Television signals contain a great deal of frame-to-frame redundancy because picture areas are scanned in every frame whether they have changed or not. That portion of the signal describing stationary images need not be retransmitted in every frame if adequate memory is provided at the receiver. The signal describing moving images must be transmitted, of course, but it requires progressively less fidelity as the motion increases. Several techniques are described for obtaining efficient transmission by taking into account the similarity in the signal from frame to frame. Basic to most of these techniques is the need to separate the signal into segments that have changed significantly since the previous frame and ones that have not changed.