DSDPlus Improve P25 Simulcast audio decoding

[brian]

I'm using DSD+ Fastlane to monitor a reasonably busy P25 Phase I simulcast system and I'm getting what I consider to be pretty poor audio decode with lots of dropouts. I'm curious if there's any suggestions for improving audio decoding performance.

I'm running the latest versions of DSD+ (2.368) and FMP24 (2.76) with direct linking. I'm running a single dongle in combined control/voice monitor mode. Signal strength is good here as I'm in the midst of several tower sites, which cause my scanners to (predictably) operate very poorly. Gain is set to around 20. PPM setting is as close to accurate as I can get it, with auto frequency correction varying between -100 and +100.

I'm wondering if maybe this PC isn't up to the task. It's an older HP Folio laptop running 32-bit Windows 7, i5 processor, 1.6MHz with 4GB RAM. Please don't suggest OP24 (I'm not a Linux guy) or SDRTrunk (it requires a 64 bit OS).

Here's a shot of the source audio:



and the event window showing plenty of control channel decoding errors.

 

[mtindor]

Post your FMP* screen. Let's see what that looks like. Your bandwidth (BW) should be at 12.5 when monitoring the CC/voice channels. DSDPlus should automatically do that, but that all depends upon whether you've overriden that.

 

[maus92]

Gain might be a little low. What's your CPU loading as shown in Task Manager - that can give you a good indication whether or not your PC has the resources to run the program. Also, have you corrected your sdr dongles for frequency / ppm? TBH, DSD doesn't have the best audio; I've found that SDRTrunk has the best, OP25 a close second, while DSD is somewhat worse when monitoring a P25P2 / TDMA system.

 

[mtindor]

I see no reason whatsover why an i5 machine would have any issue with DSDPlus+FMP24 linked. It takes very little to run that setup. I'm with Maus regarding DSDPlus not having the best audio. I agree that SDRTrunk has the best. But try using SDRTRunk or OP25 for the tasks that DSDPlus is truly meant for (in my mind). Ya can't.

M

 

[KMG54]

Yep. I run a flight aware orange amplified stick on a small Discone and have no problem. Gain is set at 39. I also have a Par FM trap inline. Lenovo think center tiny PC with a I5 7700 16 gigs of memory and a 512 gig NVME M.2 drive, no hickups at all.

 

[brian]

FMP Window:


Bandwidth automatically switches to 9.5 BW if I manually change it to 12.5 by using the "B" command. I don't have a specific bandwidth set in the batch file, so I don't know how it would otherwise be overridden.

CPU is not taxed at all, less than 1%. I can easily run other applications with DSD+/FMP24 running with no noticeable slowness.

Gain higher than 20 just raises the noise floor but not the "height" of the signal peak.

 

[KMG54]

That looks fine to me. Have you tried to update anything on the PC? Seems like a decode issue on the PC side. Ooops windows 7, more than likely can not.

 

[KMG54]

Just for giggles see if you can download LAV codecs to the machine, install them and see if it helps.

 

[GTR8000]

Bandwidth automatically switches to 9.5 BW if I manually change it to 12.5 by using the "B" command. I don't have a specific bandwidth set in the batch file, so I don't know how it would otherwise be overridden.

P25 FDMA (includes the control channel) is 9.5, TDMA traffic is 12.5

+19 ppm is quite a bit of correction...are you using an old non-TCXO dongle? That could be an issue, if the voice channels are not centered upon tuning and FMP24 has to re-center them on the fly. You might want to invest in a better TCXO dongle to keep the tuning stable.

 

[KMG54]

This is why I use flight aware sticks, very stable they need to stay on freq!

 

[brian]

Yep, definitely a non-TCXO SDR.

However, I don't see the bandwidth change when tuning voice channels. FMP24 always says 9.5.

I also see plenty of errors in the control channel decoding logs. That makes me thing it's not an issue of poor tuning while switching from one frequency to another. If I switch to passive digital monitor mode, I still see those errors.

 

[KMG54]

I will send you a stick, I found that these work great. Spartanbug?

 

[KMG54]

Hear perfectly with this.

 

[maus92]

Hear perfectly with this.
View attachment 114068View attachment 114068

SNR is def better in this example vs. the OP's setup.

 

[KMG54]

And that is with a amped dongle on a discone. Took the FM trap out and no difference.

 

[KMG54]

Keep in mind I am in north east Pickens County and getting Laurins TG's loud and clear

 

[KMG54]

My picture is perfect Brian, turn thr gain up a bit, I would say 40, see what happens.
If it makes it worse time for some ferite on your feed line.

 

[KMG54]

did not pass spelling on that one but know what I mean.

 

[KMG54]

Yet another reason to only tune the control channel on SDR, single dongle and looking at alt channels equals bad decode.

 

[brian]

So the (mostly) successful resolution to this issue was to try a different antenna. I have a portable antenna that I have tried to place in a location to optimize reception of this system with my scanners, which is an exercise in futility. But connect that antenna to the dongle, as opposed to the smaller direct-connected antenna that I was using proved to improve reception and audio, with many fewer dropouts. It's far from perfect, but considerably better than it was. Thanks to all suggestions.