Ford 1998 Windstar Owner's Manual page 69

To minimize these conditions, a stereo/mono blend circuit has
been incorporated into this system. This feature automatically
switches a weak stereo signal to a clearer monaural signal,
which improves the quality of reception.
Several sources of static are normal conditions on AM
frequencies. These can be caused by power lines, electric fences,
traffic lights and thunderstorms.
Another reception phenomenon is Strong Signal Capture and
Overload. This can occur when listening to a weak station and
when passing another broadcast tower. The close station may
capture the more distant station, although the displayed
frequency does not change. While passing the tower, the station
may switch back and forth a few times before returning to the
original station.
When several broadcast towers are present (common in
metropolitan areas), several stations may overload the receiver,
resulting in considerable station changing, mixing and
distortion.
Automatic gain control circuitry for both AM and FM bands has
been incorporated into this system to reduce strong signal
capture and overload.
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