P25 audio decode level adjustment

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Revision as of 14:30, 3 February 2011 by Ka3jjz (talk | contribs) (dropped signatures, contrary to standards)

Setting the P25 Decode Level for the BCD396T and BCD996T

Paul Opitz, the Uniden 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. It should be stressed that it's likely this discussion also applies to the BCD996T.

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 automatically 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.

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).

If the P25 audio is choppy, or switches between LNK and P25, Set P25 LEVEL to DEFAULT. This can also happen if you are in the fringes of the coverage of a P25 system, and the control channel is not being received cleanly. LNK also shows up when a P25 signal is being transmitted with no audio.

Viewing the Decode Screen

Make sure the system’s P25 Level option is set to Auto. Press HOLD/RESUME on an active digital channel on the system. HOLD appears on the display. Press F then VOL. The APCO threshold Adjust Mode appears at the bottom of the display.


The next to last line on the display now shows the digital error rate and the AUTO decode threshold start level (0-20). The last line on the display shows the decode threshold levels for the system.

Allow the scanner to monitor channel activity for several minutes. The error rate should drop for each transmission and the threshold levels should automatically adjust to a more optimal setting. Then, once the threshold level settles to a stable setting, write down these values. This is the system’s optimum decode threshold. Rotate the Scroll Control to set the AUTO decode threshold start level to a setting that most closely matches the system’s optimum decode threshold. 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. To exit when the settings are complete, press FUNC then VOL.

Setting the P25 Decode Level for the BCD396XT and BCD996XT and other tips

You can still view the decode screen but that is all you can do

To view the decode screen with the 996XT, press the scroll control then Vol.
To view the decode screen with the 396XT, press Func then Vol while you are viewing the volume adjust or squelch adjust screen.

The adjustments require a different method. There is also no need to to be in HOLD mode or be monitoring a single system. Probably the closest equivalent to 'Analyze' mode in the RS/GRE digital scanners.

  1. Hold the HOLD key while powering up the scanner. The screen will flash all of the colors twice
  2. Press Menu; go to Settings.
  3. Three options (2 new) will appear;
    1. P25 LP Filter
    2. P25 Adjust Mode (new)
    3. P25 Adjust Level (new)
  • P25 LP Filter - lets you set the scanner to apply a software filter that removes the 4kHz tone you can hear on some P25 systems. Note that turning on this setting increases the CPU load and could slightly degrade P25 decode performance on some systems. The option is on or off.
  • P25 Adjust Mode - This mode sets the algorithm for P25 decode. Leave it at 3. Anything higher than 3 will just act like 3. Anything lower will have the reduced performance of previous firmware versions on the 396T.
  • P25 Adjust Level - sets the RSSI value below which the scanner will not attempt to make P25 fine tune adjustments. If the signal is too low, RX reliability is not high enough to make P25 adjustments correctly. 50 is the default setting (1-200). From posts I've read, people have had better decoding with this value turned up.
  • P25 Waiting Time - (In the Conventional systems and Motorola site menus) On channels that contain a mix of analog and digital signals (i.e., where the Audio Mode is set to All), it is possible to have false decode problems caused by digital noise at the beginning of transmissions. To prevent this, a user-configurable P25 wait time (default 400ms; 0 to 1000 ms) has been added. During the wait time, the scanner evaluates the received signal; if it detects P25 data, the scanner opens squelch immediately. If it does not detect any P25 data, the scanner opens squelch as soon as the wait time expires.
    • Note: Any analog transmissions on this channel will lose the first part of the transmission, up to the wait time you set here. Select the number of milliseconds (after the start of a transmission) the scanner should wait while checking for P25 data. Choose a number from 0 through 1000 ms in 100 ms increments. The scanner only applies the wait time setting to Conventional or Motorola (non P25) systems, and only when the channel's Audio Mode setting is All.

Thanks to Danny6569 and others for this information

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