[App_rpt-users] Simulcast buffer value issue

[Tim Sawyer]

As others have mentioned, there is a lot of information on the internet
about simulcasting. It is necessary keep the overlap area small as
possible. Reason for that is the audio from each transmitter has to arrive
at the receiver within about 80 micro seconds. So you looking at an overlap
about 15 miles wide at best. This generally requires extensive site
engineering considering, capture effect, location, terrain, antenna pattern
and transmit power. The Radio Mobile software package does a fair job of
plotting simulcast overlap but it has a pretty steep learning curve and a
fair amount of experimentation. I've attached a doodle that may be of
interest.

On Tue, Feb 13, 2018 at 8:10 PM, Jeff Carrier <k0jsc.jeff at gmail.com> wrote:

> I really hope to be able to confirm this soon.  I've been lazy and our 3
> sites are 86 miles, 79 miles and 70 miles (in a triangle).  One of the 3 is
> over 11,000 feet elevation.  No matter what we've tried so far you still
> hear distortion in the overlap area(s) even when 1 site is 20 miles out LoS
> and the other is around 60 miles out non LoS.  Everything is linked on a
> private uW network with very low latency and basically zero packet loss.
> The transmitters (GE MIII) run about 1hz freq error with the gpsdo attached.
>
> Sometimes I just wonder if this is just multi-path from the various
> granite reflectors we have on the front range of Colorado (one of them is
> Pikes Peak)
>
> de K0JSC
>
> ------------------------------
>>
>> Message: 6
>> Date: Tue, 13 Feb 2018 20:43:42 +1100
>> From: Hayden Honeywood <haydenph91 at gmail.com>
>> To: app_rpt-users at lists.allstarlink.org
>> Subject: [App_rpt-users] Simulcast buffer value issue
>> Message-ID:
>>         <CAC0VLD0-9mc=T8BkvMwx6PqWXqtUXCYY2odbjc2ajjDiVLKSBg at mail.gm
>> ail.com>
>> Content-Type: text/plain; charset="utf-8"
>>
>> Interesting thoughts Tim... perhaps worth documenting on the wiki?
>> Even though it was just "in theory", what kind of sync error was it?
>> Were we talking microseconds or even more?
>>
>> I have a receiver on a yagi pointed at a distant simulcasted site. I
>> have noticed on occasion, I can hear the distant simulcasted site
>> start to send audio underneath the carrier of the site that is closest
>> (and strongest to me). The distant site is also the master site, and
>> has the lowest latency, but I'm talking two words worth of audio is
>> send before my other site starts sending audio. Once they are both
>> transmitting, I  have not noticed any timing issues (i.e. distortion
>> etc). I'm not sure if this issue was related to what I posted
>> previously about the variable audio delay on unkey.
>>
>> I'm running app_rpt on a Raspberry Pi using a cut down image we use here
>> in VK.
>>
>> http://vklink.com.au
>>
>>
>> "When simulcasting the audio from all (non captured) transmitters needs to
>> arrive at the receiver with in an acceptable time frame (80 us). The DAC
>> theory was that it didn't start sending audio at correct clock cycle every
>> time. That would cause the audio to be out of sync between RTCMs causing
>> simulcast distortion.
>>
>> As I said, that was the theory. The new theory is that the external clock
>> source was causing the DAC to trigger inappropriately. I spoke with
>> someone
>> I met here on the list (Kevin I think its was) who is using a different
>> external clock and reports perfect simulcast operations"
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-- 
Tim WD6AWP
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