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Frame Rates

It's amazing how heated arguments can become on rec.games.video.nintendo when posters started talking about frame rates. People argue about what 'frame rate' means, what is a good or bad frame rate, how high a frame rate must be so that one can't see any flicker, details of different TV standards, what fields means, etc. I partly stopped posting on rgvn because of such arguments; correcting posters' mistakes never seemed to reduce the amount of nonsense being posted - it just became too annoying after a while. So, if you see someone on rgvn who hasn't a clue what they're talking about, then point them here.

My personal opinion on desirable frame rate is simple: as high as possible please. :)

In the real world, people vary wildly in terms of how low a frame rate can be before it becomes irritating. For me, I find a rate as low as 20 somewhat irritating, but then I'm used to 72Hz non-interlaced monitors and thus probably more prone to notice lower frame rates. There is no perfect frame rate - everyone's different - the only sure thing is that the higher, the better.

(note: fps = frames per second)

I've seen many arguments about frame rates; often, people will quote some very strange numbers, such as 7 or 27. To explain further, here's an extract from an email I received from an SGI engineer:

"Now, in double buffered mode you have to wait for the vertical retrace events of the monitor. This means that on a 72Hz screen your max fps is 72. If you can draw your scene in the time between two retraces, you get that rate. Now, what happens if you *always* miss the retrace by a microsecond? How many fps will you get? 71? No. You get 36, because you always need 2 retrace periods. This is the quantization effect. It is especially visible if the dividing factor is small (1-4). If you need much more time to draw a frame, you will see fps rates that are non-integral fractions of your monitor's refresh rate. If you are really interested in the time your frame needs to be drawn, you want to run in single buffered mode.

Our high end vis sim people (Onyx division) have a slogan: "60Hz, 30 hurts"

Performer's main task is to do all the dirty tricks to keep the redraw time (incl buffer clears) of frames below 16 msec. 17 msec drops the refresh rate to 30Hz, which is "unacceptable" for real time simulations (so people say).

If the above isn't clear enough, then I have an alternative article that goes into further detail giving examples, etc.

The following is a copy of an excellent post by Micheal Cranford of Intel Corportation on the subject of frame rates, flicker fusion rates, fields, etc. It answers most of the questions that people have on the subject.

Subject: Flicker Fusion Rates, etc. (was Re: QUAKE poorly done or demanding?)
Date: 2 Oct 1996 05:55:47 GMT
Organization: Intel Corp., Hillsboro, Oregon
Message-ID: <52t053$f33@news.jf.intel.com>
NNTP-Posting-Host: pdx289.intel.com
Summary: more than you ever wanted to know about this topic
Keywords: NTSC, PAL, TV, film, flicker fusion frequency

Chris Clarke wrote:

>jmp@tdk.dk (Jan Magdal Panduro) wrote:

>>But tv's don't run at 25 fps, they don't use frames at all! PAL tv's >>run at 50Hz "interlaced", and that's a good deal more "smooth" than >>25Hz non-interlaced. In fact, as long as the animation frames sync. >>with the screen update, the animation will be percieved as smooth. >>Film in the cinema don't flicker at 24 fps, because the images are >>shown as whole frames, they are not generated "line-by-line" as tv >>images are. On the other hand, you can se some jerkyness in panning >>scenes. > >>Do you feel framed? > >One half of a redraw on a TV is called a Frame. > >So, (PAL) TVs run at 25 FPS.

The above quoted statements (and most of the previous uninformed commentary) are complete nonsense.

Point by point rebuttal:

"But tv's don't run at 25 fps, they don't use frames at all!"

PAL TVs do run at 25Hz frame rates and TVs *do* use frames. See just about any book covering the technical aspects of television.

"Film in the cinema don't flicker at 24 fps, because the images are shown as whole frames, they are not generated "line-by-line" as tv images are."

Film in the cinema does flicker at 50Hz (Europe) or 48Hz (US) because the image is blanked while the film is moved and the current frame is replaced with the next frame. Each frame is shown twice or it would flicker at 25Hz and 24Hz respectively. See a book covering the technical aspects of film projectors.

"One half of a redraw on a TV is called a Frame."

One half of the "redraw" time is called a Field and the whole "redraw" time is called a *frame*. See a book covering the technical aspects of television.

"So, (PAL) TVs run at 25 FPS."

While this statement is correct it is also a non-sequitur since it followed "One half of a redraw on a TV is called a Frame" (i.e. the conclusion does not follow since one half of the time would be twice the frequency - 50Hz, not 25Hz).

