I have a situation that I need some help with.
I would like to be able to connect 2 radios (Tx only) to one BIM on my Centracom console. I've been told that punching down more than one radio (load) will cause a problem with the 600 Ohms the BIM is expecting to see from the radio.
I've heard there are devices which could take the audio coming out of the BIM and split it so that the BIM sees 600 ohms, and so do the outputs going to the radios.
Anyone know where I could find one of these, or what they are called?
Thanks a bunch...
Connecting 2 Radios to 1 BIM
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Re: Connecting 2 Radios to 1 BIM
You can double terminate without causing problems. You just have to know your equipment. In general the sending end (in this case a BIM) is set for high impedance (hi Z), and the receiving end (in this case a radio tone remote adapter) has the 600 ohm termination (lo Z) to set the line impedance. When you parallel two tone adapters, you are double terminating, the xmit levels will be reduced, and your audio will sound mushy (less high freq content). Therefore, you set one of the adapters to hi Z allowing just the other to terminate the line. That's theoretical perfection.
What about real life? What do you do if there is no way to change the termination from lo Z to hi Z? Maybe nothing. The CEB line driver has enough output range, and the adapter has enough input range, to make up the lost level. Likewise, the mushy audio may be unnoticeable. If you're a baritone with an old headset with decent level out to a mobile with a tinny speaker, nobody will notice the loss. It won't sound any better if you had a perfect 600 ohm termination.
What if your boss is a freak, or worse - what if you're a freak, and your brain keeps tweeking about the theoretical loss of a few db and some vocal harmonics that will make the system more musical, and natural sounding? I have seen some Motorola engineers spent inordinate amounts of money seeking the holy grail of impedance nirvana. Well, you could build an impedance bridge. Hybrids used to be quite common when analog telephone was king, and those things are still around, but they are getting harder to find. You may have to roll your own from coils and resistors, or you may to use active circuits to achieve your goal. A quick Google of telephone hybrid will land you in the general area to start your research.
There is some product out there that will do the job, and do it well. If you're a broadcast engineer, and you need high quality telephone audio for your call-in show, then you want off the shelf stuff with known characteristics, high performance, and high reliability that can be duplicated over and over at every studio you maintain. If you are a regional dispatch center with lots of phone lines and microwave paths to a ton of sites shared by other agencies, you want building blocks that allow connectivity between multiple consoles over a wide area. Raven makes just what you need.
http://www.ravencomm.com/products/catal ... e-shelves/
As always, your mileage may vary.
Hope that helps.
What about real life? What do you do if there is no way to change the termination from lo Z to hi Z? Maybe nothing. The CEB line driver has enough output range, and the adapter has enough input range, to make up the lost level. Likewise, the mushy audio may be unnoticeable. If you're a baritone with an old headset with decent level out to a mobile with a tinny speaker, nobody will notice the loss. It won't sound any better if you had a perfect 600 ohm termination.
What if your boss is a freak, or worse - what if you're a freak, and your brain keeps tweeking about the theoretical loss of a few db and some vocal harmonics that will make the system more musical, and natural sounding? I have seen some Motorola engineers spent inordinate amounts of money seeking the holy grail of impedance nirvana. Well, you could build an impedance bridge. Hybrids used to be quite common when analog telephone was king, and those things are still around, but they are getting harder to find. You may have to roll your own from coils and resistors, or you may to use active circuits to achieve your goal. A quick Google of telephone hybrid will land you in the general area to start your research.
There is some product out there that will do the job, and do it well. If you're a broadcast engineer, and you need high quality telephone audio for your call-in show, then you want off the shelf stuff with known characteristics, high performance, and high reliability that can be duplicated over and over at every studio you maintain. If you are a regional dispatch center with lots of phone lines and microwave paths to a ton of sites shared by other agencies, you want building blocks that allow connectivity between multiple consoles over a wide area. Raven makes just what you need.
http://www.ravencomm.com/products/catal ... e-shelves/
As always, your mileage may vary.
Hope that helps.
Re: Connecting 2 Radios to 1 BIM
Ummm,,,no.Bill_G wrote: In general the sending end (in this case a BIM) is set for high impedance (hi Z), and the receiving end (in this case a radio tone remote adapter) has the 600 ohm termination (lo Z) to set the line impedance.
The sending end (whether it be a tone remote or a BIM) is set for 600 ohms. The base end at the other end of the lease line (or local) is also terminated at 600 ohms (kind of like a transmitter at 50 ohms, the feedline at 50 ohms and the antenna at 50 ohms).
When you parallel 2 remotes (or BIM’s), one is left at 600 ohms and the second is set to high impedance.
In this case, it is not the remotes (BIM) that we are paralleling, but the base station. You should be looking for a way to set one of the bases to high impedance. This is not typically a function that is available in remote adapters.
I wonder if you were to put 300-ohm resistors in series with both wires in the twisted pair at the base radio (on both radios). This would add 600 ohms of resistance in series with the 600-ohm base adapter making it 1200-ohms. Doing this to both radios and putting the 2 1200-ohm bases in parallel, you have 600 ohms. It would result in a 6 dB drop in level to/from the base radios, but if they are local, you probably have enough drive anyway.
