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Comms System technical info

Discussion in 'Sound, Music, and Intercom' started by tomed101, Dec 30, 2007.

  1. tomed101

    tomed101 Active Member

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    I nned some detailed technical info regarding the way that partyline comms systems are configured. For example, what each pin on the cable carries, how signals are input to the system etc. The project is to basicly create a wireless system which can be added to an existing system using the componants from standard FRS radios to carry the signals. This is a project for my own benefit, so no biggie if it doesnt work. I have worked out how to make the wireless stuff work, now I just need to know how to interface with another system. I figure, for 4 beltpacks, I will need 13 radios (4 transmitters in the packs, 4 recievers in the packs, 4 recievers at the base and 1 transmitter at the base) and 5 radio frequencies will be needed.

    Thanks in advance,
    Tom

    By the way, I am on holidays in Vietnam so I may not be able to reply immediatly to posts.
     
  2. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    what system? RTS, HME, Telex, Clear Com or another brand? Have you downloaded the PDF manuals for the type of base and com you use? they usually have the wiring diagrams in there fairly detailed....

    -w
     
  3. icewolf08

    icewolf08 CBMod CB Mods

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    I realize you are doing this because you want to, but there are lots of commercially available "Coms to X" adapters. Here is Clear-Com's page of party line adapters. I suppose the biggest reason that I am telling you this is not to stop you from doing your project, but you might find a useful adapter. Also, as Wolf said often the manuals have block diagrams, so look there if you want to complete;y build your own.
     
  4. mbenonis

    mbenonis Wireless Guy Administrator

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    The specifications for ClearCom and similar systems are posted online. This is an interesting project, although I don't know how well it will work for you due to the fact that the range of FRS radio's is quite low (regardless of what the box says). It's most likely going to be a bit unwieldy as well. But more power to you if you can make this work.

    One thing to watch out for is any radio that will be transmitting all the time. These radio's are NOT meant for a 100% duty cycle...probably more like a 5% duty cycle. While the power is quite low, if you stick the base radio in TX for more than a few minutes, you run the risk of smoking it. Just be aware of this.
     
  5. avare

    avare Active Member

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  6. tomed101

    tomed101 Active Member

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    Thanks for the info so far.
    I have used these radios in out venue before (just as standard 2-ways witout modification) and had plenty of range. They have different power settings so I can choose the most appropriate setting for the job.
    Depending on how the radios are consruced, I maybe able to remove the circuit board from the casing, remove the battery and put it in another case to make it smaller and have a combined power supply for the two radios.
    I have considerd this and considering the radios are CHEAP it may weell be a problem. I do not need high power radios and will be using a low power setting, so I may be ok. I will get one of the radios when I get them and set it transmitting and leave it for a few hours and see what happens...
    By the way, when I say cheap radios, i mean it (10 radios for $70...) With this in mind, they wont be very well made and may not be suitable for my task.
    The system which I need to interface is a Jands system. When I get a chance I will have a look and see what kind of system they use. A really easy way around all of this would be to get a cheap beltpack and hijack the headset inputs/outputs and interface teh system that way.
    From what I have read, the signal and call signal travells over pin 2 on the cable, which could create problems. The call signal looks lo\ike its 12V which could screw with the audio signal and potentially damage componants if not properly filtered out. Does anyone know how this ia achieved?
    Thanks again,
    Tom
     
  7. mbenonis

    mbenonis Wireless Guy Administrator

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    Technically, FRS radios only have one power level setting, 500 mW. If the channel you're on has multiple settings, it's NOT FRS, but GRMS, and this requires a license to use (and GMRS rules prohibit business/organization use anyway). Splitting hairs? Yes. Will you get in trouble? Very unlikely. But still worth noting.

    Sure. Put a capacitor in line with the audio signal, preferably a BFC (big f.....g capacitor) as we electrical engineers like to call it, to pass all of the audio. 47µF should do the trick.

    Mike
     
  8. mbenonis

    mbenonis Wireless Guy Administrator

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    I just finished skimming this, and this is an EXCELLENT guide to intercom systems. I would highly recommend everyone download this and stash it away for when you have an intercom problem!
     
  9. dj_illusions

    dj_illusions Active Member

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    I have a custom built radio unit to run with comms that I acquired from a broadcast OB van. they have basically built a belt pack and a uhf radio into one unit inside a box, using the exisiting circuitry of both, if you talk on the radio it is broadcast to all comm stations and the radio can be set to transmitt what is said on the comms loop or switched off. it is very useful for some shows, i have also used it as a venue supervisor to listen to comms during a client operated show.
     
