Showing FM on a P25 system

eaf1956 · Dec 5, 2018

I was listening to SAFE T here in Indiana, and noticed a few TG's showed as FM and not NFM. Since this is a P25 system I was a bit taken aback. I tried setting MOD to NFM but they still showed as FM, Not all TG's just a couple. I tried on another BCD 536HP and it showed NFM on all TG's. The only way I got the original 536 to work as it should I had to reprogram it.

The TG's that showed FM where Air Med Helicopters and DIST 35 Dispatch. Note that DISP showed FM but the cars showed up as NFM.

HogDriver · Dec 6, 2018

Modulation is a function of the sites, not the talkgroups. Since your system frequencies are mostly in the 800 range, where, as I understand, NFM doesn’t apply, it should display as FM. Apparently, Mediflights keep hitting the same repeater that is programmed as FM.

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GTR8000 · Dec 6, 2018

It has nothing to do with the system repeaters, and everything to do with how the scanner's band defaults are setup. The scanner does not "auto-sense" any modulation from a repeater. Besides, P25 modulation is the same bandwidth regardless of what part of the spectrum it's operating in.

It sounds like you might be scanning a site that has a mixture of 700 and 800 MHz frequencies, or perhaps are scanning multiple sites that operate in both bands. If your scanner's band defaults are setup so that 758.0000-787.9937 are NFM modulation, and 849.0125-868.9875 are FM modulation, you're going to see exactly what you described while scanning sites that operate on both bands.

eaf1956 · Dec 6, 2018

GTR8000 said:
It has nothing to do with the system repeaters, and everything to do with how the scanner's band defaults are setup. The scanner does not "auto-sense" any modulation from a repeater. Besides, P25 modulation is the same bandwidth regardless of what part of the spectrum it's operating in.

It sounds like you might be scanning a site that has a mixture of 700 and 800 MHz frequencies, or perhaps are scanning multiple sites that operate in both bands. If your scanner's band defaults are setup so that 758.0000-787.9937 are NFM modulation, and 849.0125-868.9875 are FM modulation, you're going to see exactly what you described while scanning sites that operate on both bands.

Still doesn't explain why two 536's side by side (same Programming) one showed FM the other NFM on the same transmission.

KevinC · Dec 6, 2018

eaf1956 said:
Still doesn't explain why two 536's side by side (same Programming) one showed FM the other NFM on the same transmission.

Because the band defaults are set different on both radios? Without checking that you'll never know if this is the cause or not.

eaf1956 · Dec 6, 2018

KevinC said:
Because the band defaults are set different on both radios? Without checking that you'll never know if this is the cause or not.

Both are exactly the same.

Ubbe · Dec 7, 2018

Receive wise it shouldn't matter if you use FM or NFM, if you don't have an adjacent channel that interfere. Upman even suggested that on DMR systems you could try to use FM instead of the standard NFM to try and improve the data decode success rate.

There's no place to modify FM/NFM for P25 in either the scanner or Sentinel? Where do you do that?
The database uses AUTO as the setting for modulation type so the scanner uses whatever are set in the scanners bandplan.

The bandplan stipulate modulation type and stepsize for each frequency range and are fully editable in the scanner so double check that.

/Ubbe

Eugene · Dec 7, 2018

You can change setting in Butels software.....use it all the time on my 436HP. It is given as a choice when entering data. In fact I changed all my trunked systems both analog and P25 to FM from the default NFM (which is what happens if you select "AUTO")

Eugene KG4AVE

Showing FM on a P25 system

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