[opendtv] Re: ClearLCD adaptive dual-pulse
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx, jeroen.stessen@xxxxxxxxxxx
- Date: Fri, 15 Dec 2006 08:52:47 -0500
At 1:39 AM +0000 12/13/06, Adam M. Costello wrote:
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,
The perception of 60 Hz flicker is caused by stimulation of the extra
foveal receptors of the human visual system. These are the low
resolution sensors in our eyes that detect motion at the edges of the
high resolution visual field of the fovea. Their primary purpose is
to alert the human to things that are entering the visual field, like
the lion that wants to eat you, or the car that just ran the traffic
light. They help guide visual search, but are not of much use in
terms of generating detail.
The NTSC and PAL television systems were designed to cover foveal
vision - about 10-11 degrees of the overall field of view. We also
sit about seven picture heights from these displays, so the
brightness intensity is not that great. We do not need to employ
visual search with these displays as the entire image is scanned onto
the fovea.
Here is an excerpt from the SMPTE Task Force Report on Digital Image
Architecture that explains the perception of flicker by the human
visual systems:
3.2.3 Thresholds for the Perception of Flicker
Above certain frequencies, flickering light sources will appear as a
continuous light source. The relevant frequency is called the
critical fusion frequency and varies with the level of illumination.
Separate flicker thresholds exist for the transient and sustained
processing channels.
Transient channels are sensitive to flickering light sources with
low spatial resolution; this type of stimulation appears as
wide-area flicker and is most noticeable in peripheral vision. At
low levels of illumination (where rod vision is used) flicker fusion
occurs at frequencies of only a few Hz; as the level of illumination
increases and cone vision is triggered the fusion frequency
increases.
Flicker from low light level sources such as a television or movie
screen typically disappears in the range of 20 to 60 Hz. As screen
size increase, taking up a larger portion of the field of vision, or
if screen brightness increases, the frequency for flicker fusion
increases.
Sustained channels are sensitive to flickering light sources with
high spatial resolution; this type of stimulation appears as small
area-flicker, often associated with moving objects. In this case the
flicker fusion frequency can be much higher than for wide-area
flicker; this form of flicker manifests itself as strobing of the
object.
An excellent example is found in the single pixel horizontal lines
often used in computer graphics. These lines do not appear to
flicker on a progressive scan computer display which is refreshed at
rates above 60 Hz; but if the same image is presented on an
interlaced video display the single pixel lines are presented in
every other field (at 30 Hz) and they flicker. This is due to the
fact that the persistence of the display phosphor is of shorter
duration than the refresh rate; higher scanning rates (either
progressive or interlaced) eliminate the flicker.
Now let's apply this to your question and to the design of other
displays that cover a wider portion of our field of view.
When we move close to a flickering display, as we do when using a
personal computer, two things happen. First, we cover a wider portion
of the field of view, thus stimulating the extra foveal sensors.
Second the intensity (brightness levels) increase significantly,
raising the threshold for flicker fusion. The solution is to raise
the refresh rate - typically into the range of 72 to 75 Hz. Europe
set ergonomic standards for computer displays that do just that.
Fortunately, the era of scanning (flickering) displays is almost
history. With displays where the "pixels" are constantly illuminated
rather than scanned, flicker is less of a problem, however, the
source of back lighting for panels or bulb for projection, is still a
concern, and the temporal rate of the source can still cause the
perception of flicker. The lighting problem is not too difficult, as
the flicker rate can be much higher than the image refresh rate.
When we get into the world of HDTV displays, we begin to create the
same problems as a movie theater, and conditions that are nearly
identical to viewing a computer display. The HDTV viewing experience
is defined as covering about 30 degrees of our field of view. One of
the most important difference with HDTV is that it forces us to
engage in visual search - i.e. rather than fixing our eyes on a
display that just covers the fovea, the eye starts to follow objects
to get a high resolution view. At the same time there is significant
stimulation of the extra foveal sensors. Theaters are typically very
dark, as this lowers the flicker fusion frequency - even with this
the flicker rate in a theater is usually 48 Hz and sometimes 72 Hz
thanks to double or triple shuttering in the projector. If you raised
the ambient light level in the theater the flicker would be very
objectionable.
The same is true for large HDTV displays, but we often expect to use
them in a room with higher ambient light levels. The perception of
broad area flicker with large 60Hz HDTV displays is not uncommon,
although the move away from scanning displays helps.
We tried to get the screen refresh rate up to 72Hz during the U.S.
HDTV process, but did not succeed. This would eliminate the broad
area flicker, but even more important, it would allow 24 fps source
to be "triple shuttered," eliminating the nasty 3:2 pulldown judder.
It would also allow the use of 36 fps as a shooting format that would
be quite adequate for most low motion programming like Meet the Press
and other talking head shows. And for high motion applications, 72 Hz
would significantly improve sharpness in motion portrayal, not to
mention slow motion replays.
Hope that this helps...
Regards
Craig
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