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Old 08-13-2014, 7:47 AM
jim202 jim202 is offline
Join Date: Mar 2002
Location: New Orleans region
Posts: 2,489

Some of the problems everyone is having with their scanners on simulcast trunking systems just might be the way the trunking system has been set up, the antennas the trunking system is using and the audio timing delays the techs have set up at each tower location. These all come into play on how well the radio signals are received out in the field.

Setting up a multi site, simulcast trunking system takes considerable skill. I haven't run into that many techs that seem to have those skills. Not only do all the transmitters need to be exactly on the same frequency, the audio delays at each site need to be in alignment so as not to cause distortion on the received signal. This is why they use GPS receivers at each trunking tower to maintain the frequency stability of the transmitters.

The big areas of concern are in the overlap region of coverage from the different transmitter sites. This audio distortion is called TDI or "Time Delay Interference" in the signal. Not only does this effect how well your scanner will function, but it also effects how well the user radios work on the trunking system.

If the TDI gets out of hand, the users out in the field will have trouble hearing the trunking system. You might think that their radios are wonder devices that will work under all conditions. But they are not science fiction devices out of a Buck Roger's movie. They are just commercial radios that need help in receiving the trunking system just like your scanners.

One of the more recent agencies that just went through an update to their antenna system and adjustment to the TDI was Mobile County, AL. They changed out the antennas and pointed them in different directions on the simulcast portion of their system than what was there before. After the work was completed, the feedback from users in the field was very positive. They could now use their radios in locations that before were either dead or garbled.
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