What is simulcast?
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Simulcast is the transmission of the same conversation on the same freqs from different locations. Scanners have a hard time with digital simulcast systems. Basically the systems, being digital, are just series of 1 and 0's, and when they arrive at your radio at ever so slightly different times (think fractions of a millisecond) the 1's cancel out the 0's, causing the issues.
If you are able to force the radio to only be able to hear one of the towers then the issue goes away. This can be done by reorienting the antenna, using a directional one, using the attenuator etc. This can sometimes be so difficult as to seem impossible.
Professional receivers like the Unication, Motorola, Harris et al, do not have this issue but are more expensive and lack the features that scanner users take for granted.
If you are able to force the radio to only be able to hear one of the towers then the issue goes away. This can be done by reorienting the antenna, using a directional one, using the attenuator etc. This can sometimes be so difficult as to seem impossible.
Professional receivers like the Unication, Motorola, Harris et al, do not have this issue but are more expensive and lack the features that scanner users take for granted.
Thanks for the explanation.
So does that mean that simulcast systems are not transmitted through a repeater? Back in the analog days, as long as one had the downlink frequency of a repeater, all was good.
Does this mean that simulcast systems are only used in very small locales where low power portables or mobiles are enough to carry the signal to the recipient?
So does that mean that simulcast systems are not transmitted through a repeater? Back in the analog days, as long as one had the downlink frequency of a repeater, all was good.
Does this mean that simulcast systems are only used in very small locales where low power portables or mobiles are enough to carry the signal to the recipient?
Actually, the opposite.
Simulcast systems usually cover larger areas, where there are multiple repeaters needed to cover an area. All the 'repeaters' transmit the voice data at the same time on the same frequency. That's why the scanners have a hard time when the signals arrive slightly out of phase due to distance, multi-path, etc.
Simulcast systems usually cover larger areas, where there are multiple repeaters needed to cover an area. All the 'repeaters' transmit the voice data at the same time on the same frequency. That's why the scanners have a hard time when the signals arrive slightly out of phase due to distance, multi-path, etc.
W8RMH
Feed Provider Since 2012
I never had any luck with GRE and Radio Shack scanners, better luck with the Unidens.
Had a BCD325P2 and returned it, poor on simulcast and deaf on VHF, plus poor battery life, great display though.
Had a BCD436HP that I liked a lot, (finally figured it out, it took a while), didn't care for the display, but again serious simulcast issues and I needed portable coverage and daily wear/use so I got a Unication G4 and sold the 436.
I run a BCD996XT with a yagi antenna for my feed and use the same antenna for my HP-2 at home. The HP-2 is also my travel/mobile scanner which works very well on my statewide system, except in the few major cities that use simulcast, and excellent on what little VHF/UHF we have left. I love the large color touch screen which works great on my dashboard.
I just noticed the date of the OP and that the OPster never responded, I wonder if they kept the 436 or went to the Whistler. The 436 is a good scanner once you learn it.
Had a BCD325P2 and returned it, poor on simulcast and deaf on VHF, plus poor battery life, great display though.
Had a BCD436HP that I liked a lot, (finally figured it out, it took a while), didn't care for the display, but again serious simulcast issues and I needed portable coverage and daily wear/use so I got a Unication G4 and sold the 436.
I run a BCD996XT with a yagi antenna for my feed and use the same antenna for my HP-2 at home. The HP-2 is also my travel/mobile scanner which works very well on my statewide system, except in the few major cities that use simulcast, and excellent on what little VHF/UHF we have left. I love the large color touch screen which works great on my dashboard.
I just noticed the date of the OP and that the OPster never responded, I wonder if they kept the 436 or went to the Whistler. The 436 is a good scanner once you learn it.
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Which one is better at decoding signals it was not made to? None. No scanner currently offered can correctly decode LSM. I thought there was an LSM discussion page but I can't find it. LSM is different than P25. It uses CQPSK moodulation, not the C4FM that scanners decode. The unication can decode both and that is why it is the only option to correctly decoding LSM. It's like asking which AM radio can best decipher SSB. Well none are made to do it but some you might let you hear a word or two. Start reading at page 48 here http://www.simulcastsolutions.com/userfiles/file/simulcastforums/motorola_simulcast_part4.pdf
https://forums.radioreference.com/g...want-lsm-work-properly-my-scanner-thread.html
You might want to visit this page, C4FM vs CQPSK | Difference between C4FM and CQPSK modulation, to understand why your statement doesn't make sense.
