Digital radio question

[policefreak]

Just curious. If two units key up simultaneously on a digital system frequency or system, what will happen. I know on an analog system the two units usually make a buzzing noise and cut each other out.

[wayne_h]

It disrupts the audio, just as with analog, and you hear what sounds like someone on the fringe of coverage. It's just chirps and bleeps.

[RocketNJ]

Or,

If two ditial radios are transmitting at the same time and a third is receiving the third may not hear anything at all if the received signal's bitstream is not able to be decoded.

This happened at FDNY when they first deployed the XTS3500's programmed for digital fireground. They since went to analog.

[N_Jay]

You will get anything from some reduction in the voice quality of the stronger signal to complete cancellation.

Trunking systems usually don't allow doubling.
Conventional systems do. Some radios can be programmed to prevent it.

[slicerwizard]

Originally Posted by N_Jay

Trunking systems usually don't allow doubling.

You haven't tried LTR or PassPort...

[wlmr]

Unsure if the most recent radio programming software still allows these choices - your milage may vary.

On a Motorola 9600 P25 system if the radios are programmed for transmission trunking they ask the control channel for permission to talk each time before keying up. If another radio is talking they're told to wait. This is done for every transmission (hence transmission trunking ). No keying over each other.

If the radios are programmed for message trunking, only the first radio has to ask. All subsequent radios just continue the message on the voice channel as long as they key up before the system times out the conversation. They can cause headaches amongst themselves as described in earlier posts if(when) multiple radios try to talk at the same time.

The final setting which allows important talkgroups to keep a message going (and prevents multiple radios from keying at the same time) is called message trunking with PTT ID. The message stays up but as subsequent radios try to key up they have to check with the system via control channel to see if it's ok to do so. (Back to one at a time - no keying over each other.)

Again as earlier posts have stated, if the radios aren't using a trunked system and/or programmed to avoid keying over each other, they can cause all kinds of problems.

One radio = one digital stream to reconstitute back into audio. More than one = difficult to impossible hear anything that resembles voice if they're both able to be received.

[SLWilson]

Originally Posted by N_Jay

You will get anything from some reduction in the voice quality of the stronger signal to complete cancellation.

Trunking systems usually don't allow doubling.
Conventional systems do. Some radios can be programmed to prevent it.

Not sure if it is the "Programming" or what, but on the Ohio MARCS system, two radios will NOT key at the same instance. If one radio is "on the air" and another one tries to key, the second operator gets a BONK noise out of their radio.

I'm assuming if BOTH tried to key at the same time, one would be just a "little" quicker on the key & block the other one and get bonked.

Additionally, if you try to key while another radio is talking on your TG, when THEY stop, you get an alert to let you know the channel is NOW clear for you to use....

I'm sure that the actual patrol post dispatchers can "override" the bonk out feature, but, I'll have to ask to be sure...

Steve/KB8FAR

[N_Jay]

Originally Posted by slicerwizard

You haven't tried LTR or PassPort...

Usually does not mean all,
and,
the conversation started discussing "digital", and there is not a digital variant of LTR,
and,
I don't recommend LTR for public safety,

[wlmr]

Originally Posted by SLWilson

I'm sure that the actual patrol post dispatchers can "override" the bonk out feature, but, I'll have to ask to be sure...
Steve/KB8FAR

Most likely varies with the system. On at least one 9600 P25 system if a radio is talking & dispatcher has important traffic they just key up and their voice replaces the field unit. They never get bonked. Dispatch Unkeys and if the unit is still happily talking the talkgroup audio goes back to him. (While the dispatcher is talking the dispatcher continues to hear the original field unit's audio - kinda a safety feature in case what the field unit is saying becomes important.)

I realize this info is rather far afield from the original two digital radios keying at the same time discussion so I'll cut it short with the above info.

[policefreak]

Thanks for all the input. True, silly me didn't think about the fact that a trunking system has multiple frequencies...although I guess two units could be trying to communicate on the same talkgroup and I bet that can cause some confusion!! {:>0

[zerg901]

Actually policefreak, I think you have stumbled onto something very profound. If a TRS could be set up to allow 2 field units (on the same talkgroup) to talk at the same time, that might be a very, very, very, useful feature.

Think of any intense foot chase or shootout that you have ever heard. Instead of 4 people doubling all over each other, imagine how much better it would be if everyone could be heard at the same time. (Instead of the "Tower of Static or Bonks" you would have the "Tower of Babble".)

Of course the very best system would have fully duplex field radios - radios that can receive and transmit at the same time. Like my $5 cellphone.

Peter Sz

[rdale]

Originally Posted by zerg901

Actually policefreak, I think you have stumbled onto something very profound. If a TRS could be set up to allow 2 field units (on the same talkgroup) to talk at the same time, that might be a very, very, very, useful feature.

