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W2LIE.net | Monitor Long Island, Inc. :: Forums :: Scanner Radio :: Long Island - Suffolk
Moderators: w2lie, Chieftaz, BobLandau, quint14, LI_Scan_Guy, cwerner

Whistler TRX-1 and TRX-2 on Suffolk's new P25 System

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Print View Posted: Wed Aug 07 2019, 11:54pm

Posted by: w2liePosts: 2760

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Location: Long Island, NY
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Is anyone using a Whistler TRX-1 or TRX-2 to monitor the new Suffolk P25 system?
If so - are you having any luck using it?
 
I know this scanner isn't made for simulcast, but there shouldn't be any reason that from Nassau County I miss +90% of the voice traffic on this system, and the inverted S never goes inverted T.
I shoudl be far enough outside of Suffolk where simulcast is not an issue for me.
 
My goal here is to see if it is an issue with my scanner, programming, or the radio in general.




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Posted: Thu Aug 08 2019, 01:38am

Posted by: GTR8000Posts: 78
Joined: Fri Nov 25 2016, 07:07am

It's actually a two-fold issue with most scanners.

The first is that there is actual simulcast distortion, whereby the scanner is getting hit with signal from two or more sites at once, and the timing is slightly off which gives the scanner fits trying to cleanly decode it. Normally this can be mitigated with some form of attenuation, or by using a directional antenna in order to make the capture from one site so strong that it drowns out the other sites. Attenuation can be in the form of the scanner's attenuator feature, by using an antenna that has less gain (e.g. using a rubber ducky indoors vs a big outdoor antenna), or by being well outside the intended coverage area of the simulcast cell.

Unfortunately, being well outside the intended coverage area gives no guarantee that you're only going to capture one site strong enough to drown the others out. It may seem like you're far enough away, but it only takes one monster site that is further away from you than the nearest site on the map, and it can still wreak havoc. I have seen this numerous times with simulcast cells around the region, where a site in the center of the county is actually stronger than a site at the edge of the county. Because of that phenomenon, your distance outside of the intended coverage area may not matter all that much.

The second issue is the modulation used with these simulcast cells. Motorola favors LSM (their own flavor of QPSK) while Harris and others favor the H-DQPSK standard. This is always the case with Phase II TDMA systems, as TDMA requires linear amplifiers and thus some form of QPSK modulation. Most modern P25 simulcasts will use a form of QPSK even when the system is only capable of FDMA, with C4FM modulation largely being restricted to non-simulcast FDMA standalone sites. In the case of the latter, while the control channel will use C4FM modulation, TDMA traffic channels will use QPSK.

The trouble with QPSK is that it's not straight FM, there is some AM involved that is fairly critical to attaining good demodulation. Unfortunately because nearly all scanners except for the SDS series use an FM discriminator, they kill that important AM component of the signal, thus crippling their ability to cleanly decode most P25 simulcasts.

Of course some scanners do better than others, so a 436HP might decode the system adequately while a TRX fails miserably. I for one never got into all of the smoke and mirrors of adjusting P25 wait times and all that crap...it's easier to just listen with an APX and call it a day lol.

[ Edited Thu Aug 08 2019, 01:43am ]
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