P25 = standard for the manufacturing of interoperable digital two-way wireless communications products.
LSM/CQPSK = modulation types both used in P25 systems.
Simulcast systems are typically used for larger metropolitan areas that have massive buildings (hospitals, warehouses, high-rise buildings).
In olden days to achieve the radio horsepower needed to penetrate such structures it would take a high-power repeater. Problem with that is the radio signal would travel much farther than needed and the downside is that frequencies couldn't be re-used nearby. For example a radio with 1,000 watts of power may reach 75 miles or more, so the frequency couldn't be re-used by another agency within that area.
So as to overcome this issue, radio system engineers came up with "simulcast" which is several radio repeaters working in concert, but at a lower power, to achieve the needed coverage for building penetration. These "several" repeaters operate on the same frequency. For example my community has a 13 site repeater system. When fire truck A is transmitting on frequency 1, that transmission is carried on all 13 sites simultaneously.
Each site may cover a radius of 5 "crow flying" miles, so each won't cover the entire community, but because of the overlapping of all the sites working together, that coverage is achieved.
It's a great thing, but costly to use and even more challenging to monitor with scanners in those areas of overlap. If a scanner listener happens to be fairly close to a transmitting site, then simulcast distortion isn't an issue so much.
In olden days to achieve the radio horsepower needed to penetrate such structures it would take a high-power repeater. Problem with that is the radio signal would travel much farther than needed and the downside is that frequencies couldn't be re-used nearby. For example a radio with 1,000 watts of power may reach 75 miles or more, so the frequency couldn't be re-used by another agency within that area.
So as to overcome this issue, radio system engineers came up with "simulcast" which is several radio repeaters working in concert, but at a lower power, to achieve the needed coverage for building penetration. These "several" repeaters operate on the same frequency. For example my community has a 13 site repeater system. When fire truck A is transmitting on frequency 1, that transmission is carried on all 13 sites simultaneously.
Each site may cover a radius of 5 "crow flying" miles, so each won't cover the entire community, but because of the overlapping of all the sites working together, that coverage is achieved.
It's a great thing, but costly to use and even more challenging to monitor with scanners in those areas of overlap. If a scanner listener happens to be fairly close to a transmitting site, then simulcast distortion isn't an issue so much.
kayn1n32008
ØÆSØ Say it, say 'ENCRYPTION'
In a simulcast system there is time of arrival delays, BUT the system is engineered to keep that time of arrival difference to a very narrow amount, a minimum and maximum amount of difference in time of arrival. It’s called ‘delay spread’
C4FM is very unforgiving, and can only tolerate a very small amount of delay until the receivers can no longer decide the data. LSM/(W)CQPSK can tolerate a much wider amount of delay with out losing the ability to decode(by real system radios) the data stream.
Simulcast radio systems very precisely control the phase of the RF signal to ensure that the RF emitted from each of the simulcast transmitters will arrive with before the maximum delay spread value, through out the intended coverage area of the simulcast cell.
While the subscribers are receiving LSM/CQPSK modulation, the transmit to the system in C4FM.
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You are completely wrong. Simulcast CQPSK can be demodulated and decoded with a zero bit error rate with any digital scanner, IF (and that's the key) you can isolate the signal from one transmitter with a directional antenna and/or your choice of location. I monitor multiple simulcast systems with a 436 and 536. Most are in neighboring counties, and so I can receive them perfectly because I am only getting signal from one of the towers. I do have occasional trouble with the system in my county, due to multiple-tower interference. Fortunately, it has two simulcast sites, and I can generally receive at least one of them clearly even when mobile.
The problem is scanners' inability to handle multiple signals that have similar power levels and are slightly out of phase, not a complete inability to demodulate CQPSK.
