DMR Talkaround vs. Simplex TDMA

W4KRR

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#1
In the AnyTone D868UV programming software, there are channel options for both "Simplex TDMA" and "Talk Around". What's the difference?
 
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#3
In the AnyTone D868UV programming software, there are channel options for both "Simplex TDMA" and "Talk Around". What's the difference?


It’s because Simplex is for Analog. Talkaround is for digital. They mean the same thing, it’s just applied differently for the different modes.
 

chief21

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#4
If I recall correctly, DMR "talk around" and DMR simplex are actually the same. Each of these essentially use BOTH time slots without any time sync and only one voice path per channel. Simplex TDMA, on the other hand, uses either time slot 1 or time slot 2 and can accommodate two voice paths per channel, the same as DMR tier 2. Because of this difference, I don't think that DMR simplex is compatible with simplex TDMA.
 

W4KRR

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#5
If I recall correctly, DMR "talk around" and DMR simplex are actually the same. Each of these essentially use BOTH time slots without any time sync and only one voice path per channel. Simplex TDMA, on the other hand, uses either time slot 1 or time slot 2 and can accommodate two voice paths per channel, the same as DMR tier 2. Because of this difference, I don't think that DMR simplex is compatible with simplex TDMA.

This sounds correct. On a digital channel, both options are individually selectable. On an analog channel, only the talkaround option is available.
 

popnokick

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#9
"Talkaround" is in reference to repeater operation. When using a repeater your radio transmits on one frequency and receives on a different frequency. If you select the Talkaround option on the radio it immediately switches the transmit frequency to be the same as the receive frequency. This can be helpful if all mobiles / portables are out of repeater range, or there has been a repeater failure. Without talkaround none of the radios within simplex range would hear each other... because they'd all be transmitting on a frequency to which no other receiver (except the repeater) would be listening. HOWEVER, talkaround has to be enabled for any given channel in the codeplug / config, otherwise the talkaround button or function on the radio will not work. On simplex channels (those that transmit / receive on the same freq) talkaround has no effect.
 

N4GIX

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#10
"Talkaround" is in reference to repeater operation. When using a repeater your radio transmits on one frequency and receives on a different frequency. If you select the Talkaround option on the radio it immediately switches the transmit frequency to be the same as the receive frequency.
Thank you, as you've just saved me a lot of typing! The very name "talkaround" provides the clue to it's actual mechanism: one is "talking around the repeater!" :D

For that matter, many radios will allow "talkaround" to be used even with analog repeaters as well.
 

popnokick

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#11
Bill - Thanks and you reminded me of something else I was going to write: "TAC" stands for Talk Around Channel. Hence there are DMR talkgroups labeled TAC310, TAC311, TAC312.... etc. In this case though the purpose of the "talkaround" is to avoid congestion of widely used talkgroups, such as the statewide or nationwide DMR talkgroups.
 

W4KRR

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#12
Thanks to all who replied. Post #4 above came closest to the answer I was looking for (I already know what simplex and talkaround mean with regards to analog operation).

After talking with some people and some more research, here are a couple of explanations that seem to answer what I was originally asking; I can't vouch for their accuracy:

"Talkaround" uses single or both time slots (I believe for compatibility with tier 1 devices), where "Simplex TDMA" forces use of both slots for compatibility; TDMA adaptive can alter the timeslots to find a free one ("polite" access feature).

And this:

When a DMR radio is in simplex mode there is no concept of time slots as there is no timing reference (aka, a rptr) in use. The signal from the radio is an FDMA signal much like P25, NXDN etc. That is, it fills the full 12.5 kHz bandwidth channel with the datastream of audio pkts. WHEN you select Dual Capacity Direct Mode (what Anytone is calling "simplex TDMA") the radios are set up to use two time slots so two different conversations can occur in the 12.5 kHz channel bandwidth. Here one of the radios in use is used as the timing reference to maintain TDMA
 
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#13
Bill - Thanks and you reminded me of something else I was going to write: "TAC" stands for Talk Around Channel. Hence there are DMR talkgroups labeled TAC310, TAC311, TAC312.... etc. In this case though the purpose of the "talkaround" is to avoid congestion of widely used talkgroups, such as the statewide or nationwide DMR talkgroups.
No. Talk around means talking on the output freq of a repeater, like was explained above. The “TAC” as in TAC310, is short for “tactical”. Many police departments have a main dispatch channel. But to keep traffic off the main dispatch channel, they will go to TAC1, or TAC2, etc. So to keep traffic off NA, after you make your contact on NA, you would go to TAC310, which doesn’t make sense because they are both on the same freq, & TS, so if one is busy, they are both busy.
 

N4GIX

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#16
So to keep traffic off NA, after you make your contact on NA, you would go to TAC310, which doesn’t make sense because they are both on the same freq, & TS, so if one is busy, they are both busy.
I was with you until the above statement. Whenever you key up the repeater on TAC310 (or any other such talk group) you *disconnect* your local repeater from whatever TG was active, and switch it to TAC310. When the person you were talking to does that from his/her end, the same happens...

...now in theory only those two repeaters will be connected to TAC310 on whichever time slot is assigned. This means that only those two repeaters are being keyed up for the duration of the QSO, unless that specific TG was already in use. That is why one should check for activity before suggesting a QSY there!

Of course, other folks could also redirect their local repeater to TAC310 as well, which in effect is like the old-fashioned "party line" Telco system.

Many (most?) repeater trustees set up TAC and similar TGs to be PTT activated, meaning that after ~15 minutes of no activity they will revert to the full-time TG(s).
 
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#17
In a perfect world yes. Consider this scenario. You make a contact on NA. To be polite, you tell the other party to go to Tac310. It takes a few seconds for both parties to do this. In the meantime, other people are putting out calls on NA, so the TS is busy, so you can not access TAC310, so the 2 guys can not contact each other there. Many repeater owners are making NA PTT activate because they keep blowing up their repeaters while stuck in transmit rebroadcasting conversations from thousands of miles away, where locals are not participating.
 

N4GIX

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#18
On both of my local repeaters (VHF & UHF) all TGs on slot 1 are PTT activated. On slot 2 every TG except True Local and Indiana Statewide are likewise PTT activated.

I rarely use either repeater any longer since setting up two jumboSPOT gadgets, one for the shack and one for mobile use, so no need to worry about switching TGs on demand... :)
 
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#20
Okay since this is D868UV simplex related I didn't want to start another thread but i'm having some issues with the 868 I just received. I programmed some digital simplex frequencies and left the "Simplex TDMA" box unchecked however the radio is still transmitting in tdma mode. (cc1 slot 1, as in the programming) I tried different configurations of talkaround, simplex tdma, tdma adaptive but I cannot get the radio to transmit in fdma 12.5khz mode.

Most of the radios im communicating with are tdma and it's not an issue however I would like to be able to transmit in fdma mode for backwards compatibility with older devices.
 
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