PictureTel Corp, the Andover, Massachusetts company that is big in videoconferencing systems and software, is doing a bit of back-filling, expanding into what gives every appearance of being very pedestrian technology compared with its own. It is buying multipoint audioconferencing systems specialist MultiLink Inc – a neighbor in Andover, in a share swap valued at about $40m. PictureTel expects the transaction to be earnings-dilutive in 1997 because of one-time expenses but sees the buy enhancing its results in 1998. MultiLink shareholders will end up with about 10% of the of the enlarged equity after the takeover.