Increasing the number of frames printed on a film to enable films shot at silent speeds to be projected at sound speed and retain the original temporal characteristics.
Silent films were shot at a range of frames per second (fps) from 12 to 18 fps. By double printing every second frame, or by combining two frames with half the exposure for each, new frames can be added to a film shot at 18 fps so that when the film is projected at 24 fps the movement appears natural rather than sped up and jerky.
In audio, the tape deformed by stretching is usually elongated, narrowed and cupped.