To be precise, it is caused by PAL TV system, and NTSC (used in the United States and Japan). There is no such phenomenon in the system.
PAL system uses interlaced scanning to shoot transmission images, and PAL system shoots 25 frames per second, but directly shooting transmission will feel flicker (the lowest frequency of no flicker is 40 frames per second), so each image is divided into two frames, so the frame rate is increased to 50 to avoid flicker.
A field contains odd lines, such as 1, 3, 5, 7, etc. , called odd field; The other field contains even lines such as 2, 4, 6 and 8, which is called even field. But when shooting, we don't take a complete picture first and then split it (if there is no sawtooth). We take odd fields first, then even fields (so the minimum shutter of the camera is usually 1/50 seconds), and then combine them into a complete picture.
Because the two pictures are obtained successively, when shooting a moving picture, the edges of the objects in the two pictures do not overlap, which will produce sawtooth.
When TV plays back a video picture, it is played by field, that is, the odd field is played first, then the even field, and then the odd field and even field of the next frame are played, and there will be no sawtooth. When the computer plays, it plays by frame, that is, odd fields and even fields are played at the same time, so there will be sawtooth.