I'm looking for detailed information on how simulcast sites work - How does the subscriber radio know (or not know) which site its talking to? How do the sites know which subscriber radio is talking to it?
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How does simulcast work?
- Thread starter Kazzaw
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That's the benefit of simulcast, it doesn't know or care which tower it is using. They system measures for the best received connection through a voter, and uses that for the received signal from the subscriber. All the transmitters are all time synced and transmit the exact same thing at the same time, so the subscriber doesn't know or care that it may be receiving from 1 or multiple sites. FM capture effect also plays a part.
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Thanks. The reason I ask is I was having a convo with a mate regarding time sync and how they work - obviously through GPS. Obviously there is something that 'delays' the transmission from each site, and the communications between site is fibre at a guess?
I've seen simulcast sites locally and all have a GPS antenna, I assume these all talk together between the simulcast sites through an IP connection, and delay the repeater TX so they all TX the same thing at the same time?
mmckenna
I ♥ Ø
Backhaul between radio sites can be fiber, microwave, telco, or IP.
Correct.
Frequency accuracy of the transmitted signal is super important, and GPS disciplined oscillators are used to keep the transmitters on frequency.
The GPS also is used as a reference for time. On IP based simulcast systems, the system will figure out the network delay and adjust accordingly.
More or less, yes, GPS is used for the time synchronization, and design/testing in field determines delay. Connection between sites can be done many different ways.
Just saw @mmckenna said the same thing above.
speedway_navigator
Member
We worked at a PSAP that had a cable modem IP feed to a simulcast paging site where the others were RF links. They design engineer could not get enough delay in the RF sites to match the time it took for the IP site to get the data. I think they had it set on a delay so the others were done before it keyed.
Ubbe
Member
The audio time sync isn't that critical as it can be 50mS-100mS off and still sound good. What's important are that the RF frequency are exact in frequency, but then if a vehicle travel at speed from one site towards another the doppler effect will screw that up, so simulcast will always be problematic with users that move around and are not stationary. Technology evolves so I don't know what the tolerances are but perhaps it is now taken care of in portables and mobiles at normal speeds at 55mph.
/Ubbe
/Ubbe
MTS2000des
5B2_BEE00 Czar
Launch time: your best friend, or your worst enemy.
In many cases, sites in a simulcast cell are frequency offset from each other by a few hertz, which makes that GPS reference even more important. This minimizes TDI in overlapping coverage areas and helps the radio "pick out" it's best signal. That's an oversimplification, but true regardless.
mikegilbert
MHz so good
speedway_navigator
Member
I miss Ed's forums at IWCE, I'm always teaching during his session. Next year I'm teaching Mon- Thur so I'll miss every session.
petnrdx
Member
I also miss the snacks Ed always had at his forums.
speedway_navigator
Member
I usually got there late, this explains why I never saw any...
freddaniel
Member
I would like to add that a fairly new IP based analog FM Simulcast standard is evolving from the original design of the late Jim Dixon called AllStar and is available at www.AllStarLink.org on the web. There are hundreds of systems in service from simple one Tx site with several voting receivers to 5 or 10 site full simulcast. Also see www.allstarsimulcast.com by Joe Moskalski, KC2IRV who operates a 6 site system. There is also a Wiki with tons of useful information.
In addition, it is not necessary to always use high sites, as they tend to create excessive overlap, which is where timing issues really expose any faults. By using earth blockage, using ground level sites, a better simulcast can be created.
Hardware is readily available to use with MTR2000 or Quantars.
In addition, it is not necessary to always use high sites, as they tend to create excessive overlap, which is where timing issues really expose any faults. By using earth blockage, using ground level sites, a better simulcast can be created.
Hardware is readily available to use with MTR2000 or Quantars.
freddaniel
Member
FYI - The correct link is AllstarLink Amateur Radio Voting and Simulcast – Using the Voter Board and Radio Thin Client Module
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