[App_rpt-users] Simulcasting with RTCM/voter

Tue Mar 14 02:49:46 UTC 2017

Hayden; How are you generating CTCSS tone?

On 3/13/2017 9:41 PM, Joe Leikhim 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
>
> 407-982-0446
>
> WWW.LEIKHIM.COM
>
>
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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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