P25 audio decode level adjustment (BCD396T)

[edit] Setting the P25 Decode Level

Paul Opitz, the BCD396T product manager, posted the threshold table for the BCD396T in an effort to make tuning easier for us. L. W. "Dan" Danz posted instructions on how to use this table to the BCD396T Yahoo group.

[edit] The Table

Setting	Low	Mid	High
0	1.54	1.60	1.66
1	1.52	1.60	1.68
2	1.50	1.60	1.70
3	1.48	1.60	1.72
4	1.46	1.60	1.74
5	1.44	1.60	1.76
6	1.42	1.60	1.78
7	1.40	1.60	1.80
8	1.38	1.60	1.82
9	1.36	1.60	1.84
10	1.34	1.60	1.86
11	1.32	1.60	1.88
12	1.30	1.60	1.90
13	1.28	1.60	1.92
14	1.26	1.60	1.94
15	1.24	1.60	1.96
16	1.22	1.60	1.98
17	1.20	1.60	2.00
18	1.18	1.60	2.02
19	1.16	1.60	2.04
20	1.14	1.60	2.06

From L. W. "Dan" Danz (WA5SKM) Fountain Hills AZ

There are some initially-confusing things about the P25 Decode Threshold level. The manual gives very adequate instructions about how to use the AUTO setting, I thought. But I'll provide some insight:

The level numbers are an index into a table of thresholds for decoding the digital data. Higher numbers are more permissive (the thresholds for decoding are wider) while smaller numbers tighten the threshold ranges. The optimum range depends on how the technicians have set up the system you are trying to receive.

DEFAULT - Means a Level (Index) setting of 8. Thresholds don't change. Most likely you can do better.

MANUAL - Means you control it all - Hold Func and press the scroll knob twice to get the screen. Adjust the setting with the knob until you get the best result for any given SYSTEM. The radio will remember this setting for that System and use it every time it has to decode digital audio.

AUTO - Means the radio attempts to fine tune the P25 audio transmission thresholds during receive. You can generally see it doing this. During the beta test, we convinced Uniden to implement a slight change to the "Auto" functionality. Specifically, we noticed that it was starting over at the default setting and then trying to correct it again at every transmission; sometimes this meant it took a long time to settle in. We went thru a couple of iterations, requiring at first - Set the threshold with MANUAL and then AUTO would use that as a starting point. This was cumbersome, and soon changed to the ability to control the starting point while in AUTO mode by using FUNC-SCROLL while viewing the decode screen. Whatever you set at this point will be remembered FOR THIS SYSTEM as the starting point for the auto-tuning function for subsequent transmission ON THAT SYSTEM.

Incidentally, we did suggest the ultimate -- for the radio to automagically remember this on its own, continually refining it and picking a better starting point if it could. But the engineers thought it would results in too many writes to the EEPROM, which has a generous capability for making changes to the programming, but the limit would be reached if every P25 audio transmission resulted in a write.

Now I can offer a slight refinement (courtesy of a very helpful Engineer at Uniden): If you're having trouble with a P16 (Moto type 2) system decoding digital P25 audio, slightly offtune the automatic setting. Frequently the radio will auto tune to a setting that is halfway between the values that you can set manually. For example, it might finally settle to a low threshold of 1.41, but you can't set the starting point to that, you can only make it 1.40 or 1.42. Well, the trick that helps stop the motorboating is to pick the tighter threshold side. In the example, instead of picking 1.40, pick 1.42 (that's a tighter threshold). (Think of it as similar to setting the squelch tighter for a conventional channel.) Even if you auto-tune to an even value, like 1.42, pick the next tighter value (1.44) to start at.

Works for me. Cuts out 80% of the motorboating that occurred prior to or after a transmission. YMMV. You guys having trouble with motorboating on the San Mateo system should definitely try this.

Posted by --Al42 21:06, 14 Sep 2005 (CDT)

Addition by Paul Opitz on 6/7/2007

One other thing to note is that the adjustment mentioned above must be done while you have good reception. The other side of that coin is that the above adjustment does absolutely nothing to fix reception issues. If you have bad decode, first ensure you are actually getting a good signal (no interference and solid receive). You can check this by tuning the frequency conventionally, opening squelch, and listening to see if you are getting dropouts or audible interference. Only after fixing any reception issues (changing antennas, location, etc) should you try to optimize the P25 decode thresholds.

Once optimized for a system, the threshold settings should not be changed, regardless of future interference or signal conditions, unless you have confirmation that the system engineers have actually made changes to the system itself (which is highly unlikely, as it would require all mobile units operating on the system also be brought in for retuning).

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