The engineers who developed television had a problem: how to deliver massive amounts of video and audio over the air waves, yet have it able to be received and acted upon by affordable electronics. One of the many decisions they made was to halve the amount of data by adopting interlacing. This halved the bandwidth required for the signal. If the original picture source is a movie, it is displayed in the cinema at a rate of 24 frames per second. With PAL TV systems, the pictures are shown 50 times per second. What happens is that the movie is scanned into 576 horizontal line, but only every odd-numbered line is sent in the first 1/50th of a second. In the next 1/50th of a second, the even-numbered lines are sent. The TV shows first the odd lines, then the even lines (which are shown in between the fading remnants of the odd lines). The whole process of showing one frame takes 1/25th of a second (that is, a four per cent shorter time than the original movie's, so PAL movies have a slightly shorter run-time than when in seen in the cinema). Each half-picture shown in 1/50th of a second is called a field. Material sourced directly from video cameras is already broken up into fields before being recorded. The whole thing is far more complicated for NTSC TV. Consumer videotapes and DVDs conform to this scheme.