When & why are two SDR's are prefered/required?

[iMONITOR]

I recall reading on occasion that sometimes people are using two SDR's for certain monitoring activites. I think it applied to digital scanning (IE: P25)? Why is that and what are the advantages?

 

[redbeard]

If you are monitoring a trunking system with a control channel, one dongle can stay on the control channel while the other follows the voice grants to the traffic channels. When trunking with a single dongle you will miss the other activity happening on the control channel while the dongle is tuned to the traffic channel.

 

[cmjonesinc]

To add to that... It's also very useful if you like to set talkgroup priorities. For example you can assign the dispatch a higher priority than a tac channel. So with two dongles if the one monitoring the control channel sees a high priority talkgroup come up it will switch from the tac back to dispatch immediately instead of waiting for the tac channel traffic to stop.

 

[sallen07]

It's also software-dependent. OP25 uses one dongle, but the channel grant traffic is also passed on the voice channels, so it still supports TG priority.

Some packages will use one dongle if all of the frequencies for the system fit within the bandwidth of the dongle (without retuning) but require a second one if they do not.

 

[iMONITOR]

Thanks guys I'm a little new at this. I had an RTL-SDR a couple years ago but didn't spend much time with it. I recently became somewhat bored with scanners (especially with all of Uniden's issues) in general. However I've been a computer geek since the 70's and combining the hobbies to this extent has peaked my interest. I have an RSPdx coming in the mail from a great friend I met here on RR! If i really get into it, I'll probably order the DUO as I can already seem some advantages.

 

[redbeard]

Given the HF coverage I find the RSP devices a waste for simply monitoring VHF/UHF/7-800 trunking, unless you are taking them portable for all-in-one space saving. I use the NeSDR SMArt units for my trunking monitoring at home. They are cheap and do an excellent job dedicated to trunk monitoring. Also the RSPs were only recently made compatible with DSD+, my software of choice.

 

[slicerwizard]

Dual dongles offer little advantage on most P25 systems. It's other types, like NEXEDGE, Con+, TIII, where constant monitoring of the control channel is required to support prioritized monitoring.

 

[enCrypt]

Thanks guys I'm a little new at this. I had an RTL-SDR a couple years ago but didn't spend much time with it. I recently became somewhat bored with scanners (especially with all of Uniden's issues) in general. However I've been a computer geek since the 70's and combining the hobbies to this extent has peaked my interest. I have an RSPdx coming in the mail from a great friend I met here on RR! If i really get into it, I'll probably order the DUO as I can already seem some advantages.

Given that you will have RSP devices, you may not need 2 SDRs depending on how far apart the control channel and voice frequencies are.
If they are within the max 10MHz bandwidth of the RSP (and your PC is "man-enough" to handle the display / processing of the whole 10MHz at once), you can setup another VRX or Virtual Receiver in the SDRUno software giving you essentially 2 SDRs.

 

[JimD56]

DUAL Dongles are a MUST on P25 "Simulcast" Systems. If you use SDRTrunk it is optimized for 2 Dongles. One keeps scanning "simulcast" control channels and the other the talk groups.
I run 2 Dongles on SDRTrunk and a 3rd Dongle for misc. VHF/UHF analog using SDRSharp

 

[sallen07]

DUAL Dongles are a MUST on P25 "Simulcast" Systems.

Depends on what software you are running. OP25 always uses one dongle, and does extremely well on simulcast systems.