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<p>Thanks Tim! <br>
</p>
<p>So the CTCSS is starting out of phase. Does this affect voice
audio and tones in that bandwidth as well? <br>
</p>
<p>I am a long time lurker, wishing to put this technology to use
someday. I was saddened to learn that Jim Dixon passed away. I
hope someone can take the torch and further develop this promising
product. <br>
</p>
<p>As you probably know commercial simulcast is very expensive to
deploy and for a single channel system the cost is ridiculous
because commercial hardware is scaled for larger networks. So
this could have applications in commercial applications as well.</p>
<p><br>
</p>
<p>Joe<br>
</p>
<br>
<br>
<div class="moz-cite-prefix">On 3/13/2017 11:01 PM, Tim Sawyer
wrote:<br>
</div>
<blockquote
cite="mid:CAG3ht9tVbi9w4zZ=abtYOSs9+Vp_V-Dg91EriS4_CA4g42u6sA@mail.gmail.com"
type="cite">
<div dir="ltr">Joe's statement about launch delay is critical and
that unfortunately is the problem with RTCMs in simulcast. The
problem is the DAC used in the RTCM does not start at the exact
time every time. I remember Jim Dixon working on this issue and
was unable to workaround the problem that is build into the
silicon. The problem is particularly noticeable when humming
tone on the TX. If you can get by with no PL and limit your
coverage overlap you can simulcast with RTCM but it's never
going to be perfect. </div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Mar 13, 2017 at 6:41 PM, Joe
Leikhim <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:rhyolite@leikhim.com" target="_blank">rhyolite@leikhim.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<p>Wow; <br>
</p>
<p>Great project. I suggest documenting everything and
permanently posting for others to review the system
design as a reference.</p>
<p>My background is in Public Safety LMR design so I have
dabbled a bit in VHF, UHF and 800 MHz simulcast both
analog and digital. I might be able to do some computer
modelling for you. <br>
</p>
<p>Here is an example, picture two sites x1 and x2 spaced
exactly 13 miles apart. <br>
</p>
<p>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 <u>bulk delay</u> 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. <br>
</p>
<p>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. <br>
</p>
<p>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. <br>
</p>
<p>As a mobile travels away from center and along or
behind the hyperbolic curves )(, the time differential
will increase, creating time differential interference
(TDI), or intersymbol interference (ISI) in a digital
system. This can be calculated at any point location by
using a map and a mileage scale.<br>
</p>
<p>A mobile very close to or behind either site (paced 13
miles) will experience the delay of 70.2 us or more.
This is somewhat mitigated by the capture effect of the
closest site and the capture ratio afforded by the
modulation index. TDI at 70.2 us is not too bad for
analog FM. 135 us, the distortion might be intolerable.
<br>
</p>
<p>Adding a third site will require consideration that its
signal may arrive earlier or later than the sites
described above, In the event the third site is closer
than 6.5 miles, the adjustment would be to increase the
launch delay for the third site to meet with sites 1 and
2. If it is further than 6.5 miles, the delay of sites 1
and 2 could be increased equally by the amount required
for site 3 to travel.</p>
<p>A variation of this scheme, is one where a central site
is predominant and is ringed by remote sites. In one
configuration, the central site has an omni directional
antenna and the remote sites have directional antennas
facing outward. The central site would be timed to
transmit first, and the remote sites would be delayed
until the wavefront reaches them. If the distances are
unequal, the remote sites would be timed +/- the time
differential of the central site reaching them.<br>
</p>
<p>There are a couple very proprietary computer programs
for modeling this, however it can be worked out by hand.</p>
<p>Some thoughts.</p>
<p>1) Don't worry about frequency offset until you are
sure the timing and other parameters are correct. Set
everything dead on with GPS to start. In commercial
systems using GPS disciplined reference frequency offset
is usually not performed.<br>
</p>
<p>2) Make 100% sure that the modulators are wired for the
same polarity. It makes no sense to have one transmitter
deviating +4 KHz while another is going -4KHz. Same goes
for voice and CTCSS modulation.</p>
<p>3) Are your transmitters the exact same make/model and
version? This can be a non starter. You can get away
with different RF amps, filters etc, but if you have a
Micor exciter and a Mastr II exciter it is going to be
funky.</p>
<p>4) The audio chain needs to be identical. Any filters,
amps etc, in the transmitter audio chain must be same at
each site.<br>
</p>
<p>5) Adjust the modulation limiting such that the
transmitter audio is never in limiting. For Part 97
work, you could get away with the limiter adjusted
beyond system deviation and set the RTCM so that
deviation is equal to 5 KHz. Voice deviation for each
transmitter must be identical, same with CTCSS.<br>
</p>
<p>For my commercial projects I recommend the entire
simulcast system be staged in a warehouse so that all
these parameters can be set at same time with same test
equipment in a controlled environment. Any errors must
track the same direction. You can do this remotely, but
noisy signals will make the setting of values less than
ideal. If you reach a level of frustration, bring it all
home and tweak it.<br>
</p>
I hope this helps!
<div>
<div class="h5"><br>
<br>
<div class="m_-2793759250008952460moz-cite-prefix">On
3/13/2017 7:15 PM, Hayden Honeywood wrote:<br>
</div>
</div>
</div>
<blockquote type="cite">
<div>
<div class="h5">
<div dir="ltr">Hi All,
<div>Thanks for all on the list who have given me
a hand with getting the RTCM/voter boards
running.</div>
<div><br>
</div>
<div>I currently have 1/3 of my system installed
at the sites. It's as follows:</div>
<div><br>
</div>
<div>A TX only site - running a VOTER board,
latest "Chuck" firmware. 9.6MHz OCXO replacing
the onboard crystal and a 10MHz OCXO to keep the
transmitter stable all on 53MHz.</div>
<div><br>
</div>
<div>On my bench at home I have an identical setup
except with a receiver in addition. Both radios
are the same, audio injected in the same spot.</div>
<div><br>
</div>
<div>I've been testing various frequency offsets
to avoid the cancelling of the signal due to the
inherent propagation effects at 6 metres. I've
been able to run my bench radio into a dummy
load and place it so that another radio nearby
connected to an external antenna is receiving
them almost the same (i.e. overlap). </div>
<div><br>
</div>
<div>Currently 1-2Hz sounds the best. I have been
encoding tone, however there is a very audible
hum, as if the CTCSS tone from each transmitter
is not in phase. The tone is 91.5Hz. I've
checked/readjusted deviation levels etc, also
checked the 9.6MHz OCXO etc but I cannot seem to
null it out.</div>
<div><br>
</div>
<div>I've also tested the Simulcast Launch
Delay.The main site is about 13 miles away, so I
put a value of 330 in there to represent a delay
of 66uS on the bench test radio. This makes a
little bit of improvement, but not an overall
huge improvement. Does anyone have an ideas that
I've missed or more information on the usage of
the Simulcast Launch delay?</div>
<div><br>
</div>
<div>Hayden </div>
</div>
<br>
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</div>--
<div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div>--
</div><div>Tim</div></div></div></div></div>
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