Thursday, April 19, 2007

Frame Rates Expained

Frame Rate:

The movies you watch are shot on cameras that record to either tape or film reel. These tapes or reels don't record any motion at all. In fact, what you are seeing is a series of still images put together and played back at various speeds to give the effect of the motion that was recorded.

Remember when you were younger, you'd get a Post-it pad and draw an little animated cartoon on it by changing the drawing on each page a little. When you flicked through the whole sequence of Post-its, you would have a simple animation. That's exactly how those cameras record things. Now lets say that you had exactly 60 Post-its, and you flicked through them in exactly one second. You would have 60 frames per second. The measurement use is simply "Frames per second" or "hertz." There are several things we need to consider in order to comprehend the importance of a "frame rate" on your TV.

  • The Frame rate the movie was filmed at, and the ability of the camera used during filming to process the frame rate well and accurately.
  • The ability of your DVD, Blu Ray or HD DVD player to "match" the frame rate of each specific movie source you watch.
  • The ability of your TV to display those various frame rates.

The images on a Film Reel are transfered to Video so that it can be broadcasted on TV, watched on a computer, authored to an HD DVD or Blu Ray disc etc. This process is called Telecine. Because film and video are so different, it's useful to know the various different frame rates, what they're used for, and how these differences affect the picture.

[Below left is actual film reel used from the movie "The Sixth Sense." Notice how there are very few differences between each frame. That's because 24 of those frames make up one second of playback and in that scene, he's actually being fairly still.]

24p frame rate is a progressive format. Most movies today are shot at this frame rate. It gives that classic "cinema" or "movie theatre" feel. 35mm movie cameras are now predominantly used in filming, and by nature they use 24fps for their standard exposure (the opening and closing of the lens, which results in recording movement).

25p is a video format which runs twenty-five progressive frames per second. This frame rate is derived from the PAL European television standard of 50i (interlaced). While 25p captures only half the motion that normal 50i PAL registers, it typically yields a higher image quality during fast motion scenes. Films broadcasted in Europe need to be converted from the NTSC 24fps to 25fps.

30p, or 30-frames progressive, produces video at 30 frames per second. Its progressive (non interlaced) nature attempts to recreate the film camera's cinematic like feel and eliminates artifacts that may be apparent in interlaced video with fast motion.

60i is what has been used for decades for over-the-air (OTA) TV broadcasts, home camcorders, and DVD's. 60i is really 30 Frames per second using the interlaced method, as at any one moment there is only ever 30 frames on screen. So really it is at 30fps or 29.97 to be exact.(They use 50i in Europe and Australia which equates to 25 frames per second, then the other 25 are filled in a split second later)

With regards to broadcasting, the World has been been using the following analog standards. These are all set to change due to the digital revolution.

  • NTSC (National Television System Committee), developed and used in the United States: 525-lines of resolution at 29.97fps
  • PAL (Phase Alternating Line): Used in most of Europe 625 lines at 25 fps
  • SECAM (SÉquentiel Couleur Á Mémoire, French for "sequential colour with memory"): Used in France, China, Russia, Belgium and other communist states that rejected the US standard during the Cold War. 625 lines at 25 fps

In the United States, that change is coming quickly due to the mandatory FCC regulation. The NTSC standard which has been running for over 60 years, will be replaced by the ATSC standard which is either "1080i at 30 fps" or "720p at 60fps." You'll find that broadcasters have a choice. ABC and FOX for example are using 720p while CBS and NBC use 1080i.

But there are some problems that are still inherent from these new standards. Let's say that you're watching a movie that was shot on Film at 24fps and transfer to the Blu Ray video format at 24fps. You're using a PS3 as a Blu Ray player. We encounter a problem. The PS3 outputs at 60fps, and does not currently support 24fps, although their is a rumored update that will soon allow this. So your TV and PS3 will use a variety of techniques to fill in the gaps. The Blu Ray disc outputs at 24fps, the PS3 accepts that signal and converts it to 60fps, and outputs the converted frame rate to your HDTV.

The problem with this is that 60, is not a multiple of 24. If the PS3 outputted at 48 or 72 fps, it wouldn't be a problem as they are multiples of 24 and thus makes it easier to fill in the additional frames. 24fps converted to 48fps for example would have 24 frames added to retain the feel of the original film. This is achieved through a process called "3:2 pulldown" where the hardware is "adding information" to make up for the discrepancies. But because we originally had an uneven number of frames, the result is possible "noise" and "irregularities" during playback. How well your hardware manages this is a measure of the products video processing attributes. Using our example, the PS3 does an outstanding job at this.

Going back to the broadcasted ATSC HD signals, we have the same problem. The TV stations will telecine the movie to their preferred resolution and frame rate and then they and the and Cable/Satellite companies will broadcast it to everyone. There is already the issue of compression. This is a technique used to shrink the HD signal, into more manageable data for transmission, while trying to retain picture and audio quality. There is also the issue that if the broadcast is 1080i and you have a 1080p signal, your TV will send that signal through a process called "Deinterlacing," or "Line Doubling" which again mean adding information which isn't really there. Depending on your equipment at home and those issued by the satellite/cable companies, the frame rate and interlaced/progressive differences will produce varied results.

A lof of the issues discussed here can be fixed by manufacturers allowing for the support of 24fps in their hardware. Often they can issue firmware upgrades on CD or via download which will add this type of functionality. The difference in frame rates between your source (BD or HD-DVD) and hardware can produce less than perfect results, while in other instances, the difference are not noticeable. The good news is that manufacturers are paying more attention to features of this kind, but all in all, it's another thing to look out for when making your purchase.