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How Video Formatting Works


Frame Rate Conversion

The easiest way to convert a film to video would be to project the film and shoot it with a video camera. This would give you a copy of the movie on video tape (this is how people make illegal video copies of movies that are only playing in theaters), but the image would have a constant flicker to it. This is because film and video have a different frame rate, meaning they show a different number of still images per second.

Most feature films are projected at a rate of 24 frames per second. This means that in one second, the projector shows 24 complete still pictures in succession. This is essentially the entire process of creating the illusion of movement with film. Video formats were designed to be used on cathode ray tube televisions, which work in a completely different way than a film projector. If you've read How Television Works, then you know that a television creates still pictures line by line, with an electron beam that passes over a phosphor-coated screen, in rows from left to right, top to bottom. When television was first developed, it wasn't feasible to create a system that could "paint" all the lines in one pass over the screen, so the cathode ray tube system was modified to paint every other line in one pass and then fill in the lines in between in a second pass. This process is called interlacing, and each complete pass of the electron gun is called a field. Technology has improved to the point where we don't have to build televisions this way, but much of the rest of television broadcasting equipment has been designed around this idea, so it is fairly entrenched for the time being.

The video used in traditional television signals takes this particular form, but specific formats vary from country to country. There are three commonly used formats:

  • National Television Standards Committee (NTSC) format: Used in the United States, Canada, Japan and elsewhere
  • Phase Alternation by Line (PAL) format: Used in European countries and other parts of the world
  • Systeme Couleur Avec Memoire (SECAM) format: Mostly used in Eastern European countries

On a PAL or SECAM system, the electron beam passes over the entire screen 50 times a second, which means the television presents a complete picture 25 times a second. This is pretty close to 24 frames per second, so if you make a direct conversion showing one complete film frame in every full video frame, the movie plays pretty well, just a tiny bit faster. The main thing you might notice is that all the sound has a slightly higher pitch.

NTSC format shows about 30 frames per second (60 fields), so it is a bit trickier. Mathematically, you can't easily spread 24 frames across 30 frames. But you can divide up 60 fields so that you show only 24 frames per second, if you use a block of five video fields to show two film frames. The math is pretty simple:

  • In one second of video, there are 60 fields.
  • So, you show five fields in 1/12th of a second (60/12 = 5).
  • In one second of film, there are 24 frames.
  • So, in 1/12th of a second, you show two frames.
  • If you record only two film frames for every five fields of video, you can create a video copy of a movie that plays at the correct speed.

Of course, you can't divide five by two evenly, so formatters have to stagger it. A video copy of a movie shows frame 1 for three fields, frame 2 for two fields, frame 3 for three fields and so on. This doesn't present motion exactly as it appeared when the movie was projected (pans aren't as fluid, for example), but the movie isn't sped up at all and the soundtrack isn't affected.

But how do you split up a movie this way? This is done with a device called a telecine. There are two different types of telecine, film chains and flying spot scanners.

Film chains are the cheaper option, but they don't make as good a copy. With a film chain, you attach a special kind of shutter to a film projector and project the image through a lens and off of a mirror to a specialized video camera. The shutter is rigged so that it projects each frame for the appropriate amount of time.

The other type of telecine, the one used for higher quality video copies, does away with the projector and the shutter. Flying spot scanners run a little light on one side of the film and a little camera on the other side of the film, so they can scan each frame of film. Once a film frame is scanned, it's very easy to divide it up into different video fields.

This process works a little bit differently with DVD movies. DVDs store movies in MPEG digital format, which compresses the movie file by using the same image information from frame to frame. For a full description of how this works, check out How DVDs and DVD Players Work.