DCDM on DMR

[DaveNF2G]

The MotoTRBO reference page says that the code DCDM D1 or DCDM D2 means that a subscriber unit is using what hams would call "direct" on the repeater output frequency with whichever slot is indicated after the D.

Does this signal actually indicate that DCDM is in use, or that it is available on the system?

I ask because two subsidiary questions come to mind:

1) How does the system know that a subscriber unit is transmitting on the repeater output?

2) Wouldn't the system signaling via the repeater interfere with the subscriber's transmissions?

 

[slicerwizard]

Don't know what reference you're looking at but DCDM really has nothing to do with repeaters. It's radio to radio simplex and would typically not take place on repeater input or output frequencies.

 

[DaveNF2G]

Which leaves the question, why does a repeater announce DCDM D1 or D2?

 

[slicerwizard]

Again, don't know what you're referring to or how you're seeing such announcements.

 

[DaveNF2G]

I have observed them on three different DMR channels licensed to two different entities using DSDPlus FL.

 

[LD723]

I have seen it befroe on my local police departments Dmr system i thought it was just a random false positvie

 

[slicerwizard]

Yup, sounds like random bad decodes.

 

[DaveNF2G]

Here is an example:

 

[slicerwizard]

That's 24 bits worth of random noise matching a DCDM voice frame sync pattern, followed by a superframe's worth of solid decode errors (because there was no actual DCDM voice frame)

 

[DaveNF2G]

"I don't think so, Tim"

I am getting these consistently enough to rule out bad decodes. I suspect they are mobile units operating on the repeater outputs and DSD+ does not know how to interpret the data so as to produce anything resembling a voice channel. Here are three more frequencies from this morning:

 

[DaveNF2G]

I am seeing these so often and on so many different systems at varying distances and signal qualities that I am beginning to suspect a bug in the software. For one thing, the bursts are always exactly the same length. They always show VOICE at the beginning, even though there is never any voice. (Voice transmissions would not always be of exactly equal duration, either.) There are always exactly 5 continuations.

This has to be some sort of data burst. One of the system operators that I monitor advertises GPS tracking - maybe a clue?

 

[DaveNF2G]

Disregard my previous post. I think I found the bug at the keyboard. LOL.

I was just updating my master database, where all of my identified frequencies are listed in sequence in one place. Some of the "DCDM" decodes are happening on channels previously identified as LTR-PassPort. As soon as I have a chance, I will check the frequencies again with LTR software to see it that is what I receive.

Evidently these are in fact data bursts, but not DMR protocols, so naturally DSD+ wasn't going to get it right.