A decoder for video signals, such as MPEG, which uses motion-compensated bidirectional predictive coding, performs concealment of lost or corrupted portions of a picture. For this purpose, it estimates missing motion vectors by combining the two vectors which accompany a bidirectionally coded frame to create a substitute vector. An encoder can be modified to enhance this decoder operation, including forcing at least one frame per group of frames to be coded using bidirectional prediction, and constraining the two vectors so that the substitute vector is closer to the wanted value.