*Here's an addendum to the article I sent earlier today from a guy who still
works for WTTW*.
Well guys, it was certainly one of our darker days. This occurred when the
TV stations that were not downtown, WTTW and WGN being the larger of those.
Stations in the downtown area were able to take advantage of the AT&T
infrastructure in the area at the time, to pass signals from studios to
transmitter via closed circuit connections. WGN and WTTW required to send
signals via microwave from their studios to the transmitter as copper
connections were very expensive at our locations. These are known as STL
(Studio Transmitter link) connections. All STL and TSL (Transmitter to
Studio) link frequencies were commonly available in FCC published books as
these are required to be licensed operations. Both stations were using 4
GHz links at the time and were actually very close in the band. STL
transmitters were typically one to 5 five watt output. However, these links
required high gain and very directional dish antennas. The beamwidth was
very, very small (typically less than 3 degrees) and so the perpetrator
needed to be aligned with the receive antennas close to the studios of each
station. WGN broadcast from the Hancock Building while WTTW has been on
Sears Tower since 1975. If you examine a Chicago map you will see that WGN
and WTTW are nearly aligned with the Loop. A single antenna in a single
location could be pointed at each transmitter site with a little alignment
for each. The static at the beginning of each broadcast could have been the
offending signal being aligned with the transmitter site.
As pointed out, this was thought to be an "inside job" because the person or
persons would have needed to know certain technical specifications in order
to interrupt the signals. Technically, the roque transmitter would have to
be capable of supplying significantly higher signal level than the STL was
providing. This is possible with a larger antenna or a higher power
transmitter. A larger diameter antenna would have drawn some attention as
these were not a normal sight in 1987. The person would also have needed a
similar STL transmitter in order to modulate the needed signals and
subcarriers on frequency.
STL transmitters used an FM modulated baseband signal to pass signals.
Audio was DSB modulated at subcarriers for each channel. We were modulating
3 audio channels at the time, left, right and second audio language. The
channels are then mixed with the baseband video which passed through a low
pass filter that cutoff video above 4.2 MHz. That combined signal was then
FM modulated. The receiver has AGC and AFC to keep everything locked and at
low noise. An interfering signal would need to overcome the primary signal
and drive down the AGC so that the primary signal would not interfere. Even
if the offending signal was not exactly on frequency, AFC would have been
pulled to the right frequency. WGN had two separate STL links in different
bands at the time so they simply switched to the other frequency. We had no
choice but to shutdown the Ch.11 transmitter until the interference ended.
While we still use our STL microwave as a backup, broadcasters now use
copper connections in the form of video baseband or IP based interconnects.
In a small way, Max Headroom prompted broadcasters to move to secure
connections.
Al
Alan J. Skierkiewicz
Engineer In Charge, WTTW/WFMT
Maintenance Design
5400 N. St. Louis Ave.
Chicago, IL 60625
773-279-2463
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