DSDPlus P25 simulcast decoding

[AM909]

Using SDRSharp 1807, VBCable, NooElec Mini2+ (R820T2) dongle, latest DSDPlus FL, Win10Pro here. When trying to decode some P25 control channels, even strong ones (>20 dB S/N), DSD+ doesn't decode reliably, either not syncing at all or not decoding some messages interspersed with good decodes seemingly at random. I think it's happening on simulcast systems only, and if I listen to the audio being fed by SDRSharp to DSD+, it sounds different than the systems that decode correctly. I've played with the SDRSharp audio gain and bandwidth, tried reducing RF gain to try to isolate the strongest site, etc.

DSD+ says "FM demodulated PSK signal detected. Use a FMPA/FMPP/FMP24 link and correct channel bandwidth to decode."

Does this mean SDRSharp can't be used for systems like this or is there a configuration item or plugin available to decode these signals more reliably?

 

[Reconrider]

Do you have dsdplus fastlane, or the free one that is public?
If you have fastlane, just use dsd and plug the frequency in and listen to the system that way. SDR# is pretty heavy now on the latest updates.

If you don't have dsd fastlane, give sdrtrunk a try. I never could get sdrtrunk to dsd working great enough to sound good for my liking.

 

[kj6psg]

DSD+ says "FM demodulated PSK signal detected. Use a FMPA/FMPP/FMP24 link and correct channel bandwidth to decode."

Does this mean SDRSharp can't be used for systems like this or is there a configuration item or plugin available to decode these signals more reliably?

It's telling you the solution: use FMP24, which is provided with DSDPlus Fastlane (since it's telling you about CQPSK being a problem, I assume you have Fastlane). FMP24 demodulates CQPSK directly, while using SDR# with a virtual audio cable gives a FM demodulation of a signal that isn't actually FM/FSK; that's why simulcast reception is so poor on FM-based devices.

 

[AM909]

It's telling you the solution: use FMP24, which is provided with DSDPlus Fastlane (since it's telling you about CQPSK being a problem, I assume you have Fastlane). FMP24 demodulates CQPSK directly, while using SDR# with a virtual audio cable gives a FM demodulation of a signal that isn't actually FM/FSK; that's why simulcast reception is so poor on FM-based devices.

Thanks for the confirmation. I'm asking if there is an SDRSharp-based solution. FMP24 doesn't have many of the features that are provided by SDRSharp and its plugins.

 

[AM909]

Looking at the Notes.txt, as of DSD+ 2.145, "The -mp (PSK optimizations) command line option has been removed; Optimizations for FM demodulated PSK signals are now automatically applied." This leaves some question as to the meaning of the "FM demodulated PSK signal detected. Use a FMPA/FMPP/FMP24 link and correct channel bandwidth to decode." message I'm getting. Is it saying that it only applies the optimizations if FMP* is used, and that it currently is not applying them, or does it mean that it has detected PSK, it is applying the optimizations, and use of FMP* is only an additional suggestion (and that the optimizations are therefore not effective in the SDRSharp→VBCable→DSD+FL scenario I'm using)?

I suppose I could find and roll back to 2.113 and see if manually specifying -mp helps. Or has someone been down this road before (successfully or not)?

(To clarify, yes, I'm using "DSD+ 2.324 / Fast Lane Release")

 

[Forts]

If you want DSD+ to work properly with simulcast systems, you have to use FMP24. Feeding it audio from anything else is going to cripple it.

 

[redbeard]

If you want DSD+ to work properly with simulcast systems, you have to use FMP24. Feeding it audio from anything else is going to cripple it.

This is the way

 

[CanesFan95]

Even with FMP24, I've always had broken / inconsistent audio decode. Doesn't matter the signal strength, gain settings, or what. Been that way for years.

 

[awasser1]

sdr trunk or op25 works great with p25 simulcasts

 

[redbeard]

I live near 4 simulcasts on 3 different bands and FMP24 has been flawless in all cases where my non-SDS scanners were useless. Any issues anyone is having with it may be computer related.

 

[air-scan]

I live near 4 simulcasts on 3 different bands and FMP24 has been flawless in all cases where my non-SDS scanners were useless. Any issues anyone is having with it may be computer related.

Why so dismissive? Why downplay the issue mentioned? You don't know that. These issues keep coming and its getting numerous. Just because you're not having issues doesn't mean everyone else with issues has computer problems. TDMA doesn't sound right in DSD+ FL at all. Phase I sounds decent but needs better. That does NOT indicate a computer issue!

 

[kj6psg]

I live near 4 simulcasts on 3 different bands and FMP24 has been flawless in all cases where my non-SDS scanners were useless. Any issues anyone is having with it may be computer related.

Issues with simulcast don't stem from the fact that a system is simulcast or whether a system is CQPSK. Rather, it's from timing mismatches between multiple received simulcast sites resulting from speed-of-light and/or intentional delays. Simulcasts work well when you're inside the coverage area, where the transmitted signals' timing can be controlled to arrive at the receiver simultaneously, but are unpredictable when you leave the coverage area. Some out-of-area situations work, particularly those where only one site can be heard; and some do not, particularly those where you have a distant, higher site in-line with a lower, nearby site. Moving a few thousand feet can cause drastic changes in receive performance. Proper CQPSK demodulators are far more forgiving with timing mismatches than signaling recovered from FM demodulation, which is why public safety radios can easily hear signals FM-based P25 scanners get no reception of when outside the simulcast's coverage area. CQPSK itself is far more forgiving with timing issues than C4FM (which has particularly tight timing mismatch windows), which is why CQPSK is used almost exclusively among simulcast systems. The tradeoff is in transmitter and receiver complexity, but P25 radios (at least Motorola's) have software defined receivers at the IF to properly receive CQPSK.

Regarding the original question, SDR# can't demodulate CQPSK itself, so the only way to get CQPSK from SDR# to DSDPlus is through FM demodulation. In theory, a plugin could be written that implements FMP24's CQPSK demodulation, but no such plugin exists.

 

[redbeard]

Why so dismissive? Why downplay the issue mentioned? You don't know that. These issues keep coming and its getting numerous. Just because you're not having issues doesn't mean everyone else with issues has computer problems. TDMA doesn't sound right in DSD+ FL at all. Phase I sounds decent but needs better. That does NOT indicate a computer issue!

My post was a statement of fact on my experience. Any dismissiveness, downplaying, or what I do or don't know are all constructs of your mind and are not reflected in any of my words. Also your additional complaints regarding the audio quality of DSD+ are not related to FMP24, which does not do the actual audio demodulation.

 

[Paco-Jerte]

for me SDRTRUNK works great for Simulcast here in L.A. No audio drops, if any using a discone antenna.

With DSD+ FL FMP24CC works good with a telescopic antenna (pointing at the site) but not so great with a discone.