Owner's Manual for the vehicle. With a quick reference ... - E38.org

186nRadio reception
BMW Active Seat*
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AM provides substantially longer reception
ranges than FM. AM broadcast signals,
propagated at ground level in the
form of surface waves, are also reflected
from the ionosphere as atmospheric
waves.
Frequency-modulation (FM) provides
substantially better sound quality than
AM. However, because FM transmissions
rely on line-of-sight broadcast
waves, their effective reception range is
limited.
Although numerous factors combine to
impose inherent limitations on the reception
quality available from mobile radios,
specially designed systems can
be employed to minimize their effects:
With radio stations which broadcast
over multiple frequencies, the "Radio
Data System" (RDS) ensures that the
frequency with the best reception quality
will automatically be selected.
The Diversity Antenna system employs
several FM antennas integrated within
the rear window to provide three separate
sources for receiving broadcast
waves. An integral processor automatically
selects the antenna with the best
FM reception quality at any given time.
Because the ongoing antenna selection
process is completed within milliseconds,
it remains inaudible to the radio
listener.
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The Active Seat is an advanced development
in BMW seats. These seats are
engineered with orthopedic considerations
so that you will be less fatigued
during longer trips and sitting for long
periods without moving. The seat is no
longer a passive link between the road,
the running gear, and the driver. Instead,
by means of an active modification
to the seat's surface, the seat
causes weight transfers which are slight
and imperceptible. The basic seating
position is not changed by this.
In the seat's upholstery, beneath the
surface of the seat, there is a cushion of
fluid on the left and right. The fluid is
slowly circulated back and forth between
the cushions by means of a
pump.