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Author Topic: SIMULCAST PROJECT Part 2  (Read 2715 times)
The Guru
Posts: 72

« on: January 21, 2012, 06:10:17 PM »

Part 2
Propagation Delay or Audio Time Alignment:
The communication industry now uses 5.4 u sec and not 5.3 u sec. This works better in real world. If you do the math in Example 1 with 5.4 µsec instead of 5.3 µsec, you will see very little difference in the final answers. (The differences in time delay are almost the same)
Industry testing has also found that differences up to about 70 µsec will still give intelligible speech albeit somewhat distorted. But passing PL tones and Touch Tones may be too distorted to be reliable. We do not pass Touch Tones through the repeater now. (Our repeaters strip the Touch Tones before TX).
Transmit Frequency:
Testing has shown that simulcast TX must be very close in frequency (+/- 1 Hz). If you are not transmitting a PL, some receivers may accept a larger error because the low frequency heterodyning will be removed by high pass filters in the receivers. Unfortunately not all of our amateur radios are the same.
In Example 1 all the TX powers equal, the antenna heights are the same, feed lines are the same length and all the antennas are the same. This would give all the sites the same range and time delays.
All things being equal, this puts the overlap area in the middle of all the transmitters.
Without Audio Time Alignment, the audio time alignment error would be less than 42 µsec. This is not ideal but it would still be under the maximum time delay differences of 70 µsec. This may work but with distorted signals near and/or in the overlap zone.
We could build or buy audio time delay units to install in the audio path at a later date if necessary. These circuits are fairly simple and may be able to be built for under $50.00 ea. Testing and calibrating DIY units may be more complicated then building them. Purchased audio time delay units come calibrated and all you do is dial in the delay you need. Some units can be changed by remote control with Touch Tones at a cost of about $750.00 ea.
Example 2:
If we change antenna gains or patterns and/or TX power, we can move the overlap zones to areas of less importance. This would also change the Audio time alignment by shifting the overlap zone.
We can use beam antennas and/or bidirectional antennas to limit the over lap zones. If properly planned, this has many advantages by putting the power where we need it the most.
This may seem to be the cheapest way to “time align” our system, but it may not be the easiest. It may prove to be impractical. Most commercial systems use both audio delay units as well as varying the Effective Radiated Power (ERP) and antenna patterns.

Continued in Part 3

- October 2000 issue of Monitoring Times http://www.signalharbor.com/ttt/00oct/index.html
- http://k7pp.5u.com/rich_text_5.html
- http://www.gl.com/consulting_simulcastletter.html
- APW Electronics Limited, APW SYSTEMS DIV.
- Personal experience with past simulcast systems. (Good and Bad)
- http://www.allenavionics.com/ADDL/AVDL.htm#AVSeries
- http://www.convexcorp.com/
- http://www.simulcastsolutions.com/
- Online Calculators Time, Delay, Phase:
- Many months of researching Internet sights.
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