PC Video


PC Video stores video as "clips" using files.  The files will typically have the extensions of AVI (Audio Video Interleave), or MPG (MPEG - Motion Pictures Experts Group), or RM (Real Media).


The monitor and delivery system is much like television - it scans lines across the screen, left to right, top to bottom.  However there is a big difference  .  .  . computer monitors do not scan using "fields" - they scan an entire frame, from top to bottom, and do not skip every other line the way interlaced TV screens do.  This is called "Progressive Scanning".  The picture is still refreshed with 30 frames per sec - but each frame is scanned with a single field :


TV uses analog waveforms, and scans flowing colors across the screen in lines.  The PC scans lines of "dots", or pixels, across the screen.  But to the eye they are the same.


Television is strictly defined by the NTSC.  It uses an exact frame rate of 29.97 fps, which for discussion purposes is rounded off to 30 fps.  The "resolution" of the picture is described as 525 scan lines, of which 486 are visible - again for discussion purposes this is often rounded off to 480.  The scan lines are horizontal, but the number of scan lines describes the vertical height (vertical resolution) of the picture.  TV does not use vertical scan lines, so the horizontal resolution is not really defined.  Since there are two interlaced fields per frame - there are 60 fields per sec.


So the resolution of a television is rated at 525 lines.  The following diagram shows the lines for field 1.  Then once field 2 is scanned all the lines of one image (one frame) have been painted onto the phosphor.  Although you only see 486 lines, the resolution is still stated as being 525 lines (486 visible, 25 vertical retrace) 


(diagram needs to be fixed)


Unlike TV  -  the PC can store video in virtually any size, and at any frame rate.  It does not use "scan lines" to define the resolution.  Since video takes a great deal of bandwidth, most PC videos are shown in small, rectangular boxes.  The most common display size is 320x240 pixels (width x height), which is an aspect ratio of 4:3