This is just a thought, and untested. It would be an easy fix if it worked.
Re: Connecting 2 Radios to 1 BIM
Thanks for the replies--good info.
If it helps, here's a little clarification on what I need to do:
From the Console (BIM) I am currently connected to a VHF MTR2000 using tone keying for 2-tone paging. I want to also connect an 800 MHz XTL1500 to the BIM using a tone-remote adapter so that when dispatch pages, it not only broadcasts on the VHF xmitter, it also sends the page over the 800 MHz talkgroup.
My guess is it will probably work okay if I "double tap" the punch down block coming off the BIM, but as Bill_G alluded to, I'm a freak and want to do it right if I can.
Thanks again. Let me know if you have any other thoughts.
If it helps, here's a little clarification on what I need to do:
From the Console (BIM) I am currently connected to a VHF MTR2000 using tone keying for 2-tone paging. I want to also connect an 800 MHz XTL1500 to the BIM using a tone-remote adapter so that when dispatch pages, it not only broadcasts on the VHF xmitter, it also sends the page over the 800 MHz talkgroup.
My guess is it will probably work okay if I "double tap" the punch down block coming off the BIM, but as Bill_G alluded to, I'm a freak and want to do it right if I can.
Thanks again. Let me know if you have any other thoughts.
Re: Connecting 2 Radios to 1 BIM
As Bill has said you need to pay attention to the line loading. If you can locate a diagram of the second radio termination panel or circuit, you should be able
to locate the loading resistor.
Also in the BIM card, you should find the termination resistor. Depending on how you can modify the second base radio, you may or may not have to change
the termination or load resistor in the BIM card. The book with the BIM card should be in the CEB manual. Then all you nned to do is measure the audio
output on the line before you do any changes. Jot this level down and measure it again when you add the second base.
We have had a number of threads on just how to do the level settings. What I will say is that I normally use just the low level guard tone (PTT level) for
any of the adjustments I make. This way your not causing untold grief with tones going over the air. You will need a good level meter to make the
measurements. A good TIMS unit will make the adjustments a breeze. Just make sure you use the meter in the bridged mode so you don't load the circuit
down. I look for a low level guard tone of some place between a -27 and a - 33 db. What your radio shop has used could be anything. Much below this
point and you chance the radio not keying up if you have any phone line losses that get excessive.
Jim
to locate the loading resistor.
Also in the BIM card, you should find the termination resistor. Depending on how you can modify the second base radio, you may or may not have to change
the termination or load resistor in the BIM card. The book with the BIM card should be in the CEB manual. Then all you nned to do is measure the audio
output on the line before you do any changes. Jot this level down and measure it again when you add the second base.
We have had a number of threads on just how to do the level settings. What I will say is that I normally use just the low level guard tone (PTT level) for
any of the adjustments I make. This way your not causing untold grief with tones going over the air. You will need a good level meter to make the
measurements. A good TIMS unit will make the adjustments a breeze. Just make sure you use the meter in the bridged mode so you don't load the circuit
down. I look for a low level guard tone of some place between a -27 and a - 33 db. What your radio shop has used could be anything. Much below this
point and you chance the radio not keying up if you have any phone line losses that get excessive.
Jim
detblow wrote:Thanks for the replies--good info.
If it helps, here's a little clarification on what I need to do:
From the Console (BIM) I am currently connected to a VHF MTR2000 using tone keying for 2-tone paging. I want to also connect an 800 MHz XTL1500 to the BIM using a tone-remote adapter so that when dispatch pages, it not only broadcasts on the VHF xmitter, it also sends the page over the 800 MHz talkgroup.
My guess is it will probably work okay if I "double tap" the punch down block coming off the BIM, but as Bill_G alluded to, I'm a freak and want to do it right if I can.
Thanks again. Let me know if you have any other thoughts.
Re: Connecting 2 Radios to 1 BIM
Understood, and agreed. The best transfer of energy, and the best freq response occurs with properly matched impedance. But, I did qualify it with an "in general" for a reason. If your BIM is looking into a leased line that has thousands of wire feet before it hits the telco CO, then for sure terminate the line at the BIM. On the other hand, if the BIM is looking into a comparator, a telco term card or channel bank, or a microwave channel bank far less than 100 wire feet away, you can improve the quality of the audio by taking the BIM term out as long as the other end has the term inserted. Some people like the classic Centracom boominess. I like to keep the mic gains down so you don't hear a gnat pick it's teeth at ten yards but can still hear the tapping of key caps as the dispatcher reads out a warrant. Color me a total freak.RFguy wrote:Ummm,,,no.
The sending end (whether it be a tone remote or a BIM) is set for 600 ohms. The base end at the other end of the lease line (or local) is also terminated at 600 ohms (kind of like a transmitter at 50 ohms, the feedline at 50 ohms and the antenna at 50 ohms).
Re: Connecting 2 Radios to 1 BIM
Aww yeah... Thats the stuff... Bill can you also please take a look at my post in this forum re: digitac/centracom and give me some input?Bill_G wrote:I like to keep the mic gains down so you don't hear a gnat pick it's teeth at ten yards but can still hear the tapping of key caps as the dispatcher reads out a warrant. Color me a total freak.