[App_rpt-users] Simulcasting with RTCM/voter

[Joe Leikhim]

Jeff;

Yes True; It is indeed the _audio phasing_ that needs to be synced at 
midpoint. I was trying to state in as few words as possible a very 
complex topic. And yes, in the "real world" the reception will have 
exaggerated multi-path effects due to the frequency error which are 
unavoidable.  I worked on some of the early crude analog  800 MHz FM 
simulcast and my ears hurt listening to them.

Going from two transmitters to three or four (or more)  becomes very 
complex in short order. I have used computer software to simulate all 
the effects to timing offset signal strength (reliability) and signal 
capture. EIA/TIA TSB-88D describes monte carlo power weighted 
methodology used in the computer models. Some software models can do 
automatic iterations of timing to choose best fit.


Joe


On 3/14/2017 12:34 AM, Jeff DePolo wrote:
>> 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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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM

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