TMute as a binary variable, it’s either ON or OFF.

Regardless of the state of 2TMute, when the Two Tone decoder hears the tone sequence it has been programmed to detect it should (1) sound an alert tone on the speaker (usually five quick beeps), (2) activate a visual blinking indicator (either a blinking LED or an blinking LCD icon) and (3) disable 2TMute (if it was enabled).

The user should have control of the state of 2TMute by a single press of a button (or switch). For example pressing the PTT for any length of time, including just a tap, should disable 2TMute (this provides an easy way to disable 2TMute from a speaker mic, to monitor the channel before transmitting). A short press of a special purposes button (quick press of P2, for example) should disable 2TMute. A long press of a special purpose button (long press of P2 for example) should enable 2TMute. (And any press, long or short, of P2 could also provide a momentary all-squelch disabled monitor function, while that button is pressed). By putting the 2TMute enable and disable controls on the same button, and using a button on the side of the radio (and not the front panel), it should be relatively easy to enable or disable 2TMute without looking at the radio, or removing the radio from a holster or belt clip (think of the radio in a holster attached to a belt with a speaker-mic on the lapel).

Additional functionality

An additional (optional) method of re-enabling 2TMute could be via a timer. The idea is that, if 2TMute was enabled when the Two Tone sequence was detected, have a timer start timing from zero when the Two Tone sequence is detected. When the timer end-value is reached, and if 2TMute was enabled when the Two Tone sequence was detected, then 2TMute would be automatically re-enabled by the timer (again, this only occurs if 2TMute was enabled when the Two Tone sequence was detected) . The duration of the timer should be user selectable. And the user should also be able to choose if the timer is restarted from zero (or not) whenever any carrier signal is received (so that after a Two Tone decode a series of transmissions can be heard on the speaker without any operator action, provided the dead-air time between transmissions is not longer than the end-value time of the timer). The user selectable times could be as follows (in seconds): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 120, 150, 180, 210 and 240.

The Two Tone visual blinking call indicator, once activated by detection of the selected Two Tone sequence, should remain active until the user presses a button to change the state of 2TMute. The indicator is to notify the user that someone (most likely net control) tried to contact them. Only a user button press (and not the timer) should turn off the indicator.

DTMF Sequential and Five Tone Sequential Decoding Systems

For selective calling systems using DTMF (Touch Tone) or Five Tone sequential, the Two Tone decoder user interface would apply to these systems as well.