the invention relates to a device for coding video frame sequences , comprising means for measuring the spatial activity of the video frames to be coded , means for measuring the temporal activity of the video frames to be coded , means for coding the frames as a frame structure or as a field structure . according to the invention , the means for coding the frames as a frame structure or as a field structure are able to choose the field or frame structure dynamically as a function of the measurements of spatial and temporal activity of the said frame .