Simulcasting

[Gilligan]

I'm curious about something. I know simulcasting has been around a long time, and I know it works for conventional as well as trunking. But why don't the different towers interfere with each other. Even if the same audio is being transmitted, I would think that the signals would interfere with each other, kind of like when one police officer transmits at the same time as another and they "walk on each other."

 

[SCPD]

They do interfere with each other, you can especially detect this when monitoring from a long distance or are subject to multi-path where the signals reach you out of phase. It has to do with the timing of the broadcast and the audio.

 

[Gilligan]

Okay, now I can see where the signals being received at different times would be a problem. I guess that the idea is to have one site be much stronger than the other (as in how it's received by a mobile radio). Like here in OKC, there are 7 sites in the EDACS inner-city side, so I guess the closest one should bring in the strongest signal, thus overridding the others ???

 

[N_Jay]

The way to keep a simulcast system from interfering with itself is by synchronizing the signals in amplitude, phase, frequency, and timing.
Then you align them so in areas where the timing has become unacceptable due to propagation times, you design for capture (one signal significantly stronger then the other).
In areas where the signals are unavoidable close in strength, you have to make sure they are aligned in time.
Then you hope that there is some real-world compromise that makes it all work.

 

[Big_Ears]

Back in the AM days, 2 signals transmitting on the same frequency would be received as hetrodyne where as you would hear the offset between the two frequencies. In FM there is the capture effect where the strongest signal will override the weaker one. I guess the issue would be if both received signals were exactly the same strength.

 

[N_Jay]

Back in the AM days, 2 signals transmitting on the same frequency would be received as hetrodyne where as you would hear the offset between the two frequencies. In FM there is the capture effect where the strongest signal will override the weaker one. I guess the issue would be if both received signals were exactly the same strength.

The dont have to be exactly the same strength.

Capture is usually lost when the signals are within 6dB, so that is a 12 dB range.
On top of that you have to account for multi-path and fading which will be uncorrelated between the two signals. In the real world, you will start hearing some effects of the weaker signal when its mean is about 20 dB below the stronger signals mean level.