Experimentation using Private Calls in DMR…

Digital Mobile Radio (DMR) has been a vast improvement in quality and usability in Amateur Radio Communications here in Upstate New York.   With that comes experimentation and fact finding to expand Amateur Radio usability.

In my experimentation, private calls in DMR will allow calls between 2 DMR ID’s in a sort of “Back Channel,” on a certain frequency, time-slot, color code and suppressed group call talk group.  The effect of this causes radios that are on a group call talk group to ignore the private call on that same frequency, time-slot, and color code  (AKA other radios do not hear hose private calls). Now there are ways around that I have found, which will be covered in this live article.

Programming my Radio

The radio I used in this test was my Alinco MD5TGP, the new mini DMR radio based on an Anytone 868/878.  I am still working on my Hytera and figuring out how to do it on there.

To program my radio, I had to ask myself a question, how do I get this thing to “Hang,” on the private call when someone calls me or I call them? This resulted in an investigation where “Person call hang time,” was found and set accordingly.

Sketch (1).png

“So how does this setting work?”

From my testing, it basically gives the caller or callee 20 seconds to answer a private call before returning to original channel programming (AKA Group Call Talk Group). Within that 20 second window a private call contact is maintained until hang time lapses due to an idle channel, a group call comes in, or a user changes the channel, canceling out a private call.

Now, I am not sure if my radio is still transmitting on the group call talk group as a private call with a target DMR ID. In my testing, I see evidence that a group call talk group that is originally programmed into the radio are suppressed on a particular channel while a private call is in session.

“What are some Pro’s and Con’s to a Private Call?”

I can think of several at this point, both good and bad.

Pro’s

  • Private calls between 2 individuals can be used as a back channel on a particular frequency, time-slot, and color-code.
  • If urgent communications need to go out to a single individual outside of a group conversation, a private call can be used briefly.

 

Con’s

  • This will still eat a space up on a time-slot.  If a conversation is going on in simplex time-slot 1 and a public service event is going on, there will be some “Hidden interference,” going on that same frequency, color-code and time-slot that may detract from pertinent communications.  So an alternate time-slot or color-code should be used.
  • Every radio is different. so there will be a difficulty level in setting that up accordingly between different type of radios.
  • Individuals engaging in a private call should announce they are in a private call on the initial call to a callee.  This way a callee knows a private call is in progress. Otherwise this could lead to confusion and information being shared across a group call where others are listening.
  • So far, I was only able to use it on Brandmeister and over simplex. I have yet to try it over CBridge.
  • A long Private Call Hold time will allow any malicious activity to hold up your radio and cause issues for transmissions.

“How it can be defeated?”

Has yet to be tested: Use promiscuous mode?

I have confirmed that promiscuous mode does in fact allow private calls to be heard.

Conclusions at This Point

DMR and Amateur Radio go hand in hand.  This feature I have found in my radio and in all radios should come in pretty handy for public service events, should it ever be needed. It must not be used on a frequent basis as it could cause some issues for others and information meant for private transmission to be heard accidentally on a group call with a radio not properly programmed.

Through talking with other hams and understanding the necessity of private calls, which mind you, there really isn’t any, It’s really no place in ham radio other than “Paging,” someone to call you back. Simply private calling someone, in hopes their radio will give a synonymous audio tone to note a private call, for a direct message or a cell phone call, that is really as good as it gets.

So for me at this point, I have reverted my radio back to it’s original settings.

2 thoughts on “Experimentation using Private Calls in DMR…”

  1. Very interesting. Will the LARC repeater allow you make private calls ? If so does the receiving radio have to be on a channel that’s on the same time slot as sender? If so that would assume you know what time slot your party is monitoring for the call to complete.

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    1. Hey Keith! So I have not tried private calls on the repeater, but I believe it will work. But, I have talked to other educated hams about private calls and they really are difficult to manage and have no overall purpose other than paging others for a direct chat.

      The target radio needs to be on the same frequency, timeslot and color code / talk group to be able to receive a private call.

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