Tips for Repointing to AMC-18

3/16/2017 10:38:00 AM
Thanks for publishing the article on March 15 relating the experiences of an affiliate with re-pointing their downlink to the new AMC-18, used by commercial radio networks. This project could always use more publicity.
 
It is definitely worth a shot for affiliates to try re-pointing smaller downlinks, generally in the 2.8 to 3.2 meter range, to the new bird. It might just work, but affiliates need to be aware: 
  • Interference from adjacent satellites, 2 degrees away, and from cross-polarized signals on the same satellite, can add up to degrade performance.
  • The bigger the dish, the deeper the “null” you can achieve to avoid cross-polarized signals.
  • The DSP decoder chips in most modern receivers have an error concealment feature. If one frame of audio (about 1/40 of a second) is corrupted, the chip will repeat the last 1/40 second worth of audio to “cover up” the bad data. But if two or more audio frames are bad, you’ll hear the dropout as silence, a glitch, or a squeal. So if you hear one or two dropouts, there were probably 10 or 20 more that were concealed.
If you don’t hear any glitches or squeals on the day you repoint your dish, that’s fine, but you’ve only tested the dish with the interference that was in effect on that day. The adjacent satellites and the cross-polarized transponders on AMC-18 have dozens of other customers on them who come and go, and may increase their bandwidth or power at the drop of a hat. We in the radio industry will get no warning of these changes that could increase interference.  
 
If you elect to stay with a smaller 2.8 to 3.2 meter dish on AMC-18, be aware that there may come a day, possibly months or years from now, when a new source of interference clobbers a network signal that you want to receive. And you won’t be able to do anything about it other than ordering a larger dish, which could take you a month or two to install.
 
Since each radio network operator is on a different frequency, some networks may have interference while others might not. If Dave Ramsey sounds great but Rush Limbaugh has 50 dropouts per hour, you could start a whole new conspiracy theory!  
 
Plus, listeners will abandon your dropout-filled station to go listen to your cross-town competitor who has a 3.7 meter downlink!
 
Downlinks of 3.7 meters or bigger buy you an operational safety margin and peace of mind.
 
While the internet backup option in satellite receivers is viable, be aware that it takes 10 to 30 seconds for the web stream to buffer enough audio to produce a usable output for your listeners.  Short-lived outages of the satellite RF are not served well by backup streaming.
 
Monti is senior vice president of engineering for Westwood One in Purchase, N.Y.
 

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