As we have already seen, there are various kinds of frame formats [Video formats ] depending on the country and the contexts of use. It is very important to have a good knowledge of how the different formats work since it isn't uncommon to have to undertake format conversions in this job. The reference value for frames is fps - frames per second: the number of frames each second.
24 fps: this is the standard format used in films. The number of photo-frames per second, namely 24, was considered adequate for creating a motion picture exploiting the permanence time of images on the eye's retina.
25 fps: is the European standard ratified by the EBU (European Broadcasting Union). It has been standardized both for black and white and colour TV. This value has come about because European alternate current has a frequency of 50 Hz. Each image is drawn on the screen line by line, in particular if we are on the nth photo-frame, the odd lines will be taken from the latter, whereas the even lines shall be taken from the following photo-frame (n+1). This way at each current cycle a photo-frame is shown (in the first semi-cycle for the odd lines, and in the second semi-cycle for the even ones).
The most widespread image encoding system in Europe is the PAL system (Phase Alternating Line). Unfortunately a different standard is used in the USA, which means that the format has to be converted from one to the other depending on the context of use.
30 fps: this is the standard American NTSC (National Television Standards Committee) for black and white television. This is because in the USA alternate current has a 60 Hz frequency. It is also called "non-drop" mode, and we'll find out why in a moments.
29.97 fps: It is called Drop Frame time code and is the standard format for American colour television. The reason for such an inconvenient number is that a 30 fps frequency for colour images gave some aliasing problems with certain colours. However, by slightly slowing down the photo-frames you got a correct colour reproduction. To create a time code with this format you'd use a 30 fps mode format in which 2 frames are literally dropped each minute, hence its name "Drop" Frame time code. This ensures that after one hour the frame-rate will be exactly 29.97fps.