[net.micro.amiga] Why the amiga flickers... {big time reposting}

spencer@usc-oberon.UUCP (06/14/86)

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This has all been posted before, if you already have read it move on now...
From sdcrdcf!hplabs!pesnta!pyramid!ut-sally!topaz!caip!cbm!grr Sat Feb  1 14:37:19 PST 1986

> Randal Spencer      Student DEC Consulting - University of Southern California

> I really need a good description of exactly why is it that the Amiga will
> flicker in the 400 mode.  I understand Television reasonably well, enough
> to understand that television is a set of 60 fields of 262 lines of video
> which extend from the above the top of the visible portion of the screen
> to just below the bottom. There is 1/60th of a second worth of motion and
> then the next frame is drawn, this time the lines are drawn between the
> previous lines.  This process is done so as to keep the screen from fading
> before the electron beam gets to the bottom of 512 lines after a 1/30th
> of a second.  When the Amiga is in 200 mode I assume that it is drawing
> on the first field and then not on the second?  

In 200 mode, the same image is displayed twice during the 1/30 period on
the very same lines.  The size of the scan line is large enough that you
do not notice any gaps.

> Perhaps the real problem that I could not figure out in my own head is,
> if the Amiga had to flicker to remain compatible with NTSC, why is it
> that even the highest resolution video of the day will not flicker like
> the Amiga?

The basic flicker is not really the fault of the amiga, it is caused by the
relatively short persistance of the color phosphors.  Monochrome phosphors
are available in varying persistences - from very long like used on medical
heartbeat displays to very short for oscilloscope photography.

Since only standard color phosphors are readily available, when two alternate
scan lines are very different they are each fading out 30 times a second,
which is perceived to be an alternating or flickering effect.  Because a
normal TV image has, on the average, very little contrast between scan lines,
you do not notice this while watching the A-team.

As has been pointed out before, you can minimize the effect by dimming the
ambient lighting and turning down the brightness.  Many of the commercial
color graphics displays run at 30 Hertz, non-interlaced, and must be used
in darkened rooms to minimize eyestrain.

> I run Setlace it sure seems to start shaking the formerly rock steady
> lines around on the screen.

I'm not sure what the problem is here, but anyway if you find a source of cheap
long persistence color monitors, please let Amiga and the world know about it!!!
-- 
George Robbins - now working with,	uucp: {ihnp4|seismo|caip}!cbm!grr
but no way officially representing	arpa: cbm!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)


From sdcrdcf!hplabs!pesnta!pyramid!ut-sally!topaz!harvard!h-sc1!breuel Sat Feb  1 14:39:29 PST 1986

> The basic flicker is not really the fault of the amiga, it is caused by the
> relatively short persistance of the color phosphors.  Monochrome phosphors
> are available in varying persistences - from very long like used on medical
> heartbeat displays to very short for oscilloscope photography.

No, using high-persistence monitors is not a solution. When you
scroll text or move your mouse-pointer around, it looks terrible
on a high-persistence monitor (just look at the IBM screen).
To make this perfectly clear: the reason why 70Hz interlaced monitors
do not appear to flicker is NOT that the persistence of the monitor
is matched with the refresh-rate, it is that the human eye/brain
cannot perceive flicker above 30Hz.

The way to get good high-resolution displays is not to use high-persistence
monitors, it is to use higher frequency displays. The LISA, the Mac, and
the Atari ST show that this is possible economically, at least for
black and white displays. Call me spoiled or whatever, but I have
gotten very used to my 700x500, flicker-free, low-persistence LISA
screen, and I'll not switch to a computer that doesn't have a similar
display quality.

Now, again, the reason why I am posting this is not to annoy Amiga owners
or to encourage the purchase of Atari ST's, but simply the hope that
Commodore will add a 640x400 70Hz mode to their otherwise great machine
if there is suficient demand for it. I just can't see how people can
reasonably argue that using a computer in a dimmed room, sitting back
3 feet, using a high-persistence monitor, or drawing pixels on top
of one another can be more than a bad compromise, given that a real flicker
free high-resolution display is not all that hard or expensive to make.

						Thomas.


From sdcrdcf!sdcsvax!ucbvax!nike!caip!daemon Sat Feb  1 15:05:13 PST 1986

>From: "ROBERTS, JOHN" <roberts@nbs-vms.ARPA>


   It seems that a high proportion of the postings on this mailing list
are concerned with high-resolution mode flicker, so I thought it would
be worthwhile to post the following.

