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The design and realization of filters in communication systems have been influenced by various technologies over the past years. Beyond the traditional LC-filter technology, a sophisticated crystal-filter and, more recently, a mechanical-filter technology have been developed. Each filter technology is applied in multiplex communication systems according to its inherent technical and economical advantages. With the economical acceptability of integrated circuits, two more filter technologies begin to compete with the established filter concepts. Active filters in thin-film realization show promising aspects for certain applications and for mass production. Digital-filter techniques seem to be, understood well enough that it is worth looking for their application in multiplex systems. The various filter techniques, new and old, are set into the perspective of their economical use in multiplex communication systems.