  10. tomed101

    tomed101 Active Member

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    If all goes to plan, what I will have is in each beltpack, the guts of two UHF radios, one continuously transmitting and the other receiving. I will make my own battery pack for each one so that I can have enough power for continuous transmission, and to save space. I am assuming that if I remove the casing of the UHF, the speaker and the battery pack, the remaining components should be relatively compact. If so, I will make a case out of sheet-metal and mount allof the components inside.

    For the base station I will have one UHF receiving from each beltpack and another transmitting to all beltpacks for the listen channel.

    From what I have read, I will need a big capacitor on the signal line to remove the call signal (possibly trying to use the call signal to trigger a call on the radios, but that is a low priority) input the filtered signal line into the mic input of the transmission radio in the base (possibly requiring a resister to drop the level to an acceptable level). The transmission radio will send to all beltpacks. The output from the receive radios in each beltpack will feed the headphones. The mic on the headset will go into the mic input on the transmission radio in the beltpack, which will then transmit on it's own frequency back to the base receive radios. The output signal from the receive radios in the base will be mixed together, be dropped to line level and put back into the party line.

    From what I can see, the number of beltpacks will only be limited by the number of available frequencies.

    Another low priority will be to make the wireless base into a wired base. Correct me if I am wrong, but from what I have read, to make a partyline base, all I need is a 30V transformer feeding the power pin on the line, and a 200ohm resister between the ground and the signal line. Also a 12V LED (or lover voltage connected through a resister to lower the voltage) connected to the signal line for call signal. A 12V call signal could also be sent down the signal line for call. This would obviously have to be switched so that when the unit is not being used as a base, the 200ohm resister does not interfere with the rest of the system.

    Also a headphone amp and mic pre-amp could be connected to the signal line on the partyline at the base so that a local headset could be connected.

    I'm sorry if this doesn't make much sense. If you can comment on the design, it would be most appreciated.

    Tom
     
  11. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Tom, given you are in Brissie (at least when not on holidays), I should hope you were NOT planning to use FRS radios. Doing so would be ILLEGAL... Use of Australian UHF CB stuff on the other hand would be OK, but the frequencies US FRS stuff runs on are allocated to 'real' users who could be relying on their frequencies for emergencies etc. and hence it's a big no no...

    Send me an email and I'll send you the stuff you want on Ezicom (if you click my name at the top of this post, you'll get a drop down menu, email is one of those options...) It is either pin 2 power and pin 3 audio or vice versa and it's too late for my brain to be working properly and I don't feel inspired to look it up this late either.


    Gutting radios sounds like hard work. Could you not stick it on VOX and just run in simplex? The real problem I think would lie in making sure your antennae were still the right length and all that and that you hadn't inadvertently created extra ones with wire links that could be the issue...

    The whole process of constantly transmitting from all your beltpacks I have a nagging doubt will be asking for trouble... In the end, many a professional show run on simple Motorolas, interfacing them to comms ain't all that hard if you know how to and have the right stuff... It's when you have > half a dozen different channels interfaced it becomes somewhat more interesting...
     
  12. BNBSound

    BNBSound Active Member

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    There's a few things about powering your belt packs up that are going to trip you up.

    The battery life for any radio is going to be calculated on the 8:1:1 ratio. That is to say: 80% squelched, 10% recieve, 10% transmit. That same ratio goes into how they spec the final transistors for the RF output. I understand you're trying to get a full duplex system going, but you're more than likely going to fry the TX radio. There's also a good possibility that even if the TX is on a different channel than the RX you'll get what's called front end overload and won't receive much. It's the reason wireless mics and guitar stuff doesn't work well near radio and TV stations.

    One way around it would be to use radios for TX that have a VOX feature. If they're opening too much you could use the Talk switch on the com pack to open the mic and thus the VOX.

    There's also lots of easy ways to get radios interfaced with com base stations. You might want to look into that and just get slightly better radios for your guys to wear. With some good ear pieces and isolation mics you'll be in pretty good shape.
     
  13. tomed101

    tomed101 Active Member

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    I had read something about that, I cant remember what they called it but, I was planning on getting a few really cheap radios and having a play, and seeing what happens. My main concern is that the TX chip cant handle the constant transmission, in which I might have to resort to another method, like the one you mentioned.

    Tom
     

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