ISSUES RELATING TO FLICKER IN SCANNED DISPLAYS


PHOSPHORS

   For a number of reasons, I think that phosphors exhibit close to
exponential decay in brightness with respect to time at moderately
high levels of stimulus. Shown below is a rough sketch of what such
a curve would look like. "Brightness" may be defined as the number of
photons emitted per unit area per unit time.

    |
    |
    |   x
    |
  B |
  R |
  I |
  G |     x
  H |
  T |
  N |       x
  E |
  S |         x
  S |            x
    |                x
    |                       x
    |                                 x        x
    |
    +---|-----------|---------------------------------
       (A)         (B)      TIME

	|_____|
    one refresh interval
    (for example)

   Note that for an exponential decay and a high initial stimulus, a
large part of the decay takes place very shortly after removal of the
stimulus. For the refresh interval shown, there is a large difference
in brightness between the initial level and the level just before the
next refresh. If the refresh interval can be lessened (faster refresh
rate), then this difference in brightness will be reduced. This is
not always possible, however. High-persistence phosphors attempt to
address the problem by changing the shape of the curve to reduce its
slope. This has the disadvantage that if too slow it may cause moving
images to appear to blur. Another approach would be to reduce the
initial level of stimulus (by turning the brightness down). For
instance, if the phosphors are stimulated to the level found at point
(B) instead of point (A), then the absolute decrease in brightness
before the next refresh will be greatly diminished. I think that the
high-speed response of the eye is sufficiently linear so that a
decrease in flicker will be observed. In addition, I suspect that
many phosphors exhibit increased persistence at low brightness levels.


HUMAN VISION AND PERCEPTION

   The process of vision begins when incident photons pass through the
lens of the eye and strike the retina. There they are absorbed by
special dyes in various types of sensor cells and induce photochemical
reactions. These reactions in turn stimulate nerve cells, which
conduct limited preprocessing then send frequency-modulated signals
to the visual centers of the brain.
   The sensor cells are called rods and cones, and are distributed
unevenly over the surface of the retina. The cones are sensitive to
color, produce sharp images, and are concentrated mostly in a small
pit at the center of focus, which means that only a small part of
what you see around you is sharp and in good color at any given time.
(The brain can build up a larger detailed image by moving the eyes
to scan an incoming image.) Cones have the ability to detect motion
(changes in brightness), and need relatively bright light to work.
There are different kinds of cones sensitive to different ranges of
the color spectrum. These ranges overlap, and have their peak
sensitivities at the colors red, green, and blue. This is why these
are the primary colors to humans, and any color can be imitated by a
combination of these three.
   Rods, which are mostly scattered over the areas of peripheral
vision, are not sensitive to color, and in their configuration do
not produce particularly detailed images. They can work in extremely
dim light, and are highly sensitive to motion, such as flicker would
induce.
   The chemical properties of the rods and cones and the timing of
nerve impulses affect perception of flicker in several ways. First,
images continue to appear with decreasing intensity after a stimulus
has been removed, which for example makes a flash of light appear to
last for much longer than it actually does. Second, a steady stimulus
leads to the creation of a negative afterimage, which can eventually
interfere with an incoming image. Third, observed flicker will probably
be different for direct and peripheral vision. Fourth, if a stimulus
flickers at a rate faster than the photochemicals can respond or the
nerve impulses can be modulated in a nonuniform manner, the stimulus
will be perceived as nonflickering.
   The visual processing centers of the brain function as an extremely
sophisticated pipeline processor or systolic array. (Humans can
process many images thousands of times faster than a Cray, in spite
of having only a tiny fraction of the circuit switching speed.)
The first stages of processing put together 2-dimensional images,
detect motion, etc. Higher levels group images into patterns, and
determine relative motion of patterns. Much higher levels construct
large-scale detailed three-dimensional images, identify objects,
recognize the faces of individuals, etc. All of these levels are to
varying degrees automatic, and largely prewired. Flicker and other
unusual features of scanned images can interfere with the first
stages of image processing in an annoying manner. It is incumbent upon
designers of electronic displays to produce images that interact
well with the visual processing capabilities of the users, or which
the users can learn to use effectively.


