Unitrunker: Decoding P25 from discriminator

[N9JIG]

I have been playing with Unitrunker recently and configured a couple receivers using PSR600's and they seem to work just fine on P25 signals as well as M36 systems.

I then created a receiver for my R8500 using the Discriminator type and it works great with M36 systems but not P25.

I have P25 checked on the VCO screen and tried all 3 sampling rates.

I am within spitting distance of the tower and it is strong in my area.

Is there a reason why it doesn't seem to work with discriminator audio on the P25 system?

Thanks!

 

[SCPD]

Some iCOMs have a small amount of filtering. I know this is the case for PC/R1500 and PC/R2500. The fix is to bypass the T pad or L pad circuit - usually by placing a jumper over a resistor.

 

[nd5y]

I have decoded regular P25 and simulcast control channels with Unitrunker and a tapped Pro-2006 with no problem.
Did you do a tap on the R8500 yourself or does it have a factory discriminator output?

 

[SCPD]

I'm going by memory of past discussions. I do not own an R8500. You'll have to dig through the forum archives for a more definitive answer.

 

[SCPD]

Try this ...

Discriminator Output IC-R8500

 

[N9JIG]

I do have the jumper set to Discriminator and it works great for PL tones, M36 trunking with Unitrunker etc. I just cannot get it to work with P25.

If anyone knows what resistor to shunt to bypass the filter I would appreciate it!

 

[SCPD]

Rich - to confirm this is a filtering issue - please post an image of the waveform from either the VCO tab or from UniScope.

Another possibility is an impedance mismatch. I use a 47k pot to adjust.

Reading up on the R8500 - it has 12 khz and 5.5 khz FM IF filters. Try both.

 

[N9JIG]

I tried it on a BCT15 with a direct discriminator tap (no capacitor or resistor in line...) with the exact same results. On the R8500 I tried both FM filter bandwidths.

The tap is on the R8500 is the built-in tap with the jumper set to Discriminator. On the BCT15 is was a wire direct to the DISC pad in the main board.

Here are the pics:

First properly decoding an M36 signal.

Second is the scope on a P25 signal.

Last is the VCO settings.

 

[SCPD]

Hi Rich - please read this. Your audio input should have a level / volume adjustment.

Audio Recording Level Adjustment

 

[N9JIG]

Got it!

I had been using the Mic jack and first I tried various levels, setting the level down to 2 got me some decoding, with a Health as high as 12, but for the most part it was below 5 and spotty.

I switched over to the Line-In jack and set the level at various points before finding a happy medium at about 62. I get a health level at about 80-90.

Thanks!

 

[AM909]

That was my first reaction to the scope display, too – level too high.

It seems like the sweet spot for decoding is about 10% of the scale peak in the DSD+ Source Audio window. Is there a way of adjusting its scale to make the waveform more visible, and to make it more obvious what the correct level might be? I'd like to see it decode over a range of maybe 20-70% of peak (probably about 1/7th of the current scale).