[opendtv] ClearLCD adaptive dual-pulse

  • From: "Adam M. Costello" <opendtv.amc+0+@xxxxxxxxxxxx>
  • To: jeroen.stessen@xxxxxxxxxxx, opendtv@xxxxxxxxxxxxx
  • Date: Wed, 13 Dec 2006 01:39:55 +0000

[Resending with a different From address; the first attempt seems to
have vanished.  Sorry if it reappears and you get both.]

Mr. Stessen, thank you for your very informative posts to the opendtv
list.  I am intrigued by ClearLCD, and especially by a tantalizing
remark you made a while back that we can "have our cake and eat it too",
referring to the tradeoff between 60 Hz flicker and 120 Hz judder.
I'm not in the business, just an interested consumer who is hesitant
to "upgrade" my CRT TV to a display technology with superior spatial
resolution but inferior motion portrayal.  ClearLCD is the only non-CRT
TV technology I've heard of that even attempts to retain the impulse
characteristic of CRTs.  (Is anyone developing scanning LED backlights?
I haven't found anything on the web beyond speculation.)

I guess ClearLCD's solution to the flicker/judder problem is related to
something called "adaptive dual-pulse" backlighting, but I have been
unable to find any details about it.  Can you (yet) tell us anything
about that?

From the name, it sounds like the backlight switches between 60 Hz
scanning (one pulse per refresh) and 120 Hz scanning (two pulses per
refresh, 180 degrees out of phase, so that two horizontal stripes are
illuminated at a time, 1/2 screen height apart), depending on whether
the video currently contains any moving objects that the eye might
track.  When switching from 60 Hz to 120 Hz, does the duty cycle remain
the same (the pulse width is cut in half), or does the pulse width
remain the same (the duty cycle doubles, and the lamp is dimmed to
1/2 intensity)?  What is the pulse width (in milliseconds)?  When the
backlight is dimmed for dark scenes, is that accomplished by reducing
the duty cycle or the intensity?

If the pulse width is 4ms (I'm guessing based on various imprecise
statements I've found), that's a lot better than a full frame time of
17ms, but still a far cry from a CRT, where the phosphors have largely
decayed after 0.1 ms and almost completely decayed after 1ms (right?).
If something moves the full width of the screen in two seconds (which I
wouldn't call especially fast), that's one pixel per ms, so a 4ms pulse
would blur away about 3/4 of the horizontal resolution of the moving
object (versus 16/17 for an always-on backlight, versus almost no blur
for a CRT).  Is work underway to further reduce the duty cycle?

By the way, why is 60 Hz flicker a problem?  I've been watching NTSC TV
all my life, and never noticed a flicker (and still don't).  But I can
see the flicker on a CRT computer monitor set to 60 Hz refresh.  What's
the difference?

Thanks for any explanations you can offer,
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