Can Radio Transmit To 2 Different Channels At The Same Time?

[nokoa3116]

I am not really well versed with commercial radios but I have a question that is related to Motorola in this instance.

I noticed that the city of San Jose Fire Department in California moved to P25 Phase 2 trunked system a few months ago. They were previously on analog VHF. I noticed now that the VHF channels are still active, and both dispatcher, and field units can still be heard on it. Both of them sound pure analog. While on the P25 they sound like, well regular TDMA P25 voice. Another department in the county does something similar but for them it's different, analog sounds normal, but on the P25, dispatcher is P25, while field units sound like the analog voice transmission was taken through P25. So you can occasionally hear their static, and the tail burst at the end, but as P25. For that department I thought that something is set up on the system, a patch perhaps? How does this work, do they just have all the radios transmit on 2 channels with different vocoders at the same time, is that a feature that exists with the APX radios? Even more so, I was also confused when there was a mutual aid incident where a Cal Fire helicopter responded and was tuned to an old analog command channel, but was heard on the new digital one also.

There are also some other departments in the county where the analog frequency still has the dispatcher on it, so you can hear the dispatcher both on digital, and analog. But you can only hear field units on the digital.

 

[N4KVE]

Simulcast.

 

[Hit_Factor]

Two transmitters are patched together to send the same voice signal out, but on different frequencies and modes. This patching happens at the dispatch center.

The radios carried by personnel or in vehicles only transmit on one freq at a time.

 

[sfd119]

Simulcast.

....no. Reread OPs post.

It's probably a patch between the two systems, old and new. It happens a lot when users are being migrated.

In terms of the other department where you have the dispatcher on both systems but units only on one side, they are probably simulselecting the channels which means their voice is being transmitted on both channels simultaneously. This is done usually at consoles.

 

[n3obl]

Dispatch consoles can either patch or "multi-select" transmitters at same time.

 

[ka3aaa]

radio transmitters only transmit on one frequency at a time.

 

[techman210]

I’ve seen some GE Mastr Pro transmitters do this with ease, but only within 500 kHz of the designated transmitter frequency!

 

[davidgcet]

yep it is patched at dispatch or in some instances possibly even on scene with equipment in a scene commander's vehicle.

 

[nokoa3116]

Thanks for the replies. What got me most curious about is the actual sound. For San Jose Fire Department I heard unit's radios they receive P25 voice. But if I listen to their old analog I can hear them, and it doesn't sound like the P25 voice was taken and sent as analog, as their voice sound just like it would if it was still analog. How would patching work with this situation, do they just always transmit analog voice, and then the console takes that audio and encodes it to P25 voice and sends both out?

I mentioned that the County Fire channel does that. Unit's sill use analog on their radios, but it also gets transmitted to the P25 system, via patch I guess, but you can clearly hear that the analog voice audio was just taken as is and encoded into P25. Some transmissions they make are really dirty, and you can also hear the tail squelch or what not in the P25. San Jose FD does not sound like this.

 

[paulears]

The snag with trunked systems is that whenever you listen to them, you’re never quite sure if it’s local or re-linked distant traffic that’s actually on another frequency somewhere else. Combined analogue and digital repeaters are off the shelf products that simply repeat what appears at the input. What cannot happen is analogue and digital on the same channel at the same time. The two modes are not combinable like this, so if you hear it in analogue, your source locally is analogue, carrying decoded digital.