Great signal strength but P25 audio sometimes cuts in and out...

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rpurinton

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I’ve heard that problems can occur when a scanner receives overlapping digital signals from multiple towers in simulcast. Is this true? I have 2 different model scanners having the same problem.

The problem is intermittent but happening on about 50% of the transmissions. Half are crystal clear and the other half you only get every other second of audio.

The scanner is in a building under one if the towers, so signal strength should be immaculate.

In an attempt to reduce the signal strength from the other simulcast towers I first tried putting the global attenuation to the and adding physical -6db attenuators and finally just removed the antennas completely. Signal strength is still “too good”? I am still having the problem. I haven’t tried shielding the scanners yet but I guess that’s the next step?

is this a common problem? Are there any workarounds I haven’t mentioned?
 

MTS2000des

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Neither of those radios can properly handle CQPSK and thus, P25 decoding will vary greatly using such hardware.
The only answer is to utilize a receiver that is designed for it, the Uniden SDS series are the only commercial scanners that can. Others use the Unications, some have great results using SDRs and software.
 

marksmith

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You are experiencing what is known as simulcast distortion. Receiving the same signal from different towers, partially canceling each other out. Previous responder is right that only Uniden SDS 100 and SDS 200 are designed to receive these types of signals.

You can look up simulcast distortion in the wiki section for some suggestions to help receive on other radios, but most are just tricks to avoid receiving 2 towers and not really a solution.
 

ofd8001

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Unless you have some kind of priority going on where the scanner is checking other frequencies for activity or weather alerting, what you describe sounds like simulcast distortion.

I understand that right below an antenna is kind of a dead zone, so taking your statement literally you might still be getting signals from other towers. If you can get a little farther away, a couple of hundred yards, see what happens then. If it clears up, it is simulcast distortion almost certainly.
 

gmclam

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Ironically, with this type of issue, often a little less signal strength might makle the signal more solid (when on a non SDS scanner).
 

ofd8001

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Yeah, simulcast distortion is a problem of too much signal (multiple sites being received) and many scanners cannot decode all the incoming intelligibly. So they either garble, or just give up trying and one hears nothing.
 

rpurinton

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Neither of those radios can properly handle CQPSK and thus, P25 decoding will vary greatly using such hardware.
The only answer is to utilize a receiver that is designed for it, the Uniden SDS series are the only commercial scanners that can. Others use the Unications, some have great results using SDRs and software.
so I see the prices on SDS and looks like a nice price of $700. I would need two and for now $1400 is a bit high.

so since I’m using these to do Broadcastify streams and I need a PC any way, what’s the SDR solution look like? Costwise for hardware and software? Obviously need something to support p25 phase 1 trunking and properly implement CQPSK, also need to scan thru about a dozen talk groups on 1 scanner and 2 talk groups on the other.

also bonus if I can run both broadcast streams in 1 PC
 

jim202

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Let me ask this question about the tower your under. What other transmitters are there at this tower? Your problem may be another transmitter that is causing either overload or intermod that is causing your problem.

Have you tried taking your portable scanner and moving away from the tower to see if it makes a difference? I would suggest going at least a half mile away and see if it makes any change in the problem your having.

You have not mentioned just what frequency range the system is that you are having problems with. It your looking at an 800 MHz. trunking system, then there is a possibility that your getting pounded by a paging transmitter on 900 MHz.
 

rpurinton

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Let me ask this question about the tower your under. What other transmitters are there at this tower? Your problem may be another transmitter that is causing either overload or intermod that is causing your problem.

Have you tried taking your portable scanner and moving away from the tower to see if it makes a difference? I would suggest going at least a half mile away and see if it makes any change in the problem your having.

