Steven J. Crowley, P.E.
During the inauguration there were press reports of Jumbotron problems — pixelated video and distorted audio — at the Washington Monument.
Yesterday the Washington Post reported that the problems could have been due to “interference with a microwave signal” that brings video and audio to the Jumbotrons.
CP Communications, the company charged with delivering the signals to 12 viewing screens on the Mall, said only the unit near the Washington Monument experienced problems. Unfortunately those problems occurred shortly before President Obama took the oath of office, triggering rounds of boos and groans by viewers camped out to watch the festivities.
Tom Sharkoski, CP’s engineering manager for the Mall event and a general manager in its Philadelphia area office, said the screen’s malfunctioning video and audio was not the only problem. Crews in the same area also found themselves unable to communicate on their walkie-talkies during the breakdown, Sharkoski said.
Sharkoski said company officials may never be certain what caused the problem or identify a source of the interference. The company had a dedicated channel for the feed and did not discover any equipment failure, Sharkoski said. “We have not been able to pin it down on what caused the trouble,” he said.
Without surveying the Washington Monument grounds with a spectrum analyzer, which might get one pulled into a black unmarked van and driven away, let’s see what we know.
According to authorization the FCC granted CP, it was to operate on 2200-2290 MHz. I can’t tell exactly where the transmitter was, but it presume it was near the Capitol, and the Jumbotrons down the National Mall had directional antennas pointed at the Capitol.
The emission designator is 8M00D7W. This means they were using about 8 MHz of bandwidth. The D7w code, in this context, I interpret as meaning this was a single digital transmission in which the audio and video were combined. Looking further at an exhibit accompanying the application, we see that COFDM was to be used and that two 8MHz channels were required, with a minimum channel center spacing of 10MHz. Theoretically, CP could have used one channel for video and one for sound, but that would seem to only complicate things, and it would be inconsistent with the D7W designator. I suppose two channels were used for redundancy. Effective radiated power is one watt, which seems on the low side on its face, but it’s a short path, probably line-of-sight, and I can see the numbers making sense depending the the equipment used.
One odd thing I found is this note made by FCC staff and placed in this application’s file:
CP Communications must notch out the following; 2214 MHz thru 2216 MHz 2284.5 MHz thru 2285.5 MHz
In other words, CP was to not transmit on those two narrow bands. I’ve never seen this type of condition before for an application in this band. I presume the bands are used by some type of federal radio system to be protected. The 2200-2290 MHz band is allocated primarily for federal use. It’s used by NASA and the military for satellite communications. It’s also used for telemetry when flight testing aircraft and rockets. Why did the FCC tell CP to avoid those two bands during the inauguration? Did the communications in those bands somehow interfere with CP? Why wasn’t this condition made part of the authorization? Did CP get the message? If not, it might have operated on those frequencies not knowing there were other, possibly interfering, transmissions in their band that day.
There’s also the possibility that this was an equipment problem unrelated to interference. (CP’s role seems limited to providing the radio links, not operating the Jumbotrons.)
I don’t know what to make of the report of walkie-talkie interference, as we don’t know which frequencies they were on, but it’s an odd coincidence.
CP does a lot of these video feeds on an ongoing basis in this band, and in other bands, and I’ve never heard of any similar interference problem. That’s because, in most places, you don’t have much space communications or flight testing going on. I expect CP will be back in four years with a more redundant system, and the crowd will experience clear video and sound.