unknown function, possibly a video encoder as a similar chip Bt856KPJ is found on the [[Apple Pippin]] with "VIDEO ENCODER" on it. Some info online indicates Bt8069B is a Line Interface Unit handling the RX/TX for T1/E1 protocol.
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Brooktree Bt8069-Series Line Interface Unit (LIU). A line interface unit (LIU) is a semiconductor device that implements the T1 or E1 protocol in hardwired silicon.