Number of TalkGroups Affect Scanning?

[cory_rentz]

This is a general question, but location may be important - Olmsted Co., Rochester area (Stewartville).
I'm fairly new to scanning and trying to wrap my head around all the trunking stuff. Using the ARC-XT software; I got my BCD996P2 programmed with the Olmsted Co. Simulcast (ARMER) frequencies and added the appropriate talkgroups that I was interested in (mainly Law Enforcement/State Patrol.) I have a tuned antenna (806-866Mhz) on the roof, and running ultra low-loss cable (50 ft.). However, I don't seem to be picking up much of anything.

I'm wondering if the number of talkgroups that get added slows things down, or if they just act as a "reference" for when the control channel is activated. Often enough I can see the signal icon light up at freq. 853.6125, but I'm not hearing anything. My assumption is it's communication in a talkgroup that I haven't added. I also seem to get a few "Copy That" transmissions, but not getting the other side of the conversation.

For reference, I have the following talkgroups added: (295 Total)
Statewide Interop, MNDPS BCA, MNDPS State Patrol, MNDPS 911, Olmsted Co., SE MN Interop, SE MN EMS Hailing, Mayo Clinic Medical Transportation

Thanks in advance for any insights.

 

[johnmoe1]

Be aware that Olmsted County dispatchers often flip on encryption if there is something interesting going on. The response to the encrypted message is usually something like "<unit #> copies."

 

[wtp]

Olmsted Co. Simulcast	Olmsted, MN	851.050	851.150	851.300	851.5375a	853.2625	853.5125a	853.6125c
				853.775	857.4875	858.4875	859.4875

and there is that simulcast word that many of us don't like.
and usually is the reception problem.
try taking the radio for a ride close to a tower.

 

[doctordialtone]

This is a general question, but location may be important - Olmsted Co., Rochester area (Stewartville).
I'm fairly new to scanning and trying to wrap my head around all the trunking stuff. Using the ARC-XT software; I got my BCD996P2 programmed with the Olmsted Co. Simulcast (ARMER) frequencies and added the appropriate talkgroups that I was interested in (mainly Law Enforcement/State Patrol.) I have a tuned antenna (806-866Mhz) on the roof, and running ultra low-loss cable (50 ft.). However, I don't seem to be picking up much of anything.

I'm wondering if the number of talkgroups that get added slows things down, or if they just act as a "reference" for when the control channel is activated. Often enough I can see the signal icon light up at freq. 853.6125, but I'm not hearing anything. My assumption is it's communication in a talkgroup that I haven't added. I also seem to get a few "Copy That" transmissions, but not getting the other side of the conversation.

For reference, I have the following talkgroups added: (295 Total)
Statewide Interop, MNDPS BCA, MNDPS State Patrol, MNDPS 911, Olmsted Co., SE MN Interop, SE MN EMS Hailing, Mayo Clinic Medical Transportation

Thanks in advance for any insights.

In answer to your question, the scanner monitors the control channels only. The number of talk groups won't effect scanning speed.

Only the number of Control Channels will have an effect

Control Channel Only - The RadioReference Wiki

wiki.radioreference.com

 

[ofd8001]

Expanding a little more. . .

Unlike conventional scanning, the number of talkgroups does not affect scanning speed. The number of active sites will. Since you are scanning just the one site (hopefully all the others for the ARMER system are not programmed or locked out, etc.) you will "scan" as fast as possible. (If you were scanning multiple sites, that means the scanner is checking multiple frequencies for control channel data.)

That was the short answer. If you want to know the long-winded why answer:

With conventional scanning, the scanner "listens" to each frequency that is not locked out, checking for active transmissions. If it receives one, the scanner stops and you hear the conversation.

Trunked scanning works a different way. The scanner "listens" to all the frequencies assigned to a site and when it "finds" the control channel, it stops on that frequency and monitors the control channel data. This happens very fast because there are so few frequencies to check (even fewer if you program only the primary and alternate control channels).

During that monitoring the data stream on the control channel, the scanner "notes" the data which would include the talkgroup ID of the channel grants. The scanner will then compare the received talkgroup ID against the list of talkgroup IDs you have in your scan list. If there is a match, the scanner will "tune" to the voice frequency assigned to the conversation and you will hear it.

As noted above, if the conversation is encrypted, you won't hear that. If you are in a simulcast situation where your scanner is receiving signals from two or more sites, it may have a problem decoding the signal. That can either sound like garble or no audio though you may see full signal strength on the meter. (I'm in perfect "simulcast hell" where I live - there are three towers/sites that overlap my home and my 996XT decodes very little audio sufficient to hear.)

More of a suggestion - you may wish to consider adding the statewide interoperability talkgroups too, such as the L-TACS. A lot of interesting things happen there. For example, if there is a pursuit, whatever talkgroup is involved is patched to an L-TAC talkgroup. And potentially you could even hear pursuits happening away from your area, because of the "magic" of the system.

