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Some Notes on
By Richard Cohen K6DBR
and Steve Bosshard NU5D
I just thought I would write a few notes on Simulcast for those going to attempt it. This is by no way complete. It is just a few thoughts to consider. I think a good RF guy and a very good software guy could come up with a workable system. Here's the details of the GE system this was done to.
Each carrier was less than 10 Hz apart, preferably less than 2 Hz. This was done with high stability oscillators. GPS disciplined oscillators are now the best option.
Every transmitter must have the audio amplitude response curve (300 to 3000 Hz) within 0.5 dB at all parts of the audio curve. Once you measure the audio curve for each transmitter, you choose the worst one and pull the others into alignment with it. We used an 11 section audio filter. This was the hardest part to tune in. Think about doing it on 17 transmitters (LA County Sheriff Dept). My suggestion is to limit it to three transmitters (unless you have no overlap areas).
Time delay in overlap area:
First you need to measure where the overlap areas are. This can be done by placing one tone on one transmitter and a second tone on the second transmitter. Then drive to the test area and repeat for all combinations of transmitters (3, 4, ...). Now a delay line must be added to each audio being simulcast. We used an addressable FIFO to select the amount of delay. You want zero relative time difference at the center of the crossover area. Then as you move to the fringe of the crossover area, there should not be too much difference or the audio will sound crappy. We used less than 1/2 a bit of a 9.6 Kilo Bit signal. If the crossover area is too large, you need to do some propagation adjustments.
All processing was accomplished at a central location and then sent to the transmit sites by digital microwave. The digital microwave network needed to be synchronized too, not just the standard 1.54 MBT, but the leading edge of each recovered signal had to be the same. Since we were transmitting data as well (9.6 MBT), we could not be off by one bit. A special mod was developed for the NEC Mux. This function (Synchronizing the transmitters) will be a significant challenge. Once the transmitters are synced, the delay can be added so the signals match in the crossover area.
Alignment of the system:
The best way to do the alignment of the system is to have one receiver in the crossover areas. Feed the audio back to the central point and tune the network.
There ya go. Simple.
The following was contributed by Steve NU5D:
There is a new technology that uses CDMA cell phones in place of GPS to discipline the master oscillators - I have never tried this. We use Simulcast Solutions GPS oscillators. The radio is Western Multiplex 6 GHz, 4T1 / Glenayre with Harris Intraplex Multiplexer. The mux has bulk delay for the T1 Streams and the GPS provides a precise 1pps and 9.6kb/s for resync. Much simpler. Also, all of the radio exciters are GE M3; they fall within tolerance out of the box and do not require equalization. This is on 20 kHz / 855 MHz channels - 4 sites simulcast.
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This page created 31-Dec-07.
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