ScanDiego.com

Today on the San Diego city trunked sytem, PD Dispatch patched together Western and Northern talkgroups (1616 to 208). When patching is done, are they using one frequency? i.e. When Dispatch gives an "all units all frequencies" broacast, they certainly can't be using all the frequencies, just all the Police talkgroups relative to the transmission....right? *scratches head*


-Craig

They are using old terminology, they have no idea what freqs they are using. They are using all the dispatch talk groups. That would be 6 different freqs. When patching 2 or more talk groups they still should be using 2 (or more) different freqs. But the dispatchers dont know, dont need to know, what freqs they are using. Thats what the fancy trunking computer decides.

There are 19 freqs in the SD city trunk not 6. When patches or multi selects go out, the traffic is carried on one freq only. The trunking system itself spreads the audio over as many talkgroups as are selected at the dispatch console.

JoeyC wrote:There are 19 freqs in the SD city trunk not 6. When patches or multi selects go out, the traffic is carried on one freq only. The trunking system itself spreads the audio over as many talkgroups as are selected at the dispatch console.

There are only 6 primary PD dispatch talkgroups though, which was what I was refering to.

If it uses one frequency for multiple talkgrouns, I'm curious as to how that is accomplished. Patching and multiselecting is done through the radio console (centracomm 2's at SDPD). A patch or multi select is done through the console sofetware as far as I know. Anything on the screen can be patched... Cross band, from one trunked system to the next, to open phone lines used as intercomms, trunked to non trunked, etc. Certianly in cases like those, esp cross band, the same freq can not be used.

I guess I'll ask my radio tech the next time he wanders by.

Obviously if there is a patch between SDPD and CVPD for instance, one freq is tied up to carry the SD part of the patch on the SD system and a 2nd freq on the RCS south since that is where CVPD is. Patching SDPD Western D1 to Eastern D1 should only involve 1 freq since this is the same system.

I would imagine since the control channel datastream carrys the information about which channel Western D1 is on at this moment, the same would apply when the dispatcher selects a patch between the two.

Well I don’t know how all this stuff works in a technical sense.
And I am no authority on how all these patch’s work (I wish I knew more)
But I can tell you that the San Diego Fire Department uses a lot of patch’s
When dispatching call’s on 7-A like adding in the Administration TG,
or other TGs like 4688 not exactly sure what 4688 is but they patch to it,
And whenThey do I’ve seen as many as 3 separate frequencies on the San Diego system
simultaneously putting out the same call.
And one other tidbit I know when there is a really big call that having so many units logged on to one call
seems to be hard on the old computer, so I would not think that it would be able to see all the unit’s at once,
and needs to keep it all separated.
And that 6 separate frequencies are used to put out that all units.
So color me wrong and tell me how this all this stuff works.
And if any body really knows how SDFD Uses It’s Console.


Steele 8)

When two or more talkgroups are patched on the same system, it is carried over one frequency only.

The controller directs radios to the proper frequencies.

Sean set up a conventional 800 system for the city a few times over and
You too can see that more then one 800 frequency is being used for fire dispatch’s.
And that really does not explain, what it is that fire is doing when they make a dispatch.
Or what TG 4688 is ?
But thanks for the input, and all of your help


Steele 8)

Steele wrote:Sean set up a conventional 800 system for the city a few times over and
You too can see that more then one 800 frequency is being used for fire dispatch’s.
And that really does not explain, what it is that fire is doing when they make a dispatch.
Or what TG 4688 is ?
But thanks for the input, and all of your help


Steele 8)

What I explained is how the Motorola specs for type II trunking works. Any multi select or patch is directed by the controller to a single frequency.

It is possible that the fire dispatcher is actually using the console in some way that they are actually dispatching on two different talkgroups via some multi select on the console rather than thru multi select on the Motorola system. A simplified example would be wiring two buttons together to activate two talkgroups. The system sees that as two different PTT requests.

I wouldn't trust too much what you see on the display of any scanner. Remember, these were reverse engineered and coded by a third party. We all know they didn't implement a type II system correctly, or scanner users wouldn't have problems with this. The scanners have several bugs or oversights. A Motorola radio absolutely has no problem hearing 7A or two patched PD talkgroups. It's totally transparent, whereas the scanners go off into never-never land when 7A comes up. This is the proof that the scanner firmware is deficient in some way.

It seems that you're saying to setup two scanners and watch the frequency displayed. I wouldn't put too much credence in what the display shows. Remember, the scanner is confused in some situations. I've also noticed the scanners get data errors in decoding the control channel quite often. I always hear medics popping up on LE S CMD (for example) and different talkgroups that they don't have in their radio and all other combinations of users being where they shouldn't be. They're not actually there, it's just the scanner decoded something wrong, most likely due to a bit error in decoding the CC.

4688 is probably just a talkgroup that we never identified and wasn't used until recently. It's a valid type II group, (div/16). It's not a resultant group because of a bit flag.


I'll make some inquires and see if I can get the scoop on FD's dispatching protocols.

I have had 4688 in my scanners for quite a while, and got it off of some list labeled as Simulcast. The only thing I have ever heard on it is dispatch of calls. I am certain that I have never heard any type of "regular" traffic.

Perhaps it is a talkgroup that is programmed for "receive only" devices or radios so that those people can hear fire dispatch, but not need to hear "regular" traffic or to respond to fire dispatch. Just a guess.