Newsgroups: comp.sys.amiga.advocacy
Subject: Re: Interlace video (Was Re: PC (OS/2) / Amiga debate)
From: lkoop@TIGGER.STCLOUD.MSUS.EDU (LaMonte Koop)
Date: 6 Sep 92 14:08:44 -0600
Reply-To: lkoop@TIGGER.STCLOUD.MSUS.EDU
 <1992Sep3.153516.42760@kuhub.cc.ukans.edu> <rtyy!k@rpi.edu>  
 <187t3iINNk58@matt.ksu.ksu.edu> <1992Sep4.234230.18567@mpifr-bonn.mpg.de>,<18ccmaINN53p@matt.ksu.ksu.edu>
Organization: St. Cloud State University, St. Cloud, MN

In article <18ccmaINN53p@matt.ksu.ksu.edu>, nan@matt.ksu.ksu.edu (Nan Zou) writes:
>mlelstv@speckled.mpifr-bonn.mpg.de (Michael van Elst) writes:
>>nan@matt.ksu.ksu.edu (Nan Zou) writes:
>
>>>But let's face it -- interlaced video is bad for the eyes, the fact that it
>>>facilitates video work is irrelevant.
>
>>Face it, you don't need what interlaced means. You think interlace=flicker,
>
>I don't merely "think" interlace => flicker, my experience tells me so.
>
>>this is not true. The Amiga generates an 25Hz (PAL) or 30HZ (NTSC) framerate
>>in its interlace mode. _This_ flickers. But the interlace _reduces_ flicker.
>>If you could see a _non-interlaced_ 25 or 30Hz display you would know that.
>
>WHAT?!?!  Interlacing reduces flicker?  That's a new one to me.  My under-
>standing is that when the display is interlaced, every other scan line is
>drawn in one vertical refesh cycle, the other half scan lines are drawn in
>the next refresh cycle.  So an interlaced display at 80 Hz is actually
>equal to a non-interlaced display at 40 Hz.  How do you explain that inter-
>lacing reduces flicker?

    Well, interlaced video was originally developed as a method to increase
available scan line usage without needing overly high scan rate frequencies.
Every other line on the screen is redrawn during a vertical retrace, allowing
2x as many scan lines to be used at a given frequency.  This works fine with
high-persistant phosphor monitors, but does produce the 'flicker' effect on
short-persistance monitors.  However, if a display were run non-interlaced
at the same frequency as an interlaced display (provided similar monitor
phosphor setups), the non-interlaced display would flicker just the same.
Moreover, the flickering seen would be a 'rolling' or flashing flicker which
is a great deal more visually disturbing than an interlaced display's 
showing.

>No I've never seen a non-interlaced 25 or 30 Hz display, but I have seen a
>60 Hz non-interlace display and a 60 Hz interlaced display (both at 800x
>600), the answer is obvious to me.

   I would imagine that most likely the monitor phosphor this was viewed on
was a medium to long-persistance type.  Also, you have to be careful about
how you define the scan rate.  On some specifications, the frequency rating
is given as the strict number of times the beam re-traces the screen, and
not the display.  In this case, an interlaced display is given at 60 Hz, 
but the actual image full refresh rate is only 30 Hz.  On a 60 Hz non-interlaced
display, the screen (and thus the full image) is retraced at the full 60 Hz.
Thus, the interlaced display is doing the same number of scan lines with
only a 30 Hz full refresh rate.  Bring the non-interlaced display down to
30 Hz and it will not seem so attractive.

                 ----------------------------------------
           LaMonte Koop -- SCSU Electrical/Computer Engineering
 Internet: lkoop@tigger.stcloud.msus.edu -OR- f00012@kanga.stcloud.msus.edu
              "You mean you want MORE lights on this thing???"
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