Re: VGA to Monitor GS (Was Re: Apple IIGS on a VGA monitor...the hard way?)

[Joseph Lee <nuNOUBEgundam@netNOUBEcom.hotmail.com>]

Dave Althoff <dalloff@freenet.columbus.oh.us> spawned virtual particles writing:
: Joseph Lee (nuNOUBEgundam@netNOUBEcom.hotmail.com) wrote:
: : Reason interlaced flickers more than none is that each field is on the screen
: : for 1/30 of a second.  This isn't too short a time.  However, when you're
: : swapping between 2 slightly dissimilar pictures in 1/30 of a second.  It's

: Each field is re-drawn once each 1/30 second, meaning a 1/60 second
: refresh.  I don't know about yours, but my television set uses an

Right.  Each field is drawn every 1/30th of a second and lasts for a
duration of 1/60th of a second, leaving 1/60th of a second to draw the
other field to fill in.  The total is 60 frames per second if each field is
a separate entity; 30 frames frames per second if each 2 fields is the same.

: interlaced display, and since NTSC is an analog format, the fields are
: necessarily different.  But when I fire up the Amiga and tell it to use

Analog has nothing to do with it.  It just draws the even lines first and
then odd lines next, and cycle.  This creates that up/down motion flicker
which is easily picked up by the eye at that refresh rate.

: its high-resolution display, I get objectionable flicker on a 15Khz
: monitor.  The only explanation I can come up with is that the computer's
: display generator is incapable of generating two clean fields that line up
: properly with one another within 1/30 second.  So the computer normally
: compensates by generating the same field twice.

No, your Amiga's high-resolution (640x400?) display is interlaced.  By all
explanations of what interlacing is and what 15khz scan-rate does, of course
it flickers.  Even the IIGS's 525x480 display mode (VOC) flickers even when
it's the same old Finder screen.  It flickers because it's interlaced.

Even if both fields are identical, they are still 1 scan-line vertically
apart, and so they don't "line up".  Non-interlaced doesn't flicker the
same way because each field does "line-up".

: Backwards again.  Yes, the refreshes are in 1/50 second, but it is 1/25
: per FRAME.  Note that 1/25 > 1/30 which means that the rest of the world

Yea, I got it backwards.. :)

[snip]
: 60 fps is plenty fast, as the folks at Showscan will be happy to
: demonstrate...most eyes cannot see the flicker at that rate.  Interesting,

Wanna bet. :)  I have a spectrum analyzer/cd-player for the psx that
runs at 5, 10, 15, 30, and 60 frames per second.  It's very much
impossible to not see the graphics that appear on the screen only for
1/60th of a second.

Also, seen a spoked wheel spinning?  At what rate is each spoke / # of
spokes passing the same point when it's spinning so that the spokes
appear to stand still when the wheel is spinning at a rapid clip. I bet
it's faster than 60 spokes per second.  The eye works on the brain's clock
cycles which should run somewhere sub 1/60th of a second.  I wish somebody
could quantify this number.

[snip]
: don't know about you, but I have become so accustomed to watching the 60
: field/second video signal that I can see the flicker at 48 FPS.

48fps isn't fast enough to capture large displacement realtime-motion. Of
course it flickers, and worse is that it's filmed at 24fps. :)  There's
been 60fps+ films; not very successful.  Video is a 60fps format; notice
how it looks less dramatic (and different colors) than film because it
doesn't have the dramatic blurring of slow frames. :)

: But then, I frequently amaze the TV production students by spotting their
: flash frames going by at 30 FPS, so maybe it is just me...

They should know better.. :)
-- 
Joseph nugundam =netcom=com/==\=IIGS=/==\=Playstation=/==\=Civic HX CVT=/==\
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