[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...


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