Can Simulcast create Multi-path distortion?

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TecEnduro9

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When I'm in Bedford, Va I typically have Region 2000 and the associated Simulcast freq's enabled. My audio is somewhat choppy. Wondering if having two or more sources for the same broadcast adds to problems in the audio?
 

fredva

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Have you updated your scanner from the Radio Reference database lately? If I'm not mistaken, Region 2000 is the name of an old EDACS system that was replaced by a new P25 Phase 2 system. I believe the new system is Central Virginia Regional Communication System or something close to that.

I'm going to make the assumption that you have the new digital system and just used the old name. There are at least two types of simulcast: One is a conventional frequency that simultaneously broadcasts what is heard on a certain talkgroup on a trunked radio system. These conventional frequencies are often kept alongside newer trunking systems to set off traditional, analog pagers and they tend to be VHF or UHF frequencies. I have a feeling that it is these frequencies you are referring to. If so, the programming of those frequencies in your radio will not cause choppiness while listening to the trunked radio system.

What you are probably experiencing is a different type of simulcast. This simulcast occurs within the trunking system itself when multiple towers are sending out the same transmissions on the same frequencies. All the towers share the same set of frequencies. If you search the Virginia forum, you'll find other people listening in the Bedford area complaining about choppy audio and missed transmissions. The scanners currently being sold do not have the proper demodulator to handle this type of simulcast. However, Unication digital pagers do, and Uniden is claiming a new, soon-to-be-released scanner model will handle this type of simulcast better than any other scanner.
 

jonwienke

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Simulcast by definition is multipath. The only difference is that instead of multiple reflections of a single signal off trees or buildings, you have multiple transmitters broadcasting the same signal on the same frequency, which does the same thing to an even greater extent. Reliably extracting usable audio under multipath or simulcast conditions requires something called I/Q demodulation, which no currently available scanner does.

Unication G4 and G5 pagers do I/Q demodulation, as does the soon-to-be-released Uniden SDS100 handheld scanner, and professional-grade Motorola and similar radios.
 

kb4cvn

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Simulcast “noise” is called: Time Delay Interference, or simply TDI for short. EACH AND EVERY SIMULCAST RADIO SYSTEM HAS IT, regardless of design or manufacturer. Simple physics of the technology.

Some background:
ALL simulcast radio systems by nature have TDI. In the system’s design, timing and synchronization is critical to have a usable signal. In the old days (pre digital voice and GPS), simulcast systems used either microwave or leased telephone lines to convey audio to/from the radio sites and the control point. The audio delay was at times a nightmare to keep synchronized, and produce as little TDI as possible in the desired coverage areas. In the case of leased telephone lines, it often drove Technicians to drink! Phone Company reroute a line, changing the distance, it changed the physical length of the hard-wired path and therefore the transmission time to the Radio Site from the Control Point. You had to re-synchronize the entire system.

Enter in Global Positioning System (GPS) technology! All the RF Sites and the Control Point could be GPS time locked and stay in sync all the time, and even more accurate than wireline/microwave could ever achieve. Life is good! That, coupled with computerized mapping technology allowed system designers to “steer” the worst TDI zones to desired locations, where they did not need coverage. Through antenna patterning (panel antennas, etc.) and creative TDI timing schemes, these Worst Case TDI areas can be moved to places like over a lake, or the top of a mountain where nobody lives, or the side of a hill were coverage would never been needed.

But, the takeaway is there is some TDI in all simulcast systems, even in the “good” areas. Modern DSP based two-way radios are designed to ‘scrub’ this background noise, oftentimes were the User is unaware of it. Consumer grade receivers (IE: Scanners) only recently developed this level of sophistication, but at a higher monetary price to the user for the new technology. Hopefully this price will come down over time…

In the case of Bedford County, WE ARE LUCKY that it is ONLY a six-site simulcast system. It could be a lot worse for hobby listeners! Down in Miami-Dade County (Fla) their two P-25 systems are twenty-site, each. Plus several gapfiller standalone sites. (…I wrote some of the user radio programming for Dade in 2011 before the EDACS to P-25T migration.)

Back to CVRCS25, if you are close enough to a RF Site, you might not have an issue with a consumer grade receiver. Otherwise, common suggestions are to use one or more of the following mitigation solutions:



  • Directional Antenna; yagi or a corner reflector type, with the corner reflector being the better of the two.
  • Attenuation load on the receiver’s antenna input.
  • Both.

Also, don’t forget that Conventional radio systems in our area can be simulcast too. As an example, Campbell County has upgraded all of their radio system to support simulcast, AND Project-25 conventional. (I installed the new antennas at two of their radio sites: Johnson Mountain and Altavista.) You can hear limited analog TDI now in some spots. When they eventually move to P-25 Conventional, hobby listeners will notice it.


For anyone not familiar with Simulcast Systems, here are a few items to read:

http://www.simulcastsolutions.com/u...ums/technology_simulcast_NB_Workshop_2011.pdf

https://combausa.com/en/blogs/mitigating-tdi-time-delay-interference-on-public-safety-das-systems


Another somewhat more complex solution is to use an actual P-25 radio which supports P-25 simulcast to listen with. This is what I do. Several others have followed. The newer Unication Receivers have addressed this requirement with a quality product. But, alas they are expensive…
 

UPMan

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To make things more confusing, simulcast is also used to mean transmitting the same signal on two or more different frequencies simultaneously. In the context of the similarity between simulcast distortion and multipath distortion, it means transmitting the same signal from two or more sites on the same frequency simultaneously.
 

K6CDO

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To make things more confusing, simulcast is also used to mean transmitting the same signal on two or more different frequencies simultaneously. In the context of the similarity between simulcast distortion and multipath distortion, it means transmitting the same signal from two or more sites on the same frequency simultaneously.

Those of us who manage trunked radio networks with one or more simulcast cells refer to the transmission of the same audio signal on two or more different frequencies simultaneously as 'multicast,'
 

kd7kdc

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SAME content at the SAME time on DIFFERENT frequencies is MULTICASTING.
 

jonwienke

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Multicast = multi frequency

Simulcast = same frequency
 
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