Video Frame rates


//// Mike Fulton <75300.1141@compuserve.com> shares his views on
     video frame rates:

[] The frame rate at which a game updates its screen doesn't
   necessarily correspond in a particular way to the television or
   monitor's frame rate.  In fact, ideally the game's screen refresh
   _should not_ be tied to the television's frame rate, because
   otherwise the game plays differently on different types of TV sets.

   And a TV really does do 60 frames per second, although a broadcast
   TV signal is only 30 frames per second.  Televisions use a method
   called interlacing to increase the apparent vertical resolution of
   the image.  The electron gun of a picture tube paints the display
   60 times per second, but it can only display about 262 lines of
   vertical resolution in each frame, which isn't really very good.  A
   broadcast TV signal wants to have about 525 lines of vertical
   resolution, so what they do is have the signal show only 30 frames
   per second, but each frame is broken down into two 'fields'.  One
   field contains all the odd-numbered scanlines for a frame, and the
   next field contains all the even-numbered scanlines.  Each field is
   displayed in one of the picture tube's 60hz frames, and the result
   is that the eye sees a higher resolution picture than you'd
   otherwise get.

   Interlacing is a tricky thing, however, and if you have significant
   differences in brightness or color between one scanline and the
   next in the same frame, you get a flickering effect on screen
   because the same part of the screen is changing back and forth 60
   times per second.  You don't notice this much in regular TV stuff
   because they use tricks like anti-aliasing to get around it.
   However you do see it in some instances, like in the overlay
   lettering for some mail or phone-order commercials.

   So while a regular broadcast TV signal is only 30 distinct images
   per second, each image is actually using 2 frames of the picture
   tube.  I don't know for sure if this works on a TV, but I suspect
   that if you don't interlace your signal this way, and settle for
   lower resolution, you can get the full 60 frames per second.

   I don't know what the actual frame rate of Wolfenstein 3D on the
   Jaguar is, but I do know they had to slow it down to keep you from
   running into the walls.  We got a new, higher-resolution version
   today and it's looking pretty good.  You also have to consider that
   the Jaguar can connect to higher quality monitors as well as
   televisions.


PROWLER The Atari Console Disk Magazine February 27, 1996

Copyright (c) 1996 All Rights Reserved Issue No. 01