Exploring Continuous Four Frequency Modulation (C4FM) and Linear Simulcast Modulation (LSM) in VHF Conventional Simulcast Systems

[jstorch]

Hello,

I represent a small public safety communications agency situated in central Washington, on the east side of the Cascade Range. Currently, we are in the process of transitioning to the P25 standard. Our focus has turned toward understanding the implications of Continuous Four Frequency Modulation (C4FM) and Linear Simulcast Modulation (LSM) within VHF conventional simulcast systems.

While there is ample information available on trunked, UHF, and 700/800MHz implementations, I have encountered challenges in finding substantial discussions specifically related to VHF and its use in conventional simulcast systems. I am particularly interested in learning from real-world experiences—both successes and pitfalls—regarding LSM in VHF conventional systems. Often, vendors emphasize success stories, but I believe there is valuable insight in understanding the journey, including any unexpected hurdles.

If anyone has practical knowledge, lessons learned, or insights related to LSM deployment in VHF conventional simulcast systems, I would greatly appreciate hearing from you.

Thank you for your time and expertise!

Best regards,

John Storch

 

[R8000]

My professional recommendation is to be very careful here. This is a hobbyist website and you will get feedback from some with no professional background and simply just have a scanner at their desk. They think because they got their favorites list working, they are qualified engineers.
The public safety system will be built to serve the end users, not the scanning community. Folks will chime in on LSM and bash it because their BCD996PII can't decode it properly. You may want to reach out to radio system admins in your region through more official means for feedback.
Good luck!

 

[mmckenna]

I agree with R8000.

I was reading the responses you were getting over at APCO and they were good. LSM looks like the way to go for what you are doing. The engineering will be similar if it is UHF or VHF. But this is the place where investing in a good professional consultant can be a good idea.

As R8000 said, this is a hobby radio site. There are a few professionals here, but you really need to filter what information you get on this site. There's a big knowledge gap between programming a scanner or ham radio and actually building out a public safety radio system.
It can be a good place to get information, but having a well tuned B.S. filter is required. I've seen quite a bit of potentially dangerous information shared on this site.

There are better websites for asking these sorts of questions.

 

[jstorch]

Appreciate the perspective, yes, posting on several forums. Mainly I'm looking here for grassroots responses from the hobbyist community. My experience is the 'industry' responses tend to, well, be Pollyanna. Intent is to follow-up on information, understand each response posted is a perspective. I'm finding these forums tend to have the most direct perspective about things like FirstNet, Kenwood Nexedge, PSERN, etc. I've found some hobbyist post are from an end-user of the system.

 

[mmckenna]

Appreciate the perspective, yes, posting on several forums. Mainly I'm looking here for grassroots responses from the hobbyist community. My experience is the 'industry' responses tend to, well, be Pollyanna.

Yes. Sometimes those with unlimited budget have some great ideas. Not always workable for all agencies, though.
A good consultant will take budget/realities into account. Consultants can be expensive up front, but ultimately pay off in the long run.

Still, there's lots of good information to be had , as long as you filter accordingly.

Intent is to follow-up on information, understand each response posted is a perspective. I'm finding these forums tend to have the most direct perspective about things like FirstNet, Kenwood Nexedge, PSERN, etc. I've found some hobbyist post are from an end-user of the system.

True. There is some good info, just keep your filters up.
I usually try to talk to our officers as much as I can about their experience with the radios. Usually that's my best feedback. Since scanners tend to have worse receivers and often poor audio, they are not always the most ideal form of feedback.

Good luck with your search!

 

[Echo4Thirty]

As someone who has experience building out several systems from both an end user and manufacturer, get a consultant. Also get one that is not the one that the vendor recommends.

That said, I have experience in VHF simulcast in both C4FM and LSM. I will just say that LSM was invented for a reason and is the standard over C4FM when it comes to simulcast. Can C4FM be made to work? Of course just as plain ole analog FM can. LSM was created specifically for simulcast and IMHO it enhances performance for subscribers that can properly utilize it over receviers that are only able to demodulate the FM component of the emission.

In closing: Get an independant consultant who can look at your specific geography, technology and needs.

 

[mmckenna]

In closing: Get an independant consultant who can look at your specific geography, technology and needs.

