. Though these are not required for digital video, when ITU-R BT.601 – which codified
the digital versions of the NTSC and PAL video formats – was created the standard was designed to
allow for the capture of a portion of these signals. For example, blacker-than-black was preserved as
many video engineers considered it critical to capture the rise time out of blanking and into the active
when converting an analog signal to digital. These aren't as important in a pure digital
workflow, but preservation of these aspects of the analog was considered critically important.
In the analog domain, whiter than white signals were allowed simply because a hard limit on voltage
was not placed on the video standard. (Note that the value of "video white" was specified.) Though a
limit was typically placed on signals broadcast over the air, analog video recording equipment
allowed higher voltage whites to be captured up to the limits of the recording medium. This allowed
videographers to "open up" the lens and capture very bright regions such as clouds and specular
highlights on the surface of the water without losing picture information. The ITU-R BT.601 standard
allows for the preservation of these up to a limit that is approximately 108% of peak white.
In addition to the analog signal preservation requirements, the engineers designing the format
realized that digital video processing, filtering, and compression could cause ringing and blooming
artifacts which primarily manifest themselves in the highest and lowest bit values. By having a
"blacker than black" and "whiter than white" region of the digital signal they could keep those signal
artifacts outside of the voltages/bit values of black and white and reduce their negative impact on the
This was implemented in ITU-R BT.601 by assigning black (referred to as video black) a value of 16
digital (using 8 bits per pixel) and white (referred to as video white) a value of 235 digital (again
using 8 bits per pixel). Because of this, when you display a video signal on a monitor, this higher
value of black must be compensated for or your blacks will be lighter than they are supposed to be.
(The fact that video white is a lower value than peak is considered less critical as it can be
compensated for by adjusting the monitor brightness.)
This "16–235" signal range is often referred to as the digital video mapping. And the "0–255"
signal range is often referred to as the computer or graphics mapping. (In 10-bit digital video the
range is 64–940 for digital video mapping and 0–1023 for computer/graphics mapping.)
Usage and Application
As the HP DreamColor LP2480zx is an LCD monitor, it uses 0–255 video signal mapping. Because of
this, some adjustment must be made to the monitor's Black Level or your blacks will be too bright and
the image contrast will be reduced. In order to properly adjust the black level, you should display a
test pattern on your video editing system that includes a "PLUGE" pattern. For example, the standard
"SMPTE Bar" pattern includes a PLUGE pattern in the lower right hand region. There are also full-field
(full screen) PLUGE patterns available on some test pattern generators. Note: the bar patterns
provided by some software video editing systems do not include a PLUGE pattern, making it difficult
to properly set the black level. In some cases third party hardware solutions for these systems provide
a properly-formatted SMPUTE bar pattern that includes the PLUGE portion.
The following illustration shows what a correctly-formatted SMPTE bar pattern should look like. If your
system's built-in pattern does not contain the gray vertical bars in the lower right region, you must
capture bar from a signal generator and use that bar pattern. (Note: the brightness of the PLUGE
pattern has been exaggerated so that it is easier to see in a printed document.)
Though it is beyond the scope of this document to discuss the characteristics of an analog or digital video signal in detail, synchronization
voltages (either –286mV or –300mV, depending on the video format) and blanking voltages (0mV) are used to both align the various lines of
video in the analog signal and to turn off the electron gun drawing the image on a CRT monitor when the gun scanned from the end of one line
back to the beginning of the next.
Rise time is defined as the portion of the signal where the voltage transitions from blacking to picture and is an important timing in analog signal
transmission. Active picture is defined as the region of the image that contains video information; and he area outside of active picture is typically
referred to either as the blanking region (for horizontal areas) or the sync region (for vertical areas).
Using the HP DreamColor LP2480zx for professional video applications