You have not mentioned just what frequency range the system is that you are having problems with. It your looking at an 800 MHz. trunking system, then there is a possibility that your getting pounded by a paging transmitter on 900 MHz.
I’m not sure about what’s on the tower, could be lots of things. It’s on top of the public safety building so it may only run the city’s p25. We are currently trying to find a new home
@rpurinton You need an SDS100 or SDS200.
i think I might go SDR would be allot less than $1400 to upgrade
 

rpurinton

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I’ve already disconnected the antennas. Thought about putting a metal bucket over them but I’d rather get the right gear.
 

fredva

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so I see the prices on SDS and looks like a nice price of $700. I would need two and for now $1400 is a bit high.

so since I’m using these to do Broadcastify streams and I need a PC any way, what’s the SDR solution look like? Costwise for hardware and software? Obviously need something to support p25 phase 1 trunking and properly implement CQPSK, also need to scan thru about a dozen talk groups on 1 scanner and 2 talk groups on the other.

also bonus if I can run both broadcast streams in 1 PC
I have OP25 software running on a Raspberry Pi mini-computer with a NooElec SDR dongle. I believe the total cost was about $57. So cost isn't really the challenge with SDR - it's investing the time to get the software set up and tweaked to receive the systems. What I use is Linux-based, and Linux is different than Windows. I wouldn't discourage anyone from trying SDR to save money, just be aware it's not a plug-it-in-and-it-works situation.
 

rpurinton

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A few have mentioned the lack of a proper demodulator for simulcast. Some mentioned CQPSK support. I checked the database and it actually shows P25 phase 2 H-DQPSK. Is that the same? Oddly the system as far as I knew was only Phase 1 is this maybe just a database glitch? What’s a possible way for sure to know?

I will probably get a uniden sds for myself but I feel like it would be a shame to dedicate 2 of them just to support the internet streams.

looks like SDR usb dongles is the way I want to go for the ultimate solution but will I be able to decode this system correct, and filter certain talk groups into separate audio channels is the question, I would think so. I’ll move my discussion for now to the SDR forum since that’s probably where I’ll find the most help. Thanks all
 

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jonwienke

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Strong signal but crappy decode is a classic symptom of simulcast. The tower you're under is probably not part of the system you're having trouble with.

You need an i/q digital receiver to properly parse simulcast. The SDR sticks have them, as do the SDS scanners. But any conventional scanner is going to have trouble.
 

rpurinton

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Thanks all for your thoughts. I was able to confirm our system is running LSM Simulcast CQPSK. I’ve decided to ditch the whistlers and setup SDRtrunk.
 

rpurinton

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I just wanted to share for any one else struggling with simulcast,. I bought 2 SDR dongles and setup SDRtrunk. The install was painless, I just put in my radio reference login and it programmed the p25 system and all the talk groups in just a few click, it even configured all the streaming settings and I had the broadcastify stream going in no time. This solution cost about $60 instead of $700 for the SDS100/200. There wasn't really anything hard about it. Don't be scared of SDR especially if you're streaming broadcastify, it's pretty awesome. One computer could run a bunch of streams with different talkgroups routed to different streams. Plus it is 100% digital and the clarity is wonderful!
 

bobruzzo

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I just got my SDS200 and while it is a fantastic radio, I STILL have some simulcast distortion present!! I thought this rig was immune! I must live in a bad area. I get all my P25 zones nice and strong. I made filter adjustments and switched from using a Diamond Discone to a dedicated 800Mhz mag mount antenna which did improve things. I am thinking a dedicated 800mhz antenna may help a lot.
 

dextergiii

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I just wanted to share for any one else struggling with simulcast,. I bought 2 SDR dongles and setup SDRtrunk. The install was painless, I just put in my radio reference login and it programmed the p25 system and all the talk groups in just a few click, it even configured all the streaming settings and I had the broadcastify stream going in no time. This solution cost about $60 instead of $700 for the SDS100/200. There wasn't really anything hard about it. Don't be scared of SDR especially if you're streaming broadcastify, it's pretty awesome. One computer could run a bunch of streams with different talkgroups routed to different streams. Plus it is 100% digital and the clarity is wonderful!
After reading your post I decided to give SDRtrunk a try... I was surprised at how much better it sounds compared with my DSD+ setup, and the integration with RR made it that much better.
 
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