Do trunking scanners have 2 receivers?

[GroundLoop]

Just a noob question about how these trunking scanners work..
I understand that mobile radios have two receivers -- one to monitor the control channel and one to receive audio.

Do trunking scanners do the same, or tune away from the control frequency to receive Call audio?

Just curious.

 

[Awesomeman92]

That's a very good question, and one I never really thought about before...If I had to guess I'd say they had separate receivers, but then again you can't monitor more than one talkgroup on a system without rescanning the system (to my knowledge, anyway, may be wrong on that).

 

[nd5y]

Scanners do not have two receivers. Neither do most mobile radios.
It is not necessary for trunking subscriber units to operate full duplex or have two receivers.
The control channel is not monitored during a voice call.

 

[jackj]

At first thought, I would have to agree with you ND5Y. Then I thought about changes that happen during a QSO, going from one tower to another, switching RF channels, etc. How would the mobile or scanner know about these changes if it doesn't monitor the control channel during a QSO?

 

[nd5y]

Somebody who actually operates a trunked system will have to answer that. As far as I know multi-site trunked systems don't have seamless roaming like cellular telephone systems do.
They can't switch users to another channel or site while the user is transmitting. The radio waits until it receives the end code on the voice channel and then goes back to the control channel. If it doesn't hear the control channel then it scans for another one which might be at a neighbor site.
Some types of systems can send subaudible channel data over the voice channels but as far as I know that is only for priority scanning and not site roaming.

 

[DaveIN]

Maybe the Wiki article will help: Trunking Basics - The RadioReference Wiki

Your trunking scanner parks on the control channel and listens for the voice channel grant, displays the talkgroup ID and tunes to the voice channel identified in the control channel data. Once the comm is finished on the voice channel, the end code is received (unless ignored), and the delay expires, the scanner returns to the control channel and continues to the next voice channel, if the comm continues, or picks up the next talkgroup and repeats the process.

If you have an alpha tag assigned to the TGID, the radio sees the TGID in memory and displays the alpha tag you have assigned along with the comm.

If you have multiple systems programmed and there is no activity on the control channel in the amount of delay time set, the scanner continues on to the next system.

 

[fineshot1]

Somebody who actually operates a trunked system will have to answer that. As far as I know multi-site trunked systems don't have seamless roaming like cellular telephone systems do.
They can't switch users to another channel or site while the user is transmitting. The radio waits until it receives the end code on the voice channel and then goes back to the control channel. If it doesn't hear the control channel then it scans for another one which might be at a neighbor site.
Some types of systems can send subaudible channel data over the voice channels but as far as I know that is only for priority scanning and not site roaming.

This is correct. There are no conversation hand offs in trunking systems like it is
done in cellular systems. When the users conversation ends and the radio scans
back to the control channels if it finds a better serving control channel than the one
it was just parked on it will switch to that(in a multi-site system) but other than
that there are no hand offs.

 

[jackj]

Okay, thanks for the info gentlemen. I kinda figured there would be a hand off if one mobile left the coverage of one tower and entered another during the conversation. If it noises up and drops the QSO then the user would just reestablish on the new tower I guess.

 

[GroundLoop]

Very interesting.

Does this also imply that if I have two talkgroups of interest (TGID1 and TGID2), and one of them starts a call (TGID1), the scanner will jump to that frequency and tune the audio.

If, during that call, TGID2 also keys up, will the scanner miss the start of that call (since it was tuned to TGID1 channel)? Will it ever find it after TGID1 is done talking, or will it go silent?

(Does the control channel repeat the active call information, is it only at the start?)

 

[jim202]

Very interesting.

Does this also imply that if I have two talkgroups of interest (TGID1 and TGID2), and one of them starts a call (TGID1), the scanner will jump to that frequency and tune the audio.

If, during that call, TGID2 also keys up, will the scanner miss the start of that call (since it was tuned to TGID1 channel)? Will it ever find it after TGID1 is done talking, or will it go silent?

(Does the control channel repeat the active call information, is it only at the start?)

You will only get one talkgroup at a time. You will miss the second talkgroup conversation as long as the first one is still talking. The scanner needs to hear the control channel in order to be told, just like the users on the trunking system, which voice channel to go to for the talkgroup being monitored or selected.

If your concerned about monitoring the second talkgroup at the same time as the first, get a second scanner.

 

[wa8pyr]

Somebody who actually operates a trunked system will have to answer that. As far as I know multi-site trunked systems don't have seamless roaming like cellular telephone systems do.
They can't switch users to another channel or site while the user is transmitting. The radio waits until it receives the end code on the voice channel and then goes back to the control channel. If it doesn't hear the control channel then it scans for another one which might be at a neighbor site.
Some types of systems can send subaudible channel data over the voice channels but as far as I know that is only for priority scanning and not site roaming.

I manage a trunked system, and I can answer that question....

Real radios do not have two receivers. Nor do scanners.

When a radio is receiving a talkgroup call, it is monitoring low-speed subaudible data on the voice channel. This low-speed data gives information such as active priority talkgroups. If RF channel switching is to take place it is handled by the controller when making a channel grant, after the current transmission ends.

As far as site switching, this is handled between transmissions. As ND5Y noted, if a user's radio is not receiving an adequate signal strength from the control channel, it searches it's list for an active control channel it can hear, then goes through the process of affiliating with the site and registering with the controller; there might be a lag of a few milliseconds while switching sites before the radio receives a new channel grant, but this is generally unnoticeable to the user.

However, when a radio is transmitting, it's pretty much lost to the world until it starts receiving again.

 

[rdale]

You will only get one talkgroup at a time. You will miss the second talkgroup conversation as long as the first one is still talking.

That's incorrect. If you have a scanner capable of trunked priority, AND the talkgroup is on the system's priority list, it will change.