RTCM Simulcast question
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project. Regards, KC2IRV -Joe <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=icon> Virus-free. www.avast.com <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=link> <#DDB4FAA8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
Hi Joe, I can answer part of that for you. You do need a GPS with NMEA or TSIP. The RTCM needs to know the time so it can time stamp the UDP packets when sending them to the server. If you go to http://svn.ohnosec.org/viewvc/projects/allstar/voter/?root=svn and look at the VOTER Protocol.pdf and the votersystem.pdf, and look at the protocol parts, you may answer some of your questions. I know the required NMEA sentences are out there somewhere, but I don't recall where I saw them. You can adjust the launch delay on each RTCM, but I do not believe it does any sort of auto adjust. Lee On Sat, Jun 4, 2016 at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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While I don't have any hands on experience simulcasting with RTCMs, I do understand something that is not really documented, only to be found in the depths of the email posts. If you want reliable simulcast with an acceptable level of overlap warble, you MUST inject a GPS-disciplined 9.6MHz signal to the clock of the RTCMs in leu of their stock internal crystal. Otherwise, the inherent internal clock jitter of the RTCMs will cause CTCSS warble, etc in areas where your transmitters overlap. The 1PPS feed that you normally feed the RTCMS is acceptable for voting, but naturally, a typical crystal-driven clock will slightly drift in between the pulses every second due to inherent jitter, etc, and that ain't to good for situations like simulcast which demand sample-accurate performance. If I have misspoken about anything, I apologize and relish the opportunity to be re-educated. 73 SS On Sat, Jun 4, 2016 at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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You need both 1 pps and Rx GPS data to the RTCM Cheers, Jesse
On Jun 4, 2016, at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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From my understanding as well as many others, simulcasting WON'T work
Jesse, properly in overlap areas unless you replace the RTCM's internal 9.6MHz crystal with a GPSDO 9.6MHz @3.3 volts p-p. If you expect a cheap 9.6MHz crystal to derive a 1PPS signal and stabilize it between clock pulses down to a level acceptable for accurate syncing of CTCSS generation, free of intrinsic jitter, you are "optimistic" to say the least. Jim Dixon has articulated this before on the email list. Simulcasting with the RTCMS in overlap areas WON'T WORK PROPERLY without this trivial modification. Obviously this requires usage of IDENTICAL equipment at each site. Here is a pointer to an email of his. *http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html <http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html>* Jim described a possible future RTCM design with convenient provisions for plugging in external 9.6MHz, but this has not happened. 73 On Sun, Jun 5, 2016 at 1:04 PM, Jesse Lloyd <ve7lyd@gmail.com> wrote:
You need both 1 pps and Rx GPS data to the RTCM
Cheers, Jesse
On Jun 4, 2016, at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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Just throwing this out there... wouldn't it be possible (trivial?) to configure the PIC (firmware) for 10MHz external clock, and replace the 9.6MHz crystal with 10MHz in from a GPSDO? Lee On Sun, Jun 5, 2016 at 1:33 PM, Sam Skolfield <kj6qfs@gmail.com> wrote:
Jesse,
From my understanding as well as many others, simulcasting WON'T work properly in overlap areas unless you replace the RTCM's internal 9.6MHz crystal with a GPSDO 9.6MHz @3.3 volts p-p. If you expect a cheap 9.6MHz crystal to derive a 1PPS signal and stabilize it between clock pulses down to a level acceptable for accurate syncing of CTCSS generation, free of intrinsic jitter, you are "optimistic" to say the least.
Jim Dixon has articulated this before on the email list. Simulcasting with the RTCMS in overlap areas WON'T WORK PROPERLY without this trivial modification. Obviously this requires usage of IDENTICAL equipment at each site. Here is a pointer to an email of his.
*http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html <http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html>* Jim described a possible future RTCM design with convenient provisions for plugging in external 9.6MHz, but this has not happened.
73
On Sun, Jun 5, 2016 at 1:04 PM, Jesse Lloyd <ve7lyd@gmail.com> wrote:
You need both 1 pps and Rx GPS data to the RTCM
Cheers, Jesse
On Jun 4, 2016, at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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Here's another link to some more info in an email thread. http://ohnosec.org/pipermail/app_rpt-users/2015-December/013155.html On Sun, Jun 5, 2016 at 10:22 PM, Lee Woldanski <ve7fet@tparc.org> wrote:
Just throwing this out there... wouldn't it be possible (trivial?) to configure the PIC (firmware) for 10MHz external clock, and replace the 9.6MHz crystal with 10MHz in from a GPSDO?
Lee
On Sun, Jun 5, 2016 at 1:33 PM, Sam Skolfield <kj6qfs@gmail.com> wrote:
Jesse,
From my understanding as well as many others, simulcasting WON'T work properly in overlap areas unless you replace the RTCM's internal 9.6MHz crystal with a GPSDO 9.6MHz @3.3 volts p-p. If you expect a cheap 9.6MHz crystal to derive a 1PPS signal and stabilize it between clock pulses down to a level acceptable for accurate syncing of CTCSS generation, free of intrinsic jitter, you are "optimistic" to say the least.
Jim Dixon has articulated this before on the email list. Simulcasting with the RTCMS in overlap areas WON'T WORK PROPERLY without this trivial modification. Obviously this requires usage of IDENTICAL equipment at each site. Here is a pointer to an email of his.
*http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html <http://ohnosec.org/pipermail/app_rpt-users/2013-November/007704.html>* Jim described a possible future RTCM design with convenient provisions for plugging in external 9.6MHz, but this has not happened.
73
On Sun, Jun 5, 2016 at 1:04 PM, Jesse Lloyd <ve7lyd@gmail.com> wrote:
You need both 1 pps and Rx GPS data to the RTCM
Cheers, Jesse
On Jun 4, 2016, at 6:03 PM, Joe Moskalski <kc2irv@gmail.com> wrote:
I will be setting up a 2 site UHF simulcast system in the coming months and so far I have only one question to ask. The RTCM manual is a little vague on the GPS part of the setup. Normally, with other types of radio simulcast products (which I am very familiar with several) all you need to properly time the audio is a 1 pps signal from the GPS reference. The RTCM manual makes mention of the NMEA/TSIP data being connected but does not explicitly say if that serial data is needed or if only the 1 pps is needed for simulcast operation. From the other simulcast products I have worked with, you set your bulk delay to a value larger then the delay required for the furthest site and the system will adjust. Does the RTCM version of simulcast work the same way? I just want to be sure because this will determine what type of GPS locked reference oscillator I choose for the project.
Regards, KC2IRV -Joe
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participants (4)
-
Jesse Lloyd -
Joe Moskalski -
Lee Woldanski -
Sam Skolfield