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February 14, 2000

j.ello | by the byte

MPEG Video, part 1

     Ok, we're talking video this week... MPEG video.  If you've watched a DVD movie... if you have DirectTV or another satellite provider... or if you've downloaded video on the net, you've certainly dealt with some version of MPEG.  But, before you can be impressed by MPEG, you've got to know a little bit about video really is.
     Consider the typical stream of video that comes into your TV.  On U.S. television, we see half a frame, 60 times per second.  That means there are 30 full frames, or individual images shown, per second.  (In European countries, and in movie theaters, the frame rate is 24 frames per second)  If we were to save U.S. TV video images frame by frame, to an uncompressed file... it would take about 27 MB/s (Megabytes per second) of video.  A 2 hour movie recorded that way would be a 194 Gigabyte file!  So, the natural question is how can we possibly squeeze a full length movie into only a few Gigs on a DVD?  Moreover, how can we possibly transmit VHS-quality video on the web to a modem?
     Well, basically there's a lot of information recorded on film and sent to your TV that you can't ever see or hear.  Once you trim the fat, and rewrite the good information in "digital shorthand"... you find that you can save a LOT of space and you really haven't lost much of the stuff you wanted in the first place.
     That's what the MPEG standards are all about.  MPEG stands for the Moving Picture Coding Experts Group.  The technology they apply to moving pictures is much the same as the technology applied to JPEG (Joint Picture Coding Experts Group) images that you see all the time on the web.  There is, in fact, a format called MJPEG, which is a Motion JPEG where each frame of a video is recorded in JPEG format and played like a flipbook, not unlike an animated .GIF which we see all the time on the web.  This is an effective form of compression, but as I'll show you, there are also ways of throwing out data over a block of time in a video.  This means that in an MPEG... each frame is dependant on the frames before it and the frames after it in order to achieve maximum quality and compression. 
     One common misconception is that each MPEG standard makes the earlier one obsolete.  In fact, each MPEG standard is designed for a special purpose.  

Here is a list of MPEG standards and a basic description:

• MPEG-1 was designed with for low bandwidth video of up to 1.5 Megabits/s ( about 200 KB/s).  This was approved Nov. 1992.

• MPEG-2 is the digital television standard.  It is similar to MPEG-1 but allows for higher data rates and better image and audio quality.  Approved Nov. 1994.
  *MPEG-3 are the HDTV standards which were absorbed into MPEG-2.  The MPEG-3 title was retired.

• MPEG-4 versions 1 and 2 are standards for multimedia applications & very low bandwidth video (for things like videoconferencing.. etc.). Approved Oct. 1998 and Dec. 1999.

Currently in development are

• MPEG-4 versions 3 and 4.

• MPEG-7 standards for multimedia search, filtering, management and processing.  To be approved in 2001.

• MPEG-21 the multimedia framework.

     We're gonna pigeon hole MPEG-4 for now.  As it turns out, there is a logical progression from a JPEG image, to MPEG-1, to MPEG-2 that gives us the base of all digital video available today.  Now, it took a whole lot of really smart people many years to figure out how to do this stuff, and the mathematic details are mind-boggling... but we're gonna cover all the basics this week. :).

     -j.ello

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