 

[cory_rentz]

Expanding a little more. . .

Unlike conventional scanning, the number of talkgroups does not affect scanning speed. The number of active sites will. Since you are scanning just the one site (hopefully all the others for the ARMER system are not programmed or locked out, etc.) you will "scan" as fast as possible. (If you were scanning multiple sites, that means the scanner is checking multiple frequencies for control channel data.)

That was the short answer. If you want to know the long-winded why answer:

With conventional scanning, the scanner "listens" to each frequency that is not locked out, checking for active transmissions. If it receives one, the scanner stops and you hear the conversation.

Trunked scanning works a different way. The scanner "listens" to all the frequencies assigned to a site and when it "finds" the control channel, it stops on that frequency and monitors the control channel data. This happens very fast because there are so few frequencies to check (even fewer if you program only the primary and alternate control channels).

During that monitoring the data stream on the control channel, the scanner "notes" the data which would include the talkgroup ID of the channel grants. The scanner will then compare the received talkgroup ID against the list of talkgroup IDs you have in your scan list. If there is a match, the scanner will "tune" to the voice frequency assigned to the conversation and you will hear it.

As noted above, if the conversation is encrypted, you won't hear that. If you are in a simulcast situation where your scanner is receiving signals from two or more sites, it may have a problem decoding the signal. That can either sound like garble or no audio though you may see full signal strength on the meter. (I'm in perfect "simulcast hell" where I live - there are three towers/sites that overlap my home and my 996XT decodes very little audio sufficient to hear.)

More of a suggestion - you may wish to consider adding the statewide interoperability talkgroups too, such as the L-TACS. A lot of interesting things happen there. For example, if there is a pursuit, whatever talkgroup is involved is patched to an L-TAC talkgroup. And potentially you could even hear pursuits happening away from your area, because of the "magic" of the system.

Thank you all for your answers and input - apologies for getting back to this late, work has kept me busy. I'm going to try to throw some information out, assumptions, and questions.

@johnmoe1
"Be aware that Olmsted County dispatchers often flip on encryption if there is something interesting going on. The response to the encrypted message is usually something like "<unit #> copies.""
I was not sure if Olmsted used any encryption... too bad we can't decrypt. However, here in Stewartville there is almost always two deputies on duty. I've watched them enough to know there is plenty of back-and-forth on the radios - that should not be encrypted. Yet I don't hear any or most of that. I'm also fairly certain there is a repeater tower near by, so I should hear all or most of the car-to-car or car-to-dispatch conversation, correct???

@wtp
"and there is that simulcast word that many of us don't like.
and usually is the reception problem.
try taking the radio for a ride close to a tower."
I would take a ride with the radio, if I had a mobile antenna and knew where the towers were. As stated above, I'm pretty sure there is a tower nearby, just not certain where.

@ofd8001
THANK YOU for your long=winded answer, it's always appreciated to have things explained, in a simpler manner.
How would I know if I am receiving signals from more than one site? I have a hunch that might be at least part of the issue. There is some that comes through and it seems garbled - I figured it was a signal strength issue.
I do have the statewide interoperability added, as well as several other things. The only thing I seem to get, and it's not consistent, is EMS.

I am adding some screenshots from ARC-XT so everyone can see what I have, and maybe point out any issues.

 

[ofd8001]

With a simulcast system, there is no way of knowing which site or sites you are receiving. Your scanner may be receiving from Simulcast site 1 and Simulcast site 2 or Simulcast site 1, site 2 and site 3.

As a review a simulcast system consists of more than one site all of which are transmitting on the same voice frequencies at the same time.

If you "know" your scanner is receiving traffic, but the audio is garbled or absent, odds are good that you are getting radio signals from multiple sites.

If you hear reasonably clear audio that means the incoming signal from one site is "probably" for lack of a better word overpowering the other sites, or you could be at the outer fringes of the coverage area. On the latter, you are probably receiving from one site.

X96 scanners are not the best scanners for decoding simulcast. So you have to be fairly close to a site to get good decode, or on the outer fringes.

You can always confirm a scanner is programmed properly and works properly by bringing it to about a quarter mile away. If you get clear audio it confirms you have programmed it correctly and the scanner is capable of good decode. The rest is a matter of physics and the radio nature.

Lastly, with simulcast systems, the issue is not necessarily a matter of signal strength. Rather it is about how well the scanner decodes all the incoming digital signals from multiple sites. Signal from Simulcast site 1 arrives micro seconds apart from the signal from simulcast site 2, kind of creating an "digital echo" and the scanner has difficulty dealing with the echo.

 

[wogggieee]

X96 scanners are not the best scanners for decoding simulcast. So you have to be fairly close to a site to get good decode, or on the outer fringes

I've had some success with those scanners using a yagi pointed at a single known tower to remove or reduce simulcast issues.