|Originally posted by WanMan
Curious, Guy. If the left bar is 1-IRE about black and the right 2-IRE above black, neither assume that black is correctly set, right?
Rant Mode ON!!!!
What I read on the forums about the concepts of black, encoded digital level, 0 IRE, and 7.5 IRE is a "freaking mess" to put it mildly. Many incorrect postings or partially correct postings abound and multiply that it is a wonder anyone understands what anyone else is talking about. This is worse than trying to teach newbies what reference level means.
I'll try, but my patience for this isn't very good.
1. Digital video encodes it signals with black at digital level 16 and white at digital level 235. Levels 0 and 255 are illegal for encoding any image content. Levels 1 to 15 are below black and used to handle undershoot. On a display, material from levels 1 to 15 supposed to be the same absence of light as digital 16 (black). Material from levels 236 to 254 handle above white overshoot. Picture information on pefectly mastered material has no intentionally visible image data below 16 or above 235. Black is black. Things below black are black. Nothing is brighter than max white 235. Processing and/or poor video sourcing may end up with some picture information outside the recommended range of digital 16 to 235 so preservation of those values may be of use.
2. Computer video has black at digital 0 and white at digital 255. Notice how this differs from digital video. Oh great. True black for computer graphics is at digital 0, but true black for digital video is at digital 16. Even worse, HTPC playback may or may not expand the digital video range to fill the computer video range.
3. IRE is voltage unit which was used to simplify expressions of the 0.7 volt excursion of video signal content. It was more convenient to express things in IRE than in odd fractions of of a volt. 0 IRE is at the baseline of the signal. 100 IRE is at the top of the signal range and indicates max white. If North American NTSC setup is present in an analog signal, black is represented as 7.5 IRE. If no setup is present, black is represented as 0 IRE.
4. Black means BLACK. If I say black, that unambiguously means black, true black. However, if you say 0 IRE or 7.IRE, the meaning is somewhat ambiguous because you haven't also stated whether or not setup is present. For example: 0 IRE is black if the system has no setup, but it is 7.5 IRE darker than true black on a system with setup. This is why I always refer to the bars relative to BLACK rather than an absolute IRE level. The absolute IRE level changes.
On the Avia discs, we LABEL the patterns expecting black to be output by the playback device at 7.5 IRE so the labels are correct only if the playback device represents black as 7.5 IRE and white at 100 IRE.
If there is no setup, the labeling would be slightly off. You can translate the on screen IRE labels to their zero setup IRE level using Zero Setup IRE = (Labeled IRE - 7.5) *100/92.5
So we both 0 IRE and 7.5 IRE can mean the EXACT SAME signal level as true black (video digital 16) depending on the equipment. HTPC folks probably did the most in confusing the situation by suddenly taking their "convention" of calling black = 0 IRE without also stipulating that digital 16 must also be at 0 IRE for this to be true. Any mismatch and you end up with elevated blacks or submerged blacks.
5. DVI came along and further complicated things because DVI interfaces also work with computer graphics video. Recall that computer RGB has black at 0 and white at 255, but digital video has black at digital 16 and white at digital 235. The display and source device must match in their conventions or once again you end up with submerged or elevated blacks.
6. HTPC's confuse things in the same way as DVI does because the on screen graphics and played back video may not have true black in both systems at equal digital levels. Calibrate to make black appear correct for DVD playback and you may mess up your desktop. HD played back through HTPC can have the same type of problem. The goal for each source is to make true black in THAT type of signal be black (neither submerged nor elevated) on the display.
7. No matter what the playback system, processor, or display deviced, nothing changes the on disc encoded level of black from being digital 16. In other words, you equipment doesn't magically get the disc to rewrite itself and change the values. What is encoded as black on disc is always black. It's the subsequent playback and display system that can mess things up.
8. If you have correctly understood digital levels through the above the the incredible misunderstanding it takes to come up the following statment and question should be obvious:
Calibration disc A has black at 7.5 IRE but calibration disc B has its black at 0 IRE.
No. If accurately encoded, both DVD's must have black encoded at the same digital 16. It is the playblack equipment settings that determines what IRE the true black lies. If the player has setup, then black is at 7.5 IRE. If not, then black is at 0 IRE.
Can't you put a signal on disc that is always at 0 IRE so we can compare against it?
No. No matter what digital level we encode the output IRE is varied by the player's setting. A DVD cannot overried a disc player in this regard.
To get things right, each signal stage must match the next piece of equipment's expectation for where black is on the digital range. Get it wrong anywhere along the way and you have a problem.
Labeling on the AVIA, Avia PRO, and S&VHTT discs are done to follow the convention that 7.5 IRE represents true black (digital 16) and 100 IRE represents max white (digital 235). So when I say black, I mean true black (digital 16), the black background of patterns in AVIA, or something labeled as 7.5 IRE in AVIA.
I guarantee that within a month there will be enough incorrect postings to render what I just wrote a useless reed in the wind. This is a topic that people have a knack for clearly, convincingly, and thoroughly misunderstanding.