Ubbe
Member
Also concider the doppler effect. When a patrol car are on its way to an incident and they have a transmit tower behind them and one in front them, the doppler effect will change the received frequencies conciderably. Users of simulcast systems will never be 100% satisfied as they always will be the victims of interference from another tower at one time or another. When rushing to an incident the radio communication might not fully work until they stop the car at the scene.
Are there any simulcast systems outside of US? I think it is only the high cost of frequency licenses that have made simulcast systems neccesary. Technically is better to have different frequencies for each tower in a radiosystem.
/Ubbe
kayn1n32008
ØÆSØ Say it, say 'ENCRYPTION'
Absolutely there are simulcast systems outside of the US. I can think of a few in Canada. Both phase 1 and phase 2.
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DisasterGuy
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DisasterGuy
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In a properly engineered simulcast system this isn't really the case. The need for simulcast also goes well beyond any cost of licensing (government agencies are exempt from filing fees anyway). Spectrum is a very limited commodity. To use something other than simulcast on a 10 channel trunking system with 10 sites would require 100 frequency pairs. Couple that with the fact that every time a radio would determine it's need to move to a new site (in P25) it would need to deregister and the register to the new site causing a brief drop in coverage. If you have heard complaints about OpenSky systems, it is because OpenSky is engineered exactly as you describe using a cell type strategy of non-simulcast sites.
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From what I've been reading isn't LSM just an acronym for CQPSK?
Not true, given the fact that in any multi-site system, there are multiple sites whose coverage areas do not overlap. You'd only need 10 times the maximum number of sites with overlapping coverage areas. The problem is similar to calculating the minimum number of colors needed to avoid giving adjacent states on a US map the same color. You don't need 50 colors, you can actually get by with 4. The absolute maximum number of unique channel pairs you'd need for any size system is 70, if the sites were arranged in a perfect honeycomb grid on flat unobstructed terrain.
It's not an issue for the thousands of P25 systems that do not use simulcast, or have a mix of simulcast and non-simulcast sites, like Ohio's MARCS or West Virginia's SIRN. It's also not an issue for agencies whose entire jurisdiction falls within the coverage area of a single site.
DisasterGuy
Member
Have you ever modeled 40dB interference contours in a relatively small county that needs many sites? As a general rule you aren't going to see reuse in municipal or county systems.
Also, when you are building a system for strong in-building coverage, the number of sites required would indeed cause a radio to register / deregister as you would be seeing a tower change every few minutes as opposed to the limited amount you see in regional cell implementation.
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Project25_MASTR
Millennial Graying OBT Guy
Doppler effect isn't honestly that noticeable. Consider a car traveling away (or towards) a tower at 100 mph (160 kph, ~44.4 m/s) and a general frequency of 150 MHz. The actual doppler shift is going to be around 22 Hz. At a 2.0 ppm frequency stability requirement, that's 375 Hz of allowable frequency error and 22 Hz is well within those requirements. The reality of it, Doppler Shift is next to negligible for land based vehicles.
It's not always the high cost of licenses that have made simulcast necessary for many but the added simplicity. Say you build out a 10 site 800 MHz system covering a specific county. Say you did the math, and assessed you could get away with 10, 5 channel sites in a traditional wide-area arrangement. One of the problems you will instantly notice, mobiles are holding onto sites longer than they should and therefore, not releasing the channel resources as they should be or when scanning adjacent jurisdictions because no one is affiliated on the same talk group you are trying to scan at the site your radio is affiliated to, there is nothing to scan so you won't ever hear them (causing you to hit the roam button and roam to that site where you run into the channel releasing issue again). You'll find, to make everyone happy, you'll have to build out to nearly the same capacity at each site as you would if you just went with simulcast for the whole county but you'll have so much adjacent spectrum that it makes coordinating with other counties to potentially build out a large wide area system next to impossible due to availability and intermod.
If you've designed a system in such a way that you're hearing signals from sites on the opposite side of those immediately adjacent to the site covering your location at power levels causing interference, you've failed as a system engineer. If the sites are in a perfect hexagonal honeycomb pattern, you should be able to hear a maximum of 7 sites--the one you're in, and the 6 sites surrounding it. If you're hearing anything beyond that, you've misconfigured the system.
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