In standard interlaced TV each Field displays one-half of a Frame by only scanning alternate lines. Thus the odd Fields display the odd numbered lines and the even Fields display the even numbered lines making up a full Frame of interlaced video. The real purpose of the interlace is to reduce the required transmission bandwidth and the downside to this scheme is at least two-fold (there are other disadvantages that I won't go into here).

  1. If the video image lacks sufficient redundancy (i.e. adjacent lines are not essentially identical) the resulting image will have large amounts of Frame-rate flicker (as opposed to field-rate flicker which is at 2X the frame-rate flicker). You can frequently see this if (for example) horizontal features are captured by a single field of the video camera (window blinds, car grills, etc). These will be displayed in only one field and produce unmistakable flicker.

  2. Even with perfect redundancy (i.e. identical line pairs) any horizontal motion will result in "spearing" (ragged edges) caused by the temporal displacement of the two Fields. You will notice this when the camera pans horizontally across high-contrast straight vertical features.

The US standard for interlaced Black and White TV uses a 60Hz Field rate and a 30Hz Frame rate and Color TV uses a 59.94Hz Field rate and a 29.97Hz Frame rate (the latter values are actually 1000/1001 times the Black and White rates) and the corresponding numbers for PAL TV are 50 and 25Hz respectively.

Computer monitors today are generally non-interlaced (i.e. progressive scan) to reduce the artifacts present in interlaced video. Obviously, the bandwidth of computer displays is far less restrictive than broadcast TV so higher data- rates are very practical. The VESA standard for computer displays "specifies" progressive scans at Frame rates of at least 72-75Hz (at a minimum) to reduce the flicker to acceptable essentially undetectable levels. At high brightness levels most people will perceive a 60Hz display as flickering quite badly in their peripheral vision.

Film in the US is displayed at a 24Hz frame-rate (i.e. each Frame of film is displayed for about 1/24 seconds). To reduce the flicker to acceptable levels each Frame is actually displayed twice (i.e. double-shuttered) which raises the flicker frequency to 48Hz. European film is displayed at a 25Hz frame-rate and is also double shuttered to reach a 50Hz flicker frequency.

The Flicker Fusion Rate (the frequency where flicker becomes undetectable) of the human visual system is a function of retinal field-of-view, brightness and color. This is also sometimes referred to as the Critical Flicker Frequency.

Since human peripheral vision is more sensitive to flicker, sitting closer to a monitor will increase the field-of-view and the resulting Flicker Fusion Rate will increase making the amount of perceived flicker also increase. You can see this effect by changing the viewing distance from a 60Hz computer monitor with a bright white background. Looking to one side of the monitor will also expose your peripheral vision to the display, increasing the perceived flicker.

Increased brightness also increases the Flicker Fusion Rate and this is why PAL TVs (with 25Hz Frame rates) must operate at lower brightness levels than NTSC (US) TVs (with 30Hz Frame rates) to produce acceptable levels of flicker. You can see this effect by operating a computer monitor at a 60Hz Frame rate (for example) while increasing and decreasing the brightness level of a solid white background image.

Displaying small objects at very low brightness levels produces undetectable flicker levels even when using sub-10Hz frame rates and this effect was commonly used with calligraphic DVST (Direct View Storage Tube) terminals nearly two decades ago. These vector displays showed very thin lines that were very dim at oftentimes very low update rates. Theatures are able to use quite low frame rates (24Hz to 25Hz) because the images are so dim. Have you noticed how blindingly bright it is outside when you leave a movie theature in the daytime? - your eyes have accommodated to the low luminance levels while watching the film.

To watch movies (i.e. film) on television a device known as a telecine must be used to convert between the differing frame rates. In the US a 3:2 pull-down is used (24Hz to 60Hz) and in Europe double shuttering is used (25Hz to 50Hz). In 3:2 pull-down every-other-frame is shown 3 times and the alternate frames are shown 2 times. US films are shown in European theatures by simply speeding up the projectors by about 4% (24Hz to 25Hz frame rates). The slightly speeded-up motion and increased sound pitch are usually accepted.

At very low brightness levels the color Flicker Fusion Rates diverge with red requiring the lowest refresh rate and blue requiring the highest refresh rate.

Recommended reading:

There are over a hundred other books covering the above topics in sufficient detail to destroy uninformed claims. You could also contact a video engineer or visual perception scientist.

Micheal Cranford, Resident Skeptic Intel P6 Architecture Labs

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