When is DMR not digital?

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When is DMR not digital?

Postby G4RMT » Sat Jan 07, 2017 12:56 pm

Weird one this - I have DMR project coming up for a client, and he doesn;t need much in the way of cleverness, just a swap of his analogue system to a digital one, and just a few radios on the system in total. 8, I think.

I noticed the TYT-8500 DMR repeater, and then spotted this page http://www.gb3gr.co.uk/index_files/Page1193.htm.

Inside the 8500 appear to be two modified ANALOGUE radios! TYT-9000Ds, a custom control board and that is about it. If this is a tier II DMR system then how on earth does this work in terms of the two timeslots? Can it be so simple as a discriminator output from the first radio, straight into the second? Surely not. Fair enough, this would indeed pass the signal through intact(ish) but would this really allow two radios on the same frequency at the same time to be combined and retransmitted without issues? I thought DMR repeaters also retimed everything being passed through and two mobile analogue radios could not possibly do this, unless the clever stuff is done to the data stream before being passed to the analogue radio to retransmit? This would also allow analogue straight through operation, but how would CTCSS tones be managed? The receiver output would be checked by an external board and identified as analogue with CTCSS? This would be a function of the receiver, so how could it also be bypassed for digital input?

The GB3GR website page is scant with the information.

If DMR can be passed by discriminator output, then can I pursuade the existing repeater to simply pass it through, if I knock off the CTCSS? Has anyone tried this?

Loads of questions and I don't remember us talking about this before? The DMR repeaters with 'cleverness' are clearly very different, but are these repeaters just a fudge/bodge?
G4RMT
 
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Re: When is DMR not digital?

Postby Bert Ringroad » Sun Jan 08, 2017 4:38 pm

G4RMT wrote:I noticed the TYT-8500 DMR repeater, and then spotted this page http://www.gb3gr.co.uk/index_files/Page1193.htm.
Inside the 8500 appear to be two modified ANALOGUE radios! TYT-9000Ds, a custom control board and that is about it. If this is a tier II DMR system then how on earth does this work in terms of the two timeslots?


Big "if", isn't it? Presumably the "custom built control mechanism with makes home to an ARM processor, two audio/digital processing chips" handles this. The TH-9000 docs say it supports 12.5k/20k/25k bandwidth, so a 12.5k DMR signal can fit inside one of those - perhaps that's what they're doing? Maybe they've customised the TH-9000 so it's just wide enough to pass DMR? Without actually seeing what this control mechanism does, we're just guessing. If you program the two radios and not the control unit, then that suggests the control unit doesn't do much clever at all.

G4RMT wrote:Fair enough, this would indeed pass the signal through intact(ish) but would this really allow two radios on the same frequency at the same time to be combined and retransmitted without issues?


It might not be perfect but perhaps it's good enough? Ideally the digital signal would be recovered and re-generated. If a receiver wanting to use slot 2 hears a conversation on slot 1, will it take sync from what it's hearing on slot 1 and avoid stomping on it? One thing we can be reasonably sure about is that the output of this repeater would be keyed at all times so long as there's an input on either timeslot. This isn't as problematic as something keying up on the input frequency in both timeslots but only using one.

G4RMT wrote:are these repeaters just a fudge/bodge?


Entirely possible - wouldn't be the first "DMR" product to make it to market that doesn't quite follow the letter of the spec, would it? At the lower end of the market, it seems to be a case of "right, it basically works, let's ship it before we find any bugs!". "DMR" doesn't appear to be trademarked, so any idiot can sell something as "DMR" and if it isn't DMR, the consequences are minimal.
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