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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