AVS Forum Special Member
Join Date: Nov 2003
Location: Stockholm, Sweden
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 101 Post(s)
This text from Spears&Munsil sums it up quite good. My underlining.
One important thing that we haven’t yet mentioned is that the video system has two different white levels. The white level that is talked about most often is “reference white.” Reference white is the top of the nominal video range. However, there is room in the video standard for above-reference values, which are sometimes called “whiter than white” or “super white.” This area of the video range is supposed to be reserved for “overshoots” and occasional excursions outside the nominal range. Very little of most video frames strays outside the reference range, but often the stuff that does stray outside the range makes a difference in the image. Most notably, when displaying bright saturated colors and/or highlights on bright white and near-white objects, one or more of the red, green, or blue channels may stray into the overshoot area. For this reason, professional video monitors are always calibrated such that they have room to display the above-reference range.
This is the basic quandary: if you calibrate so that reference white is the brightest level the display can produce, you maximize the contrast ratio, which is measured by comparing reference white to reference black. But you may be missing some amount of picture information, albeit small, that the professionals working on the video saw.
How exactly to resolve this quandary involves considering what you consider most important and what your viewing conditions are. If what you want is the punchiest possible video, or you routinely watch video in a room with a significant amount of light, you may want to consider deliberately calibrating to maximize reference white and clipping the above-reference range. If you want to make your display look as much like the pro monitors used to master video, and you’re viewing the video in a dim or dark room, you’ll almost certainly want to preserve the above-reference range.
You can split the difference if you like. Each additional chunk of brightness above reference white adds a little bit less to the image than the previous chunk, so you might decide to preserve some of the above-reference range but not all. This isn’t inherently wrong. However, it’s worth noting that we nearly always calibrate our displays to preserve the entire above-reference range. We think that most modern displays have enough inherent contrast that sacrificing a small part of it to the above-reference range is worthwhile