Do I need 2 Dongles?

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auti

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I want to monitor one P25 trunked system with one control channel. Can I do this with just one? I seen videos on youtube with people with one then I see post saying you need two? Can I do this with just one?
 

KD8DVR

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Yes, depending on software and the difference between all frequencies in the system you want to monitor. Get 2.

AntiSquid Disclaimer: All comments are personal opinion only and may not indicate a claim of actual fact.
 

br0adband

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The general recommendation - because the RTL-based USB sticks are so cheap in terms of price, roughly $10-25 depending on which ones you choose - is yes, get two of them so things are easier to do because of having two of them to work with. It technically is possible to monitor a trunked P25 Phase I or even Phase II system using OP25 but that's a Linux-based monitoring solution and it's not a very simple thing to get running. You can monitor a trunked P25 Phase I system with just one SDR device but it won't be nearly as efficient as using two sticks ends up being with one dedicated for the control channel and one dedicated for hopping the voice channels.

You're not limited to just P25 Phase I these days because of DSD+ and other protocols it supports, but the only way to monitor P25 Phase II still to this day is either a physical scanner with support for that or OP25 and OP25 does a really damned good job of it, especially in terms of the audio quality it produces. If it was just easier to set up I believe more people would probably make use of it even in spite of the Linux necessity.

But definitely get two of 'em just in case. :)

EDIT:

DOH, forgot about SDR Trunk which iirc is capable of monitoring trunked P25 Phase I systems using just one USB stick as well but don't absolutely quote me on that. I can't find the actual thread for that program (it's Java-based) so perhaps the author DSheirer will chime in here or you can PM him for some info. The website for the program is here:

https://github.com/DSheirer/sdrtrunk
 
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M105

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Get two dongles and DSD+ fastlane. Best money you ever spent on the scanning hobby when you connect up a decent old computer and antenna.
 

SCPD

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br0adband's advice is dead-on. Even if the software you choose can handle voice and control channel data from the same device - you are still limited to the SDR's usable bandwidth.

Having a second SDR avoids that.
 

KD8DVR

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br0adband's advice is dead-on. Even if the software you choose can handle voice and control channel data from the same device - you are still limited to the SDR's usable bandwidth.

Having a second SDR avoids that.
In my area, there are systems with frequencies over 6 Mhz apart. No way to just use one.

AntiSquid Disclaimer: All comments are personal opinion only and may not indicate a claim of actual fact.
 

br0adband

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Just for the record: if you want to monitor a P25 Phase I or II system you can do that using OP25 and with just one dongle - note that in this situation with this software the bandwidth capability of a given SDR device is not relevant because OP25 works like an actual scanner does with just the one tuner aka VCO, it hops from the control channel to voice channels and back just like a physical scanner does - without any issues whatsoever. Based on my own experience of using OP25 for a short period of time it was more than capable of handling Phase I or Phase II using one single RTL-based dongle - as noted earlier OP25 is not the easiest thing to set up because it requires a Linux base to work from and it's something you have to compile yourself on the given hardware for best performance and it effectively requires a dedicated computer to run it.

There's been many attempts by many people (myself included) to get OP25 working inside a Linux guest OS virtual machine running on a Windows host OS with VirtualBox and VMware but from what I gather nobody has ever been truly successful getting it to work properly and it's not necessarily the fault of OP25 itself but more that virtual machines are taxing the host hardware beyond the point where it'll work efficiently. While it's possible to get OP25 to work in such a situation, the resulting audio performance is typically very poor, scratchy, with a lot of dropouts, whereas running on bare metal hardware with a proper Linux installation will ensure the best possible performance.

Because of the rather complex setup a lot of people simply won't ever bother with it unfortunately but it's understandable because of that effort required not only the dedicated machine. Some of us have way too many computers (yeah, I know) and some of us can spare a laptop for such purposes or even dedicate an old tower of some kind for it. Compiling OP25 is a rather CPU/RAM intensive purpose so the recommendation is to have at least 4GB of RAM and a dual core machine - even the best dual core machines could take 2-4 hours or even longer to get the compiling of everything done completely, the newer and more powerful the CPU the less time it'll take but if you have something like an older Core 2 Duo laptop or desktop laying around with 4GB of RAM in it you could be looking at 4-6 hours or more to get it all done. On top of that will be needing to create the configuration files which can be problematic (as I learned myself) if even a space is out of place - yes, I do mean a space in the config files, or a tab out of place (shifting the data too much), and so on.

Yes I'm making it sound like it's nearly impossible but it's really not, just intensive and requiring of some focus and concentration to get it done.

Is it worth it in the long run? Yes, it absolutely is, especially when you do get it working and hear those transmissions for the first time you'll think "Cool..." like so many others have (yes, myself included again).

And you can do the monitoring, as stated, with just one RTL stick. Considering most of us already have a computer (since we're members of this site and obviously we can get online to participate), it's really quite awesome to know that you can monitor a P25 Phase I or Phase II system with a $10-25 "cheap USB TV tuner" and a free application running on a free OS and do it just as well as the best commercially manufactured scanners made by Uniden and Whistler these days.

Good luck... :)
 
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