[opendtv] Re: RGB mania ('translated' for Prin)
- From: jeroen.stessen@xxxxxxxxxxx
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 17 Jan 2005 09:39:33 +0100
Hello, Bert Manfredi wrote: > This is a fascinating thread. If I understand > what you're saying, a normal TV transmission > assigns Y values of, say, 235, both to large > areas of normal white and to tiny highlights > which should be super-white. This is not entirely correct. The target value for a white highlight may well be 235, but because of the ringing in the anti-aliasing filter on such a small transient the peak value will rise above 235, up to the highest value allowed (i.e. 254 in ITU-R.601, because 255 is reserved for sync). Thinking digital is okay, but keep in mind the analog world... > And because CRTs react to Y values in strange ways, > they will naturally blast a very powerful beam on a > tiny area identified as Y 235, to render the specular > highlight expected there, This is called "punch", and it is thought to be a positive quality of CRTs, and in fact of most other emissive displays. It is specifically not a natural quality of light valve displays. In CRT this occurs because for large bright areas the average power limiter takes precedence over the peak drive limiter. So images that contain large bright areas are limited, to perhaps 20-25% peak brightness, whereas images that contain only specular highlights on a dark background are not limited. The average power limiter is added to prevent overload of the EHT power supply and to safeguard the life time of the picture tube and limit the temperature. As a side effect it limits the blinding effect of an all- white scene, limiting it to e.g. 250 cd/m2. But in fact we appreciate this feature for its opposite effect, that the highlights in a dark scene can reach 1000 cd/m2 or so ! > even without having been told specifically that > this tiny area was actually brighter than > the large areas marked Y 235. One big IF.... the behaviour as described above ONLY happens if the user contrast has been turned up high enough for the limiters to activate. Joe Kane advises you to turn down the contrast so far that the peak brightness is between 100-200 cd/m2, and then neither the peak drive limiter nor the average power limiter will EVER be active anymore. Then the peak brightness will be proportional to the video signal, as it was probably intended !! Activation of the peak drive limiter (which acts like an automatic gain circuit) causes the nasty effect of "pumping": the appearance of a subtitle can cause the brightness of the entire scene to be reduced. This is most annoying because a local cause has a global effect. (Our patents on "contrast reserve" aim to avoid that.) Activation of the average power limiter is less annoying, because a high average power level is by definition a global cause, and it has a global effect. This is natural. It is even appreciated that large bright scenes are reduced below the level of being blinding. > So I'm trying to see how an LCD would react to this, A snow scene on a typical LCD, with 400-800 cd/m2 peak brightness, is blinding ! Not so on a plasma display, because this is severely average power limited. > to see if I can understand why it was so much harder > to adjust my LCD than it is to adjust a CRT TV. This could have several other causes as well, e.g. the black level of many LCDs is not very good due to light leakage. The black rarely goes below 0.5 cd/m2, unless the backlight is dimmed appropriately. It is not always easy to see if and how you can dim the backlight. > An LCD would paint the specular highlights > with the same intensity as the larger normal > white regions. Except for that effect of ringing of the AA filter. > So perhaps if one sets the screen to look right for > specular highlights, the effect is that all the > white or brighter regions of the image look too pushed. It would be similar to a CRT set to below the limiting levels, as per advice of Joe Kane and the ISF, except that it all happens at much higher brightness levels. > It's especially noticeable on b&w images. That's a different aspect: in order to get maximum efficiency out of a light valve it is tempting to drive it up to the clipping level. The whites will look crushed. The CRT has no real hard clipping mechanism, and even if it had then the two limiters will usually keep it well below the clipping limit. > Which makes one want to back off on the > "contrast" control (i.e. white level), which > in turn causes the darker parts of the > scene to fade to black too fast. With LCDs... the familiar "contrast" and "brightness" controls are sometimes connected to different circuits. Do not always expect the familiar behaviour ! > If this reasoning is about right, it would > describe what I was seeing. My reaction was > to back off on the "contrast" and turn up > the "brightness" (i.e. black level), to > bring up the darker parts of the image. That is a correct reaction. Your contrast ratio will be even worse, but you will see more details in the blacks. It has to do with the differential gain of the display (defined by its gamma curve) and the differential gain of your eyes (defined by a similar inverse gamma curve). Best regards, -- Jeroen +-------------------------------+----------------------------------------+ | From: Jeroen H. Stessen | E-mail: Jeroen.Stessen@xxxxxxxxxxx | | Building: SFJ-5.22 Eindhoven | PHILIPS Applied Technologies | | Phone: ++31.40.2732739 | Visiting & mail address: Glaslaan 2 | | Mobile: ++31.6.44680021 | NL 5616 LW Eindhoven, the Netherlands | | Pager: ++31.6.65133818 | Website: http://www.pdsl.philips.com/ | +-------------------------------+----------------------------------------+ ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.
- [opendtv] Re: RGB mania ('translated' for Prin)
- From: Manfredi, Albert E
- [opendtv] Re: RGB mania ('translated' for Prin)