IMAGE DESIGN TO MINIMIZE FLICKER

   In an NTSC-type display, if vertically adjacent pixels in alternate
fields carry the same image, and if the pixels are placed close enough
together, then they will both have an influence on any sensor cell
that detects them. This will lead to an apparent refresh rate of 60Hz
instead of 30Hz, and the apparent flicker will be greatly reduced.
An Amiga programmer using 640x400 mode and writing with elements one
pixel wide and two pixels high can therefore enjoy many of the benefits
of the full hires mode at greatly reduced flicker.
   Several people have expressed the erroneous impression that this
approach would give no greater detail than the 640x200 mode. Observe
the structure of a diagonal line:

		   ##              #              ##
		  ##               #              ##
		 ##              #              ##
		##               #              ##
	       ##              #              ##
	      ##               #              ##
	     ##              #              ##
	    ##               #              ##
	   ##              #              ##
	  ##               #              ##

   The first pattern represents a line drawn on a 640x400 screen using
elements two pixels high. The second pattern represents a minimal line
drawn on a 640x200 screen using single-pixel elements. The third
pattern represents a line drawn on a 600x200 screen that has been
"filled out" by using double-width elements, or a line drawn on a
300x200 screen. Note that the first line looks much smoother and more
substantial than the others. By inference, one can see that more detailed
images or images with better detail can be drawn using the first technique
than the other two, for all except patterns of horizontal lines. One could
seek to design text fonts to take advantage of this technique. It is
conceivable that one allowing a full 50 lines of text could be devised.
If not, one might still be able to obtain a 50% increase (for instance)
in the number of lines of text.


COLOR, BRIGHTNESS, AND CONTRAST OF TEXT AND BACKGROUND

   Has anyone come up with colors and relative brightness of text and
background that seem particularly useful in minimizing flicker? How
well does gray scale work?


THE 520ST IN MONOCHROME MODE

   In addition to its 70Hz refresh rate, I suspect that the ST monochrome
monitor may run in noninterlaced mode or use long-persistence phosphors.
Does anybody know?


DESIGNING NONINTERLACED MONITORS

   From page 22 of the January 23, 1986 issue of EDN magazine:

   "DSP Chips Extend Digital-TV Capabilities"
   "Additions to the Digit-2000 digital-TV chip set from ITT-Intermetall
(Freiburg, West Germany, TLX 772715) include...a video memory controller.
The VMC-2260 video memory controller stores an entire picture frame in
standard 64kx4 dynamic RAMs and provides freeze-frame, multiple
picture-in-picture, and zoom capabilities plus the ability to eliminate
picture flicker by doubling the TV's vertical scan frequency. For the
high-bandwidth color monitors required for teletext or computer display,
where vertical scan frequency doubling is not suitable, the RGB-2932
double scan processor allows you to eliminate screen flicker by doubling
the horizontal scan frequency of the RGB signals."

   It seems that something like this could be built into a high-speed,
high-resolution, non-NTSC display and used with an unmodified Amiga.
I would expect such a display to cost anywhere from the price of an
Amiga to several times that much.

(Standard disclaimers apply. Much of this information was derived from
numerous books and magazine articles that were shockingly lacking in
differential equations and such, so you might want to do further study
if you're really interested. Some of the explanations are a little more
simplistic than an actual application would call for. My description of
Amiga graphics was based on certain assumptions concerning how the display
works. If there are errors in this area or elsewhere, please feel free to
bring them to my attention in a non-flaming manner.)

				   John Roberts
				   roberts@nbs-vms.ARPA
------


From sdcrdcf!sdcsvax!ncr-sd!ncrcae!ncsu!mcnc!decvax!decwrl!amdcad!lll-crg!seismo!hao!hull Wed Feb  5 00:28:19 PST 1986

> How does the Amiga in non-interlaced mode persuade the monitor to
> exactly retrace the first field in the second field of the frame?
>                                 John Roberts
>                                 roberts@nbs-vms.ARPA

I seem to have no possibility of getting mail through to John by the "r"
command.  The mailer can't find the site, and sends the mail back along
the news path through topaz and caip.  The caip site *always* rejects it
(The deamon there hates return mail).  So I guess I'll just post the reply
here...

The Amiga doesn't have to convince the monitor to overlay the second field
of lines on top the the first.  The monitor *wants* to do that just naturally.
What is more likely a task for the Amiga is to get the monitor to interlace
properly.  To do that, the Amiga must generate "equalizing pulses" at twice
the normal line rate during the vertical sync interval, and then momentarily
invert one of the vertical sync horizontal pulses to get the monitor to sync
half a line later than normal.  At the bottom of that frame, it then must
repeat this stunt, but half a line earlier.

[If yet unproven concepts are outlawed in the range of discussion...
                   ...Then only the deranged will discuss yet unproven concepts]
        {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull
								Howard Hull


~r

{Me again, Hope that this all helped!}-- 
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....I disclaim everything, I had nothing to do with it, it's not my fault!....
Randal Spencer  - DEC, {amiga} Consulting -  University of Southern California
phone: (213) 743-5363  Arpa:Spencer@USC-ECL,USC-Oberon  Bitnet:Spencer@USCVAXQ
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