Trying to understand the structure for DMR

Trying to understand the structure for DMR

Defined on your Radio:      zones, scan-lists, rx-group, contact list, channels

Defined on DMR repeaters and bridges:     talk groups, time-slots, color-codes

A zone is a list of channels on your radio that you can select.  Some radios have dials, others have buttons to select zones.  Within a zone you select channels either manual (dial or buttons) or by use of a scanlist that goes thru all channels in a scanlist automatically. A talk group is like a VLAN tag in network lingo, or, in analog repeaters like a CCTS (continuous coded tone squelch).

Most radios and programing references I have seen, define on each memory channel a frequency, color-code, timeslot, and talk-group combination treating each TG as a channel which you can select or scan thru.  This will use a lot of channels for the number of repeaters and Talk groups one is interested in.  As your radio is scanning and a finds a busy channel, the radio will wait a "scan hang time” on that channel during which you can press PTT to answer.  If you are outside of the hang time, you may be transmitting on a different TG/channel combination as you would similarly do when scanning analog channels on a non-dmr radio.

Scan memberships are defined in each channel and you can select if scanning is to start as soon as you tune to that channel.  Radios also allow you via the menu button to initiate or turn off scanning and/or select scanlists.

But scan lists must be associated with a channel to initiate auto start of scanning.  

l have setup my radio with only two memory channels per repeater, one for timeslot 1 and one for timeslot 2 plus frequency and color code for each.  I have setup 2 RX-groups in my radio (for my area), one group for talk groups used on time slot 1 and one group for time slot 2.  I then add each RX-group to a channel with the corresponding timeslot , I also add a default contact (aka talk group ID) for transmission.

Now RX groups work like filters.  When I am on a channel I will receive all TGs (talk group) that I defined in that RX group associated with that TS (timeslot).   I also have a group hang time defined of 10 sec for instance.

When there is activity on a TG and I reply within the hang time I will talk back on the same TG.  If I like to originate a call on a TG that was not active, and that is not my default TG, I just select the channel with the proper TS and select from my contact list a TG I like to originate a call on and hit the PTT button.  From there one I would be in hang time window to reply, or I have to select the contact again.  Some radios can be programmed with shortcut buttons to select that TG.  I named my contacts to also have the TS number in it for TS1 (smaller number usually) so I don’t originate on a TS with the wrong TG.

I can also use scan list now to scan thru multiple of such combinations.  However, scanning would usually use up more battery.  Unless I move from repeater to repeater I usually don’t have to scan more than two channels.
My zones are setup by geographic selection.  I am in the SF bay area so I have a zone “bayarea” and I call my channels by the town they are in. If I like to listen locally, I would select San Jose1 to TS1, or, for wider coverage
I use San Jose 2 which covers TS 2.  All this works fine for now as there is not that much traffic.  I may revise that scheme by splitting up RX groups if it gets too busy.   Some radios have a “nuisance delete” that allow you
to “tune out” of a TG temporarily if there is too much chatter, but you always turn off the radio :)

DMR radios need to get “permission” from the repeater to talk.  If the channel/TS is busy you wont be able to talk on it.  If you were to listen only on one TG you would not hear other traffic, but your radio would show signal
activity or some LED blinking if it has such an indicator. Due to the widespread coverage we have, it is advised to to give a curtesy pause after pressing PTT or some of your initial message may not make it.

One thing that is also important to setup is that your microphone “gain” is properly setup and tested.  When listening to multiple people on DMR there is often a few that are very load, either by having a loud voice, speaking too
close to the mic, or having the mic gain to high.  There is a TG where you can listen to your own voice, see the dmr-marc website. Most commercial DMR system are setup by select group of people who control those settings
But as hams we have different programmers and different manufacturers with different ideas of what a certain gain level means. This has given rise to some unfavorable views towards some manufacturers products.

If your radio supports roaming, switching between repeaters would be automatically depending on signal quality received from the repeaters nearby.  I don’t  have such a radio and comment on it, but I think it may require your
radio to xmit once in a while to determine which repeater to use.

Hope this helps and I hope I represented all correctly

On Jun 15, 2015, at 7:59 PM, [MOTOTRBO] <> wrote:

1 comment:

  1. Anonymous6/17/2015

    correct, but I like to add some of my views as I learned about DMR with a slightly different view


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