Why are tv's running at 1080p 24p?

Answer 1

Interlaced television signals run at 25 or 30 frames per second with 50 or 60 fields per second. (A field is half of the complete frame so two fields generate a full frame). Interlaced signals are designated with an "i". 1080i, 480i etc.

Progressive signals normally run at 50 or 60 frames per second. Progressive signals are full frames without multiple fields. The designation is "p". 1080p, 720p etc.

Films use 24 frames per second. As each frame of the film captures the whole image, it is progressive so 24p identifies a progressive signal at 24 frames each second. Whenever a film is broadcast, it is changed to suit the broadcast format. However, DVDs and computer generated or stored content can retain the film's frame rate. To display 24p content, a television must be capable of handling that format. The 1080 refers to the number of lines making up the image but this could be 480 or 576 lines if the image is standard definition rather than high definition.

Although many televisions can handle 24p formats, not all of the do. Whenever a film is broadcast, it needs to be changed to a conventional format such as 1080i. There are two ways to do it. For European television, it needs to be converted to 25 frames per second. Generally, the film is run a few percent faster than the original speed and 24 frames per second becomes 25 frames per second. So, a 100 minute movie will be shown in 96 minutes. North American television runs at 30 frames per second and a technique known as 3-2 pull down is used. The first frame of the film is sampled and displayed over two television fields. The next film frame is sampled and displayed for three television fields. The process is repeated throughout the film transfer to video. The end result is that the film is converted to a television format and will last the same length of time as the original.