listening to a system trunked vs conventional

[topcop1833]

this seems like a hard topic to search for, could not find the answer, maybe just putting the wrong words in the search field. so, i apologize if this is the 300th time this question has been asked.

a system local to me recently went digital. no problem, I've got a PSR500. the system is a simulcast system, roughly 13 towers or something like that, 9600 baud, true p25. I wasn't too happy with how the scanner was receiving the audio. it was kinda choppy, sporadic, not very reliable. i say self, we have an xts5000 in the same band, lets load up the voice channels and scan it in conventional mode using the xts and give that a whirl. same outcome, almost identical, a little better consistency in the decoding of the P25 audio that i suspect is due to fact that it is a radio built for that band (800mhz btw). the gre decodes my jurisdiction (trunked 3600 baud) just fine, never had an issue. decodes a state vhf system (trunked 9600) just fine too.

So last week I had the opportunity to have my XTS, my GRE, and another XTS programmed by the county radio shop all within 1 foot of each other, and wouldn't you believe it, the "county" radio decoded 100% all of the time, the other two about 30%. Ran like this for about 10 hours. County shop even plugged my xts for about an hour to prove that my xts was capable of performing just like their radio was.

so, what gives? whats the difference between receiving a trunked audio channel and scanning it conventionally?

 

[DickH]

... County shop even plugged my xts for about an hour to prove that my xts was capable of performing just like their radio was.

so, what gives? whats the difference between receiving a trunked audio channel and scanning it conventionally?

Didn't the county shop explain? In a trunked system, a computer chooses which freq. your radio receives. Every time someone pushes their mic button it may choose a different freq. If you are in conventional mode, the radio only moves to the next freq., but someone else may be talking there, thus you can't follow conversations.
In trunking mode, your scanner follows talk groups and it doesn't matter what freq. because the computer tells your scanner to tune to the right freq. automatically.

 

[ScannerSK]

If the same identical P25 frequency is monitored conventionally or as part of a trunked system it should provide the same result. I think there is another issue.

Both radios can be programmed to use only the tower with the best signal strength or all towers (including weak hard to decode towers). In my PSR-500 & PSR-600 I use the multi-site ROAM setting with a high threshold of 99 and a low threshold of 94. This prevents my scanner from monitoring weaker towers/frequencies and provides 100% decode rate. If I were to change the low threshold setting, then my decode rate would likely drop to 30%.

Shawn

 

[topcop1833]

i totally understand the basics of trunking (i think, simulcast is a new concept to me), no issue there.
this system is a simulcast system according to RR. from what i've read that means that even though there area umpteen towers, the control and voice channels are the exact same on all towers. ie talkgroup 1234 comes out on the same freq on all 13 towers. if this is correct, then it is not an issue of listening to a distant site, something else causing the distortion and decode rate issues.

my question is when the scanner, conventional XTS and (i'm guessing) trunking XTS all receive the same freq, why does only the trunked radio receives it clearly and consistently. i played with it just now for an hour. the scanner and the xts portable are receiving the same freq, the scanner is opening up on the correct talkgroup. audio is crappy on both the scanner and conventional xts. now granted i can't see what freq the county radio is receiving, but in my comment above referencing the simulcast, i'm inclined to believe that the county radio is receiving the same freq too.

 

[grem467]

Program the conventional XTS5000 for CQPSK instead of C4FM and post the results... the long and short of it is in a simulcast environment, the scanner does not properly decode the LSM format. When the trunk system is set up, there are parameters that can get set to enable the CQPSK LSM format to be decoded properly.

 

[n5ims]

This is a common issue with scanners called Simulcast Distortion, so common that there's an article in the RR WIKI on it (Simulcast digital distortion - The RadioReference Wiki). Here's a search on this topic for the RR forums where you can find even more info on this topic and some possible work-arounds RadioReference.com - Scanner Frequencies and Radio Frequency Reference

 

[RKG]

My guess is that when you programmed the PSR, you put in the control channels for multliple simulcast sites within the parameter set for a single PSR "TSYS." This allows (or forces) the radio to select one site to listen to, and it may not have selected the best site.

The PSRs do have a "roaming" feature that attempts to emulate the "SmartZone" feature of Motorola trunking systems, as best it can within limitations involving IP and other issues, but it is not perfect.

My advice to people is to program each relevant site as an individual "TSYS," using only the control channels for that site. If you are at a fixed location, you select the site to listen to based on your knowledge of how the system is laid out, and stay there. If you're moving, to select and then re-select one site to listen to, as you move, on the same basis.