Ajustes elementales con cartas de ajuste

Aquí os paso otro LINK, es de CHRIS WIGGLES, otro más más sobre este tema, de él he rescatado este fragmento que lo deja todo totalmente claro.

2) Digital outputs and digital levels

DIGITAL OUTPUTS AND DIGITAL LEVES


I explained IRE in background section 1, so you should already recognize that IRE only describes analog waveforms, and is ambiguous as to the actual intended image content. When using digital transmission formats like SDI, DVI, HDMI, analog measurements like IRE units or mV are useless for describing the image data. As briefly explained before, digital video uses 8-bit encoding, which creates 256 unique steps from 0-255.

Video from DVDs or other digital sources, follows Studio RGB standards which encodes reference black at level 16, and nominal reference white at level 235. Outside these bounds is the footroom and headroom that contain blacker-than-black(BTB) and peak white data. Footroom and headroom is necessary for the best possible video reproduction for the reasons discussed briefly in background section 1:



Enter the PC.
If you're a computer graphics person, you may recognize the 0-255 scale of digital levels. But you may be confused because with computer graphics black is at 0, white is at 255, and that's it! There is no footroom or headroom for BTB or peak white data. Since computers and video often have to be dealt with at the same time, this creates a problem between PC levels and Studio levels.

Projectors with digital inputs have to be able to handle both video applications and computer graphics applications. They should be able to calibrate or switch their white and black levels to accommodate both Studio levels and PC levels, but some don't. Because video sources and graphics sources may be used simultaneously in some applications, video source manufacturers tend to include an option to leave the digital outputs at Studio levels, or to re-map them to PC levels.

Mapping Studio levels to PC levels can be done a few ways. Sometimes all the levels are just shifted down 16 steps, thus clipping off BTB data, but not introducing banding/contouring or clipping highlight details. If there are other PC level sources fed to the display, their whites will be substantially brighter. The clipping of BTB data is undesirable and the brightness mis-match is also noticeable, however note that there will be no banding problems. Usually the levels are expanded: digital 16 (black) is shifted down to 0, and 235 (white) is shifted up to 255 (or sometimes a value slightly lower than 255) thus expanding the numerical range between black and white to match PC levels. Note that this doesn't improve contrast in the final image. In this case, re-mapping Studio levels to PC levels will destroy BTB and peak white image data, and introduce banding/contouring artifacts because of the expansion. This is also undesirable.

Always try to maintain Studio levels whenever possible in your system. You should use Avia PRO or DVE to test whether or not levels are being clipped. Both discs contain patterns with both BTB and peak white data. Note that the current consumer Avia does not, it only contains data in the range or 16-235. Thus if BTB or peak white data is being clipped, you won't see it with Avia! If you are clipping the black or white bars in Avia, then your system is doing even more severe damage to the video by clipping above black shadow details and below-white detail: very bad! Note that Avia PRO also has some very useful ramp patterns with levels encoded at equal width so as to be completely banding-free. These are very useful for observing banding/contouring caused by the playback system.

For more reading, see this thread, especially the expert posts by Don Munsil and Stacey Spears:

¡AMEN!