BCD396T/BC996T: Decode threshold levels

[dstew67]

I've had this BCD369T for several years, and have updated it through a couple firmwares, finally settling on 3.02.02 when it came out. I'm having trouble adjusting the P25 decode threshold. This is the first time I've tried. I'm monitoring the Atlanta Public Safety P25 system, and I've been monitoring in Midtown, Downtown, on the Northwest, Northeast and West sides of Atlanta.

The problem I'm having is that it takes the radio a little bit to figure out the transmission. If it's a long transmission, I end up getting good audio once the radio figures things out. If it's a short transmission, or series of transmissions, I hear some motorboating. I've tried increasing the P25 Adjust Level above 50 at different intervals, but always end up setting it back to 50, because I can't tell what effect it's having on decoding the transmission.

I've read the Easier to Read manual at Easier to Read BCD396T/UBCD396T Digital Scanner Manual

I've also read the wiki entry on P25 audio decode level adjustment at P25 audio decode level adjustment - The RadioReference Wiki

Here are the questions I have:

1. In the Easier to Read manual, it tells you to hold on an active channel. From what I know about trunked systems, a channel refers to a frequency. An active channel would be the control channel. Does is mean rather, to hold on an active talkgroup? Holding on an active talkgroup is the only way I see the error rate.

2. The low, medium and high voltage levels in the table on the P25 Audio Decode Adjustment wiki entry doesn't match what I get when I manually set the threshold level. When holding on an active talkgroup in Auto mode, the threshold level seems to settle around 6, and the voltage levels seem to settle around 1.40, 1.60 and 1.79. In looking at the table, a 6 has 1.42, 1.60 and 1.78. Here's what I get when I manually set my threshold level:
5 = 1.41 1.58 1.74
6 = 1.39 1.58 1.76
7 = 1.37 1.58 1.78

As you can see, I can't seem to get 1.40 1.60 1.79. Does this mean I need to send my radio in for repair? Or is the goal just to get as close as possible?

3. In the P25 Audio Decode Level Adjustment wiki entry, and under the "Setting the P25 Decode Level for the BCD396XT and BCD996XT and other tips" section, it says "People have had better decoding with this value (P25 Adjust Level) turned up." But of source, this is under the BCD396XT section. Would it help me if I increased this Adjust Level to a setting above 50?

Obviously, I'm new to monitoring a pure P25 system, so any remedial help you can give would be much appreciated.

 

[Boatanchor]

I don't think even Uniden really knows how the Level/threshold settings work in any of their scanners.
The automatic threshold setting doesn't even work properly in their newest x36HP scanners.

So, since we can presume that the 'Auto' mode doesn't work even work with stable signals, it is going to have a complete meltdown trying to adjust for fast fluctuating mobile signals or weak, noisy signals.

In my experience, it is always best to set the decode level to 'Manual' or 'Default' and the level to 8 in the XT/HP scanners (the original 396T scanners may be a little different). That will prevent the scanner from trying to optimize (which it can't/won't do anyway).

On my 396XT, 996XT and 436HP scanners, I get best weak signal decode performance with the threshold set to 8, monitoring various conventional and trunking systems.

 

[W6KRU]

Boatanchor hit the highpoints, I'll just add that different systems need different levels. The simulcast sites on the system I listen to work best at 11.

Don't worry about holding on a channel. Just bring up the decode screen and monitor the Err rate during transmissions. Don't worry about voltages, just adjust the threshold for the minimum Err rate.

 

[dstew67]

Thanks, guys. My first trunked system was several years before any trunked scanners were even made, so I've been scanning a while, but this is the first all P25 system I've spent more than a few weeks listening to. All the fun of scanning is the pay off when you finally gets things right and really understand a system, so this one has me feeling all newbie again.

Anything else I should be aware of? Anything that sets P25 systems apart from a regular Motorola analog system?