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

[dalloff@freenet.columbus.oh.us (Dave Althoff)]

Joseph Lee (nuNOUBEgundam@netNOUBEcom.hotmail.com) wrote:
: Dave Althoff (dalloff@freenet.columbus.oh.us) writes:
: <snip>
: >Of course, one of the problems is that computers don't do the timing
: >necessary for interlacing very well.  Interlaced displays tend to flicker
: >because the signals don't line up exactly, so most computers use
: >non-interlaced displays...both fields are identical.  I don't know if XGA

: Actually no.  Flicker is a subjective thing dependant on sight and the human
: brain.  ALL displays flicker; just below 70hz, it's more visible to most
: human equipped sight systems.

I should have typed, "flicker more noticeably."

: 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
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
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.

: just slow enough for the visual cortex to perceive that something was there
: before it shifted.  Non-interlaced displays at below 70hz will also seem to
: flicker.  Look at PAL.  It's 1/50 of a second refreshes (1/25 per field) and
: it's fine to the rest of the world to watch with.

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
is looking at *fewer* fields each second that the Americans and the
Japanese.  On the other hand, the decreased field rate also means that
there is more time to cover the screen, which can result in higher
resolution.  I forget how many scanlines there are on a PAL monitor...

: Why is 70hz somewhat a key scan-rate?  It's quite arbitrary, just that screen
: redraws are fast enough that the eye doesn't preceive than blanking part of
: the redraw through persistence of the image shifting fast enough.  The human
: eye does not operate on a 60 fps cycle, just some arbitrary number that has
: not really been quantitively measured.

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,
though...our movie theatres still operate at 24 FPS, mostly with
two-bladed shutters resulting in an effective flash rate of 48 FPS.  I
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.

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...

--Dave Althoff, ][.
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