@jstorch, if you need some leads on a suitable consultant, I can send you a name. He designed an P25 simulcast conventional system for us. Plus, he's vendor agnostic.

 

[RFI-EMI-GUY]

Hello,

I represent a small public safety communications agency situated in central Washington, on the east side of the Cascade Range. Currently, we are in the process of transitioning to the P25 standard. Our focus has turned toward understanding the implications of Continuous Four Frequency Modulation (C4FM) and Linear Simulcast Modulation (LSM) within VHF conventional simulcast systems.

While there is ample information available on trunked, UHF, and 700/800MHz implementations, I have encountered challenges in finding substantial discussions specifically related to VHF and its use in conventional simulcast systems. I am particularly interested in learning from real-world experiences—both successes and pitfalls—regarding LSM in VHF conventional systems. Often, vendors emphasize success stories, but I believe there is valuable insight in understanding the journey, including any unexpected hurdles.

If anyone has practical knowledge, lessons learned, or insights related to LSM deployment in VHF conventional simulcast systems, I would greatly appreciate hearing from you.

Thank you for your time and expertise!

Best regards,

John Storch

My only comment is that VHF repeater systems have some design challenges versus UHF. 1) Getting duplex pairs can be a nightmare. 2) The noise level at VHF can be very high both at repeater sites and inside buildings. 3) VHF portable antenna efficiency is awful. You may want to explore why VHF is where you want to be. If it is interoperability, there are V/U dual band public safety radios as an option.

 

[mrsvensven]

I read your question on the APCO forum but didn't respond because the question is a bit vague on what you are actually trying to accomplish. Have you received two quotes and are trying to decide between them? Have you engineered your own future system and this is the last system parameter you need to define? Or are you at square one in designing a system? If you answered yes to either of the last two questions, you need a consultant or trusted vendor (or both).

While unrelated to the question at hand, as someone else mentioned you should be quite sure that VHF is the right band for you and that you have a good clean input frequency. There is a good possibility that if your repeater is located at a high site, it can hear the output of someone else's repeater on the same frequency quite far away. In most areas, there just aren't any new clean VHF frequencies left that cover wide areas such as a whole county. You don't see many agencies building new VHF wide area simulcast systems unless they are using frequencies they have had licensed for a long time. You also need to consider who is using the adjacent channels because VHF channels are only spaced 7.5khz apart and even a P25 signal is wider than that.

 

[dispatchgeek]

I read your question on the APCO forum but didn't respond because the question is a bit vague on what you are actually trying to accomplish. Have you received two quotes and are trying to decide between them? Have you engineered your own future system and this is the last system parameter you need to define? Or are you at square one in designing a system? If you answered yes to either of the last two questions, you need a consultant or trusted vendor (or both).

While unrelated to the question at hand, as someone else mentioned you should be quite sure that VHF is the right band for you and that you have a good clean input frequency. There is a good possibility that if your repeater is located at a high site, it can hear the output of someone else's repeater on the same frequency quite far away. In most areas, there just aren't any new clean VHF frequencies left that cover wide areas such as a whole county. You don't see many agencies building new VHF wide area simulcast systems unless they are using frequencies they have had licensed for a long time. You also need to consider who is using the adjacent channels because VHF channels are only spaced 7.5khz apart and even a P25 signal is wider than that.

I wholeheartedly agree with this sentiment. The VHF noise floor in my area is atrocious. Skip is a regular visitor.

 

[MTS2000des]

I wholeheartedly agree with this sentiment. The VHF noise floor in my area is atrocious. Skip is a regular visitor.

Yep, we stopped using VHF in this region ages ago as the noise floor hovers around -90dbm, difficult to get frequency pairs that play nice with combiners, and portables are always at a disadvantage due to compromised antennas.

What the O/P needs is a qualified consultant. None of this is found on a hobbyist forum, it would be akin to Boeing asking questions on how to properly engineer avionics on an aviation buff forum.

 

[speedway_navigator]

it would be akin to Boeing asking questions on how to properly engineer avionics on an aviation buff forum.

From the way parts fall of their 737 max series I wonder if they did...

I suggested John post here after his APCO post because I know the pros here have lots of been there, done that and 'oh crap what was I thinking' experience.

My list of simulcast info I've gathered is posted here.

Index of /tech/library/simulcast/