Occasional Garbled Audio using OP25

[scottabing]

I've had a complaint about an intermittent issue where audio is garbled, I'm including a link to see if anyone can identify what may be the issue.

Google Drive Link to Example Audio

Fresh install as of 5/26/20 running on pi4 with NooElec NESDR Smart v4 SDR
./rx.py --args 'rtl' -N 'LNA:35' -M meta.json -S 960000 -x 2 -q 0 -T trunk.tsv -l http:0.0.0.0:8080 -w -v 1 -V -2 2> stderr-stream0.2

This recording occurred with a yagi antenna aimed at the closest simulcast antenna site which is about 4 miles away but I seem to have the similar results with the cheap kit antenna.

It seems to only happen sporadically. In attempting to troubleshoot this, I built a second identical system and ran them side by side. I do not get identical audio on both systems, sometimes it happens on one or the other. Or would be worse on one than it is on the other.

I have very limited background in RF (don't assume I know what I'm doing) and linux but I've gotten this far by following the readme and reading in the forums. OP25 has been awesome, I didn't even know it could work this well!

Very interested in how to reduce the likelihood of these occurrences though as I've hit the end of the road on my googling.

 

[boatbod]

Please post the constellation plot (#2)

 

[scottabing]

Here is a quick recording with some transmission.

Thanks!

 

[boatbod]

The constellation plot is supposed to have 4 tight clusters in the corners. Those large spread-out "12-gauge shotgun pattern tests" are not going to yield good stable audio. Type adjusting the antenna or moving it around a little bit to see if you can increase the signal strength and tighten things up.

 

[scottabing]

That was it, seeing much tighter pattern on the constellation after reaiming. I'm pointed at a different tower site now and so far the audio hiccups and distortion are gone and it is the best it has ever sounded. For reference, here is the cheap $17 yagi I'm using to get good results: https://www.amazon.com/dp/B00EC804SO/ref=cm_sw_r_tw_dp_U_x_ScM2EbHDV5DXN So far the audio is much more stable than the omni-directional kit antennas and I think I finally have this ready for 24/7 operation and will monitor for a couple more days. Thanks boatbod for the help and all that you do for OP25 and the users.

 

[scottabing]

I'm still noticing occasional garbled audio but I'm not sure if I should keep pursuing this as it seems to happen with any combination of hardware and antennas.

Just wanted to show the difference with the yagi now moved to a 15' mast outside my house:

Does anything need adjusting?

 

[boatbod]

Your tuning is off a ways... When you turn on the datascope plot you should have the eyes centered vertically if the tuning is correct. See example as follows:

 

[scottabing]

Thanks for that explanation of the datascope.
I've made that adjustment but the issue is persisting on both my pi systems.

I'm looking for any guidance on what direction I can take to troubleshoot this further as these glitches in the audio persist.

Sometimes it will be fine for 15 or 20 minutes but inevitably the bleeps and bloops creep in and sometimes an entire transmission will be garbled.

I appreciate your patience boatbod and I'm thankful to anyone who has suggestions or can relate to this phenomenon. If this is normal, I'll go enjoy my weekend. If it isn't, I'm willing to give anything a shot.

 

[maus92]

I've had a complaint about an intermittent issue where audio is garbled, I'm including a link to see if anyone can identify what may be the issue.

Google Drive Link to Example Audio

Fresh install as of 5/26/20 running on pi4 with NooElec NESDR Smart v4 SDR
./rx.py --args 'rtl' -N 'LNA:35' -M meta.json -S 960000 -x 2 -q 0 -T trunk.tsv -l http:0.0.0.0:8080 -w -v 1 -V -2 2> stderr-stream0.2

This recording occurred with a yagi antenna aimed at the closest simulcast antenna site which is about 4 miles away but I seem to have the similar results with the cheap kit antenna.

It seems to only happen sporadically. In attempting to troubleshoot this, I built a second identical system and ran them side by side. I do not get identical audio on both systems, sometimes it happens on one or the other. Or would be worse on one than it is on the other.

I have very limited background in RF (don't assume I know what I'm doing) and linux but I've gotten this far by following the readme and reading in the forums. OP25 has been awesome, I didn't even know it could work this well!

Very interested in how to reduce the likelihood of these occurrences though as I've hit the end of the road on my googling.

That's an extremely weak signal being only 4 miles away, especially with a Yagi that will add some gain.

 

[maus92]

Thanks for that explanation of the datascope.
I've made that adjustment but the issue is persisting on both my pi systems.

I'm looking for any guidance on what direction I can take to troubleshoot this further as these glitches in the audio persist.

Sometimes it will be fine for 15 or 20 minutes but inevitably the bleeps and bloops creep in and sometimes an entire transmission will be garbled.

I appreciate your patience boatbod and I'm thankful to anyone who has suggestions or can relate to this phenomenon. If this is normal, I'll go enjoy my weekend. If it isn't, I'm willing to give anything a shot.

The other possibility for the garbling is that the subscriber is not providing a good talk-in signal. Nothing you can do about that.

 

[scottabing]

Thanks maus92, what should I expect to see for signal strength? Running 2 separate OP25 configurations side by side, one with an omni directional antenna and one with a yagi I'm noticing the garbling seems to happen with only certain transmissions but doesn't always manifest exactly the same way on both systems. So it definitely seems like it is happening with certain transmissions. I'm not sure what else I can do to diagnose this.

 

[boatbod]

If you have a sloppy constellation plot and see numerous "control channel timeout" messages in the logs then it's a fair bet the signal quality is poor. If the constellations are generally very tight, the audio disturbance may have other causes. It's also not unheard of for just one or two frequencies to be messed up while others are solid. (I see that a lot on my home system where the CC is noisy yet the voice frequencies are good).

 

[maus92]

Thanks maus92, what should I expect to see for signal strength? Running 2 separate OP25 configurations side by side, one with an omni directional antenna and one with a yagi I'm noticing the garbling seems to happen with only certain transmissions but doesn't always manifest exactly the same way on both systems. So it definitely seems like it is happening with certain transmissions. I'm not sure what else I can do to diagnose this.

The Phase 2 system I monitor has ~-38db signal strength on the control channel, with a similar noise floor. The voice channels are a bit more powerful in terms of db.

I believe the SARA system that you are monitoring does not use Motorola infrastructure based on the WACN - do you know the manufacturer? Might be Harris, which are a bit notorious with weird signalling issues. Attached is the Constellation Plot of the system I monitor: notice the tight groupings at the poles. I was lazy and didn't fine tune the SDR.

What happens if you use the crappy antenna that comes with the SDR kit? Or even a piece of wire as the antenna? You could also boost the LNA value incrementally to see it it helps.

 

[maus92]

Also, looking at the RRDB shows a number of encrypted talk groups on the system, and for Linn County - they will sounds garbled.

 

[boatbod]

Also, looking at the RRDB shows a number of encrypted talk groups on the system, and for Linn County - they will sounds garbled.

The default action of the current rx.py (as of a couple months ago) is to silence encrypted audio unless specifically overridden by command line option.