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DMR Basics, Talk Groups

Please note with the GD-77 Talk Group numbers are entered into the ‘Contacts’ section, both on the radio and in the software.

The concept of Talk Groups on digital repeaters can be confusing to anyone who is only used to normal analogue VHF/UHF repeaters or simplex FM operation.

The benefits of analogue repeaters in effectively increasing the range of any transceivers accessing them by re-broadcasting the received signals over a wider area is well understood but they are normally limited to a local area, albeit a much larger one than the transceiver could reach on its own.

Digital repeaters work in the same way but with the added (massive) advantage of being linked via the internet to all other digital repeaters using the same system all over the world.

DMR repeaters also have two channels or Time Slots which can be used independently at the same time, so there can be two separate conversations going through a single repeater simultaneously.

However if every time anyone transmitted through a repeater all the others around the world re-transmitted the same signal the network would be instantly clogged up by only two QSOs.

So the Talk Group numbers control which sets of repeaters actually re-transmit a particular signal.

Some Talk Group numbers such as 1 and 13 do indeed open almost all of the repeaters world wide so that DX QSOs or nets can be initiated between people in different countries whenever needed.

Talk Group 1 is the World Wide calling channel.

Talk Group 13 is the World Wide English calling channel.

There are also Talk Group numbers which only open the repeaters in certain countries or areas such as 235 for the UK or 2 for Europe.

These World Wide and Countrywide Talk Groups are used a bit like the ‘Calling Channel’ on FM simplex, allowing stations to contact each other initially before moving to either a Local Talk Group number such as 9, 840 or 950 if they are in the area covered by their local repeater or a User Activated Talk Group number such as 80 to 84 in the UK or 113, 119 or 123 for World Wide use.

As should be gathered from the previous sentence the Local Talk Group numbers (9, 840, 950) only use the local repeater, exactly like a normal analogue repeater.

Talk Group 9 is also used on most repeaters for Time Slot 2 and Local transmissions only.   

The User Activated Talk Group numbers (80 to 84 in the UK) are a little more difficult to explain although the clue is in the name.

Basically anyone can open (activate) the particular Talk Group on their local repeater by switching to that TG number and PTTing their transceiver, this will cause the repeater to re-transmit anything being sent to the TG anywhere in the country or region. Otherwise these TGs are ignored by the local repeaters.

If there is no local activity on one of these TGs for more than fifteen minutes the repeater will stop re-transmitting the Talk Group.

This allows two or more stations in the same country or region to have a QSO through their local repeaters without bothering any of the other repeaters in the system.

Well that is how the DMR system is meant to be used but of course not everyone operates according to the rules and there are many QSOs which take place on the World Wide or Countrywide Talk Groups instead of moving to a User Activated Talk Group (UA TG).

There are many more Talk Group numbers used world wide but not all are passed through local repeaters.

It is best to look on the web site for your local repeaters to find the TGs that they handle.

The GD-77 in normal mode only decodes signals on Talk Group numbers actually programmed into the Rx Group List active on the particular channel in use, this means sometimes the radio will light up and show incoming antenna signals but will not output any audio.

To get the radio to tell you what TG is in use there is now a system called Monitor Mode aka Promiscuous Mode which was added to the operating system a while ago as a modification to the original version which only worked in analogue FM mode.

Unfortunately the modification has not been documented in the software help system so this still talks about Monitor Mode just opening the squelch in analogue FM mode only, which is not correct.

By default a long press on the top Side Key switches the radio into and out of Monitor Mode.

The radio will then report any TG numbers used on the LCD screen, these can then be programmed into the Contact and Rx Group Lists to allow it to decode them in normal mode.

An alternative way to find the TG numbers is to use a decoding program called DSDPlus, an SDR dongle and a decent antenna.

This program has its own simple SDR tuner called FMP which uses very little CPU power to operate and allows the decoding of signals even on relatively slow computers.

DSDPlus decodes all Talk Groups, Colour Codes and both Time Slots on any repeater and logs the results so all the TGs used through the repeater can be found by examining the log files, and these can then be added to the Digital Contact and Rx Group Lists on the radio.

You can find DSDPlus as a free download with a quick Google search so I haven’t bothered to include any links or instructions on its use as there are simple help files included.