DMR 2 slot problems

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DMR 2 slot problems

Postby G4RMT » Sat Jun 25, 2016 9:29 am

I'm trying to get my head around some DMR simple system issues.

In simplex systems, I'm coming to the conclusion that the often talked about 'benefit' of two slots, and the always quoted "two conversations at the same time" is unworkable, as clearly from use, handhelds don't have the ability to sync without a constant carrier to lock too - so the handhelds often overlap and presumably packets crash and both 'channels' lose signals. With the repeaters that burble all the time, I thought that the radios would all use the incoming signal to lock to, and therefore would know when their slot time was, but occasionally I've noticed things clash. I'm not talking about ham band all over the world connectivity, but just on-site comms. Press the button and nobody answers, or the signal notes who you are listening - presumably because somebody on the input on a different slot wiped them out.

I realise simplex slots have this problem, but do some radios just have poor sync timing?
G4RMT
 
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Re: DMR 2 slot problems

Postby bigboyblue » Sun Jun 26, 2016 1:15 pm

Theres several issues at play here:

The first generation of Motorola radios, didn't (and don't) have the ability to choose slots on simplex comms. You use one freq for one 'channel' on simplex comms. The newer generation, the DP4,DP1,DP2 series, all have the ability to have a 2 slot per freq system on simplex. They choose a single radio as the timing leader, and it sends out the timing sync signals as well as sending / receiving voice. The disadvantage is that the timing radio needs to be in range of all the others.

The second issue is simply of lazy dealers. If its a repeated system, then you will still get tx clashes, as in analogue, if the settings on the radio aren't correct - 'always transmit' rather than 'colour code free'. There's lots of lazy dealers out there who put in a few settings, and send out the radios to their customers.

Third issue is, is the system you are listening to definitely a single stand alone repeater - not capacity plus. Lots of smaller sites with less than 100 radios have capacity plus, so that they can private call radios on any talkgroup. This usually leads to people scanning who miss some or all of the conversation as it flits between slots and repeaters. I've even seen single repeater capacity plus systems. Confuses the hell out of you when a conversation comes and goes like the clappers.
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Re: DMR 2 slot problems

Postby G4RMT » Sun Jun 26, 2016 10:27 pm

Thanks for that info - I'm pretty certain that it's just a single repeater, and I think a hire. Been at a local construction site for a few months now and apart from being DMR, which I found at random with just a few stabs at getting the config right. It just seems to be a bit of a gamble if people get the messages or not. Just noticed the computer typed 'note' in my post, I meant 'mute'.

I must say thanks for the comment on 'always transmit' - the consequences of lazy programming make more sense now.
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