Missing Some Traffic Because Control Channel Is Locked

[Ethan Patchen]

I'm not sure if there's just an option I'm not seeing but I would like to unlock the control channel during transmissions. I only need to hear one talkgroup at a time, and some of our frequencies are so far apart that they get rejected because the program wants to keep monitoring the control channel.

My thinking is that if I can unlock it, it can monitor the control channel but switch to a voice channel when it needs to, just like a normal scanner would do. I'm just trying to avoid needing to add another dongle.

Any advice would be appreciated.

- Ethan

[GTR8000]

You'll have to use different software if you want to do that, as SDRTrunk doesn't and can't operate in that manner. It is designed to remain locked on the control channel indefinitely, while simultaneously decoding traffic channels. So either add more dongles, or switch to something like DSD+ Fast Lane which can be setup to leave the control channel to follow voice traffic.

[Ethan Patchen]

Got it. Thanks for the reply.

I'm not willing to switch away from SDRTrunk, that's just minor to me. I'll probably add another dongle at some point.

Unrelated, but I have a question for you. I know you manage a portion of the RR database, and I'm interested to know how you get the radio information when it's not user-submitted.

I tried a FOIL request to Jefferson County asking for "all information pertaining to channels and/or talk groups on [our trunked radio system]" but the county denied it saying it would compromise the security of the system if they gave that out. I sent an appeal and clarified what I was expecting from them, literally saying that I only want a list or table of channel names and their associated talk group ID, so I don't see how they can deny that. I think they got away with denying the first one because I wanted "all information" and that would include encryption and system keys.

I haven't heard back in regards to that appeal that I sent, but considering that USPS takes about 2 days to deliver mail, and the county has 10 days to respond, I should've heard back today at the latest, but maybe it'll be tomorrow because of the holiday.

[GTR8000]

The overwhelming majority of the info that makes its way into the RRDB is user submitted via the normal submission form. We may also get info sent to us discreetly by "those in the know", and some of the info comes from personal knowledge database admins have, which gets entered directly.

As far as trying to obtain info via FOIA/FOIL, a friendly word of caution...don't poke the sleeping bear. Encryption on P25 systems is fairly cheap (if not free) and easy to implement these days, and so requests for info about their system may spook them, especially repeated attempts that require them to do paperwork. There is really very little reason to go that route when you can very easily log the talkgroup activity and figure out the usage of those not yet identified. And as for trying to figure out what the already encrypted talkgroups are, ultimately it doesn't really matter as you can't monitor them anyway. Nice to know what they are, sure, but not at the expense of overreaction to requests for the info by the agencies involved. And of course the very worst position to take is "I'm a taxpayer, it's my right to know!", so definitely steer clear of doing that. 😉

[wemana...@gmail.com]

On Tuesday, January 19, 2021 at 8:44:33 PM UTC-5 patch...@gmail.com wrote:

Got it. Thanks for the reply.

I'm not willing to switch away from SDRTrunk, that's just minor to me. I'll probably add another dongle at some point.

In re the original part of this. While the reply covers what are some of the options a complete path is:

1) Obviously add more SDR's  -

OR

2) Use an SDR with wider bandwidth(s) such as the HackRF's and AirSpys which can do 3/6/10Mhz swaths, BUT beware this comes with a computational need as well. Getting these units to work on limited level PC boxes will be the same agony.

1/2a -  be aware that some issues with multiple SDR's and/or high bandwidth SDR's can arise in the areas of the USB bus being overwhelmed even though you are not using its full capacity, this is a combination of hardware and OS issues the libs used to access USB via Java.

This is information to keep in mind if you want to try to use more than 3 of the "V3" type SDR's out there, and especially if you want to use the higher bandwidth ones on lower end PC's.

How far apart are your frequencies?

Use:

# SDR's (ROUND UP to next full number always w/any remainder!) =  (Freq Highest - Freq Lowest) / SDR Sample Rate in MSPS/MHZ

3) Alias out all TG's you don't want.

4) Prioritize the TG's you do want in the playlist editor

5) Disable data calls. This may help. As many Moto systems tend, I repeat, tend to favor data calls at the lower end frequencies and voice calls and CC at the upper end.  That is tends.

As pointed out SDRT doesn't work the way other software does such as OP25 or a scanner.

The reason a lot of these systems have wide spans is because of the Rebanding process when nexhell was kicked out of the band to their own pollution zone in the 862-869MHz ESMR band. So it was not a simple process to allocate channels close by to systems. Some also already were spaced out due to lack of frequencies when they built out. Its annoying. I've got one system like that we were handed piece meal frequencies all over the band. It was bad to start, this just made it worse.

6) if you have your CC at the highest frequency of your system, then you can also OVERRIDE the SDRT logic for tuning. This takes some experimenting to get right. You want to create a 2nd channel that is 2.4 (or what ever sample setting you are using) MHz below that to get the most of your tuning, rather than letting SDRT jump around alot. This may actually hinder things in some cases. Especially this may help if you notice most of your voice grants are near the CC, and data calls away from it.

7) Determine if your SDR will support the 2.5 or 2.68 Sample rate this is most of them. I've got some that it will run at the 2.68 Sample rate, but thats not V3's. Most of them I've tried refuse to work reliably at anything other than 2.4MSPS/MHZ. You can try it and see if it helps to reduce the number of missed calls.

Ultimately MORE SDR's is the only solution.

If you need more than 2 of the V3 type SDR's I would suggest that getting HackRF or Airspy clones is probably the best to eliminate headaches.