No, it would just result in nobody hearing anything with the dual conversations, and the 2 field units transmitting wouldn't have any idea that everybody else missed out.

Originally Posted by policefreak

although I guess two units could be trying to communicate on the same talkgroup and I bet that can cause some confusion!

No... As noted above, a trunked system will 'bonk' one of the users - conventional is the only place where this could happen.

[zerg901]

Perhaps I wasnt as 'clear' as I meant to be. (pun intended)

I am talking about putting a 2nd input freq into play when a double is about to occur. Field Unit 1 would transmit on Freq A - Field Unit 2 would transmit on Freq B - the infrastructure would allow everyone on the TG to hear both messages.

Peter sz

[JoeyC]

Originally Posted by zerg901

- radios that can receive and transmit at the same time. Like my $5 cellphone.

Peter Sz

Cellphones I've used do a poor job at full duplex where digital is concerned. That is especially true when 2 cellphones (digital) are talking to each other.

[rdale]

Originally Posted by zerg901

I am talking about putting a 2nd input freq into play when a double is about to occur. Field Unit 1 would transmit on Freq A - Field Unit 2 would transmit on Freq B - the infrastructure would allow everyone on the TG to hear both messages.

I understand what you are saying... Realize though that in a pursuit, adrenaline is up, so voices are louder. Imagine two people with higher volume voices talking at the same time to you. Can you pick out exactly (or even closely?) what each are saying? Now put those same loud voices in a small speaker and hold it to your ear. No value...

[Astro25]

Both signals usually end up muting, with some choppy audio inbetween. If your radio is capable of displaying IMBE numerical IDs and SoftIDs, you'll see each radio's respective ID popping up and flickering as well. Not much else. All depends how close they are to the receiving site of the system as well.

[zerg901]

Rdale - I think we will have to agree to disagree on your point. Maybe a pilot project is in order.

I saw a recent post on the Net from St Charles MO. Apparently their P25 system is set up to accomadate 'doubles' if someone holds their mike button for a longer period of time. It seemed to be some little known Motorola configuration. I can probably find the post if anyone cares.

Peter Sz

[REDave]

WLMR's Posts are correct.

Excellent Posts!!

[ocguard]

A trunked radio system uses repeaters. Repeaters use two frequencies; one is an input, one is the output. Dispatchers are typically connected directly to the repeater or base station, so they are monitoring the input and transmitting on the output. This immediately builds in two safety features, as stated about.
1. If the dispatcher is transmitting (on the output) and a field unit keys up (on the input), the dispatcher can hear this call coming in, and, if it's emergent, can stop transmitting and monitor the new call.
2. If a field unit is transmitting (on input, repeated on output), and the dispatcher needs to air a more urgent call, he need only key up and take over the output, and all field units (with the exception of the one transmitting) will hear the call. The dispatcher will continue to be able to monitor the incoming call.

As far as two inputs, what!? I mean, I understand the idea, but why? Which input would be chosen for the repeater-out? Which would be heard in the dispatcher's ear? One in the ear, one in the unselect speaker? Why not just switch talk groups? Or if it's a true emergency, push the emergency button and overtake the voice channel. Two inputs seems silly, and defeats the purpose of a group call.

Zerg, I'm sure the config you speak of refers to that second radio being "regrouped" and permitting the call so that someone will hear it. Two inputs into one talk groups seems unlikely. How would the system mix the voice? Which call would be repeater?

[El_Commandante]

Originally Posted by SLWilson

Not sure if it is the "Programming" or what, but on the Ohio MARCS system, two radios will NOT key at the same instance. If one radio is "on the air" and another one tries to key, the second operator gets a BONK noise out of their radio.

I'm assuming if BOTH tried to key at the same time, one would be just a "little" quicker on the key & block the other one and get bonked.

Additionally, if you try to key while another radio is talking on your TG, when THEY stop, you get an alert to let you know the channel is NOW clear for you to use....

I'm sure that the actual patrol post dispatchers can "override" the bonk out feature, but, I'll have to ask to be sure...

Steve/KB8FAR

Don't have my system manuals in front of me, however you're correct.

On a 9600 P25 (6.x/7.x) Motorola system, there IS a parameter you can
toggle in UCM that will allow/disallow preemption of a talkgroup by a subscriber.
In other words if you key up on a particular TG in use, you will get a "bonk" on
your radio (IF that parameter is set to disallow preemption).

Console priority is standard on all Moto systems. So dispatcher will ALWAYS
be able to override a field unit.

Re: description if both keyed up simultaneously, one quicker than other.
It wouldn't matter, "recent user priority" would favor the last user in the
conversation that was going on. Again this is another parameter that
can be fine tuned if needed. However, it's best left alone with the factory
defaults unless you understand what you're doing.

Re: the "alert", is simply a talk permit tone, for in inbound PTT that was
in queue. subscriber now has channel grant and is clear to send voice.

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