RadioFeed Delaying one channel in RadioFeed?

[KD0TAZ]

Hey,

I've had a feed up and running for a while now which covers my county's police, fire, EMS, and SO. It is P25, and I've run into an issue in trying to provide "complete" coverage. When EMS/FD are paged out, a pre-alert goes out over the Law Dispatch TG, then the dispatcher says something like "Dispatch to Fire/EMS Station #, medical emergency, 1234 Main St, standby for page". Then the call is actually paged out over the VHF system. When I used to live in another town that did it that way, it was no problem, because all I did was connect an analog scanner to the right channel and locked it to the pager frequency. However this city does things a little different and with that setup it's unintelligible during EMS pageouts.

For some reason the EMS pageouts (including tones) are simulcast over the P25 Fire/EMS Dispatch TG, while the FD pageouts are not. I actually didn't realize this for a while. I tried to set up the stereo feed again, but - due to processing or whatever - when EMS is paged, it is delayed on the P25 scanner about 3/4 of a second behind the VHF scanner. Since FD pages only come through VHF it fixes the problem of missing those, but EMS pages come through the stream as an "echo" and it's impossible to understand unless you shift the balance.. The delay seems to be pretty consistent, so is there a way to delay one channel in order to sync them up? I'm using RadioFeed.

 

[boatbod]

Since FD pages only come through VHF it fixes the problem of missing those, but EMS pages come through the stream as an "echo" and it's impossible to understand unless you shift the balance.. The delay seems to be pretty consistent, so is there a way to delay one channel in order to sync them up? I'm using RadioFeed.

The issue is that digital systems have a coding delay not present in analog transmissions. i.e. the time it takes to convert the microphone signal from analog to digital, encode it with IMBE/AMBE and send it to the network which turns around and re-broadcasts it. The scanner then receives the the incoming P25, recovers the IMBE/AMBE parameters and converts these to a digital pcm stream and then an analog signal to play through the loudspeaker. Typically the delay is a half-second to second, and isn't just present on consumer scanners; you can experience the same thing if you operate a pager and a subscriber radio side by side, or even two subscriber radios on the same tgid.

 

[fredva]

Once the pages go out for FD and EMS, does the radio traffic associated with those calls stay on VHF analog? Or do they switch to the trunked system for the dispatcher to give info to responding units about the call, for the responders to report they are on scene, etc? Just wondering how critical it is to include the VHF analog. On one of my feeds, an EMS station is paged on an analog frequency, but when the ambulance responds, all the info about the call is passed on a digital trunked talkgroup. I locked out the analog frequency due to interference, but because all of the info about the call is repeated on the trunked system, the listeners don't really miss anything other than pager tones.

 

[KD0TAZ]

The issue is that digital systems have a coding delay not present in analog transmissions. i.e. the time it takes to convert the microphone signal from analog to digital, encode it with IMBE/AMBE and send it to the network which turns around and re-broadcasts it. The scanner then receives the the incoming P25, recovers the IMBE/AMBE parameters and converts these to a digital pcm stream and then an analog signal to play through the loudspeaker. Typically the delay is a half-second to second, and isn't just present on consumer scanners; you can experience the same thing if you operate a pager and a subscriber radio side by side, or even two subscriber radios on the same tgid.

Oh yeah I know why it happens, I'm just looking for a way to sync the two channels better. I know it'll never be able to be truly synced, and the way I have it set up all traffic is readable, it's just annoying to have the "echo".

Once the pages go out for FD and EMS, does the radio traffic associated with those calls stay on VHF analog? Or do they switch to the trunked system for the dispatcher to give info to responding units about the call, for the responders to report they are on scene, etc? Just wondering how critical it is to include the VHF analog. On one of my feeds, an EMS station is paged on an analog frequency, but when the ambulance responds, all the info about the call is passed on a digital trunked talkgroup. I locked out the analog frequency due to interference, but because all of the info about the call is repeated on the trunked system, the listeners don't really miss anything other than pager tones.

The pages go out over analog then traffic is on a TAC TG. I know it's not really important and yeah you're really only missing pages for the most part but it's annoying that you don't know where fire calls are basically until they arrive on scene because the location is sent in the page.. As I said, for some reason EMS pages are simulcast on the FD/EMS Dispatch TG but FD are not.

 

[fredva]

it's annoying that you don't know where fire calls are basically until they arrive on scene because the location is sent in the page.

Oh ok. That's a different dispatch protocol than in most jurisdictions I'm familiar with. It is fairly common for the dispatcher to repeat the location of the call when units radio that they are responding, as well as give the age of the patient, medical history and other details that weren't given in the initial page. So your situation is a little different.

 

[jim202]

If you think that is bad, try standing next to a person that is talking on a trunking radio and others standing nearby with their portable trunking radios on the same channel. You hear the same 3/4 second or so delay.

kind of blows your mind when trying to say what you need to over the radio and hear your voice being delayed as you talk. It generally stops people mid sentence if they are not use to hearing it.

 

[KD0TAZ]

Oh ok. That's a different dispatch protocol than in most jurisdictions I'm familiar with. It is fairly common for the dispatcher to repeat the location of the call when units radio that they are responding, as well as give the age of the patient, medical history and other details that weren't given in the initial page. So your situation is a little different.

As I said, this is only an issue with fire calls. EMS does do that - but their pages are also simulcast on P25. The only opportunity to hear the location of a fire page over P25 is during the pre-alert that goes out over Law Dispatch, which you don't always hear if there's traffic on another TG at the same time. Sometimes the only indication that a fire page has gone out is when a unit responds on the TAC channel (and again sometimes the Law Dispatch TG has traffic while this is happening). When fire responds on the TAC they usually just say "Engine 1 responding to 3000 block 250th Rd". The captain is usually the first on scene and will say "Captain 301 on scene 3705 250th Rd, establishing 250th Command". In-city calls are a lot easier to figure out because the blocks are really short, but out in the country the rural blocks can be a mile or more long.

If you think that is bad, try standing next to a person that is talking on a trunking radio and others standing nearby with their portable trunking radios on the same channel. You hear the same 3/4 second or so delay.

kind of blows your mind when trying to say what you need to over the radio and hear your voice being delayed as you talk. It generally stops people mid sentence if they are not use to hearing it.

Yeah that happens with cell phones too. I called my neighbor because her cat was on the roof between our balconies, and she was still talking on the phone when she came out and it was weird hearing her talk through the air and then hearing it come through the phone half a second later lol