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The word 'Scanline' has a slightly different meaning these days to the one I'm used to
I always took the word 'Scanline' to describe 1 physical horizontal display line on a monitor,
more often these days, when someone talks about a 'Scanline' they are referring to the
'Horizontal Retrace Interval',
or (in the case of emulators) faking a low scanrate horizontal retrace interval on a high scanrate monitor

To show you what I mean, here's a simplified diagram of how an image is created on any raster monitor

Simplified scan diagram
  In the above diagram the black lines show the 'active' part of the display
(i.e. when the electron guns are on)
and the grey lines show when the electron guns are inactive and sweeping back
in readiness to display the next line of the image
(The grey lines represent the Horizontal Retrace Interval or Horizontal Blanking)

The most important thing to remember is that the speed of the electron guns sweeping across the display
are governed by the horizontal scanrate
Therefore, if the horizontal scanrate is high (as in SVGA/VGA PC monitors)
the horizontal retrace interval is small and the physical gap between one active display line
and the next is also small
If , however, the horizontal refresh rate is low (as in standard resolution arcade monitors)
the physical gap between one active display line and the next is larger

This is where the arcade emulator use of the term 'Scanline' comes into play
The emulator is attempting to create a display with a long horizontal retrace interval
which results in a noticable physcial gap between one active display line and the next

(DOS) MAME running on a PC monitor attempts to do this in two ways...
   .by using tweaked VGA modes with double the normal vertical refresh rate, but half the lines
   .by using SVGA modes which draw a blank line every other display line

The first of the above is the most successful in replicating a low scanrate arcade monitor
The second falls far short - and is also the only option available to all Windows based emulators

A tweaked VGA 'Scanline' mode is a closer emulation than a SVGA odd/even line 'Scanline' mode
but still does not look like an original low res. arcade monitor

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