[App_rpt-users] Simulcasting with RTCM/voter

[Jeff DePolo]

> First: They both need to launch the audio waveform at exactly 
> the same time T=0 in same phase.  This also means that there 
> may need to be bulk delay to ensure that the most distant 
> (with respect to the IP back haul network) transmitter site, 
> receives the entire data payload before it is transmitted. I 
> am just learning about the RTCM so I believe that bulk delay 
> is inherent to the process. 
> 
> 
> Assuming both sites are exactly synced T=0, then where the 
> wave fronts meet midway will be in exactly sync even though 
> 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to 
> propagation delay. 

That's not correct (the part about "the wave fronts").  Simulcast audio/PL
launch delay only guarantees AF synchronicity to the extent
practical/possible (e.g. within a few microseconds of each other), but
certainly not at the same RF phase.  A microsecond at 150 MHz is 150 cycles.

> The wave fronts will also form a pair of theoretical opposing 
> hyperbolic curves that meet at the center of the two sites 
> like this: x1 )( x2  with the x representing each site. 

Again, you can't look at it in the RF domain; AF sync is what you're working
hard to achieve; you'll never get it at RF.

Also keep in mind that sync at the midpoint isn't necessarily the goal.  In
a perfect world with no terrain, identical antenna patterns, identical ERP,
etc. it might be, but not in the real world.  You need to do coverage models
for each of the sites (using actual ERP's and antenna patterns), see where
the most-problematic areas are (i.e. the areas where capture effect isn't
going to hide potential sins), and make decisions as to where to optimize
and where to tolerate degradation, and then set delays accordingly.

> 1) Don't worry about frequency offset until you are sure the 
> timing and other parameters are correct. 

Agree.  Until the delays are correct (and again, that doesn't necessarily
mean equal at the midpoint), there's no sense in adding another variable to
the equation.  Offsetting carrier frequency only helps mitigate cancellation
nulls, it does nothing to mitigate delay errors.

			--- Jeff WN3A



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