[App_rpt-users] Simulcasting with RTCM/voter
Joe Leikhim
rhyolite at leikhim.com
Tue Mar 14 03:15:45 UTC 2017
Thanks Tim!
So the CTCSS is starting out of phase. Does this affect voice audio and
tones in that bandwidth as well?
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.
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.
Joe
On 3/13/2017 11:01 PM, Tim Sawyer wrote:
> 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.
>
> On Mon, Mar 13, 2017 at 6:41 PM, Joe Leikhim <rhyolite at leikhim.com
> <mailto:rhyolite at leikhim.com>> wrote:
>
> Wow;
>
> Great project. I suggest documenting everything and permanently
> posting for others to review the system design as a reference.
>
> 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.
>
> Here is an example, picture two sites x1 and x2 spaced exactly 13
> miles apart.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> There are a couple very proprietary computer programs for modeling
> this, however it can be worked out by hand.
>
> Some thoughts.
>
> 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.
>
> 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.
>
> 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.
>
> 4) The audio chain needs to be identical. Any filters, amps etc,
> in the transmitter audio chain must be same at each site.
>
> 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.
>
> 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.
>
> I hope this helps!
>
>
> On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
>> Hi All,
>> Thanks for all on the list who have given me a hand with getting
>> the RTCM/voter boards running.
>>
>> I currently have 1/3 of my system installed at the sites. It's as
>> follows:
>>
>> 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.
>>
>> 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.
>>
>> 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).
>>
>> 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.
>>
>> 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?
>>
>> Hayden
>>
>>
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>
> --
> Joe Leikhim
>
>
> Leikhim and Associates
>
> Communications Consultants
>
> Oviedo, Florida
>
> JLeikhim at Leikhim.com <mailto:JLeikhim at Leikhim.com>
>
> 407-982-0446 <tel:%28407%29%20982-0446>
>
> WWW.LEIKHIM.COM <http://WWW.LEIKHIM.COM>
>
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> --
> Tim
>
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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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