Distortion on P25 II BCD536HP

[CalmWind]

It seems my local public safety frequencies are exhibiting distortion.

The distortion manifests in two ways. First is the receiver may not be exactly on the correct frequency so the sound is garbled. The second seems to be a type of audio clipping where parts of the transmission are missing.

Most of these problems seem to happen with the dispatcher, which should be a strong signal. I have heard them change transmitters, which seems to clear up certain types of distortion. I can hear folks in other cities fine and no distortion.

If my system is being over modulated, is there a way to eliminate certain repeaters to try to eliminate the problem? I have tried Digital AGC which does not seem to help. I am running an external antenna, but it happened with the stock one as well.

Thanks.

 

[trentbob]

... welcome to the world of LSM... check the archives for LSM threads on p2 systems...

 

[ofd8001]

The first things to try are:

1. Set Site Attenuation On
2. Set P25 Threshold Mode to Manual
3. Try different P25 Threshold Level values

Sometimes Automatic Threshold and a different Threshold Level value are used. Its all trial and error to see what best combination of the three works.

 

[CalmWind]

OK, I will look for threads referencing LSM and P2 systems.

The P25 Threshold settings appear to be for Conventional Only. I have been looking at that setting which I assume is akin to a squelch setting.

The Attenuation set to ON would lower the signal by 20 db so maybe that will help if it is being over modulated.

I also see a Band Plan settings, but I would think when using a Trunk system in the database, such a setting would be correctly set as the default.

I still have quite a bit of learning to do even to understand what and how some settings are used. Maybe the other thread will fill in some of that void.

Thanks for the responses.

 

[CalmWind]

I think I finally found what "LSM" means .. Linear Simulcast Modulation...

I am adjusting the P25 Threshold and watching the errors to see if it gets any better.

The archive threads referenced seem to indicate Simulcast distortion is fairly common. I had read earlier that Simulcast depends on GPS positioning to adjust the timing of the signals. So I have attached a GPS to the radio to see if that helps.. If that is the case, simply entering a Zip Code may not be sufficient to help.

If none of this helps, the attenuation will be the next step.

 

[K4EET]

Could you be on the fringe of reception? The audio would sound like being under water. As for mobile versus dispatch, they should be the same if the manufacturer designed the system correctly (talk-in versus talk-out coverage areas) for the repeater(s). Nonetheless like "trentbob" said; "welcome to the world of LSM" if it is a multi-site system. 73 (best wishes in Amateur Radio lingo), Dave K4EET

 

[MStep]

It seems my local public safety frequencies are exhibiting distortion.

The distortion manifests in two ways. First is the receiver may not be exactly on the correct frequency so the sound is garbled. The second seems to be a type of audio clipping where parts of the transmission are missing.

Most of these problems seem to happen with the dispatcher, which should be a strong signal. I have heard them change transmitters, which seems to clear up certain types of distortion. I can hear folks in other cities fine and no distortion.

If my system is being over modulated, is there a way to eliminate certain repeaters to try to eliminate the problem? I have tried Digital AGC which does not seem to help. I am running an external antenna, but it happened with the stock one as well.

Thanks.

(First of all, hats off to you Dave (K4EET)--- it seems like only a few weeks ago you were wondering when you would have the experience to start answering a few posts rather than just asking for information. So I see you have been studying Dave--- keep up the good work!)

With regard to distortion. garbled transmissions, etc, on digital signals--- I think the guys here have offered some good explanations, but on a more general level, you are dealing with a 500 dollar scanner as opposed to professional digital communications equipment which costs considerably more. I'm sometimes amazed that we can receive both Phase I and Phase II as clearly as we sometimes do, so besides all the technical explanations and suggestions that folks here offer, you should also keep in mind the constraints that come with the cost of manufacturing a unit with these capabilities at the consumer level.

 

[CalmWind]

Could you be on the fringe of reception? The audio would sound like being under water. As for mobile versus dispatch, they should be the same if the manufacturer designed the system correctly (talk-in versus talk-out coverage areas) for the repeater(s). Nonetheless like "trentbob" said; "welcome to the world of LSM" if it is a multi-site system. 73 (best wishes in Amateur Radio lingo), Dave K4EET

Thanks for the response and I am on the Western side of the Trunk area. The closest nods are about 6 and 7 miles in the Southeast quarter. But if that was the problem, why would I be able to hear cities farther east.

MStep, thanks for the response and I am sure your assertions are at least partially correct, but I am not yet ready to settle for a "that's how it is" type solution.

Last night I was able to hear full transmissions without distortion, but not all the time. I am still working on the GPS situation because that seemed to help a little. For research purposes, I am adding other types of systems beside just the simulcast Phase II ones.. Maybe I will see some differences while I adjust with specific settings... I might even take my BCD436HP and drive around to different areas to see if the transmissions improve.

Thanks again..

 

[CalmWind]

It looks like my problem is solved by using the question K4EET posed. I would have thought on a Trunk system all the transmissions would have the same signal level relative to my location. But it appears they do not.

A new antenna, with a 5 dB gain, has stopped the distortion and brought the error correction rate down to just above 0, where before it was 35 to 75...

Thanks to all who responded.