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DVI/VGA to 3G/HD/SD Scan Converter User Guide
runs of 0’s and 1’s. Under normal conditions, only very short runs of these are produced due to a
scrambling algorithm that is used. The Checkfield, also referred to as the Pathological test signal, will
“undo” the scrambling and cause extremely long runs to occur. This test signal is very useful for testing
transmission paths.
Chroma
The color or chroma content of a signal, consisting of the hue and saturation of the image. See also
Color Difference.
Component
In a component video system, the totality of the image is carried by three separate but related
components. This method provides the best image fidelity with the fewest artifacts, but it requires
three independent transmission paths (cables). The commonly used component formats are
Luminance and Color Difference (Y/Pr/Pb), and RGB. It was far too unwieldy in the early days of color
television to even consider component transmission.
Composite
Composite television dates back to the early days of color transmission. This scheme encodes the
color difference information onto a color subcarrier. The instantaneous phase of the subcarrier is the
color’s hue, and the amplitude is the color’s saturation or intensity. This subcarrier is then added onto
the existing luminance video signal. This trick works because the subcarrier is set at a high enough
frequency to leave spectrum for the luminance information. But it is not a seamless matter to pull
the signal apart again at the destination in order to display it or process it. The resultant artifacts of
dot crawl (also referred to as chroma crawl) are only the most obvious result. Composite television is
the most commonly used format throughout the world, either as PAL or NTSC. It is also referred to as
Encoded video.
Color Difference
Color Difference systems take advantage of the details of human vision. We have more acuity in our
black and white vision than we do in color. This means that we need only the luminance information to
be carried at full bandwidth, we can scrimp on the color channels. In order to do this, RGB information
is converted to carry all of the luminance (Y is the black and white of the scene) in a single channel.
The other two channels are used to carry the “color difference”. Noted as B-Y and R-Y, these two signals
describe how a particular pixel “differs” from being purely black and white. These channels typically
have only half the bandwidth of the luminance.
Decibel (dB)
The decibel is a unit of measure used to express the ratio in the amplitude or power of two signals. A
difference of 20 dB corresponds to a 10:1 ratio between two signals, 6 dB is approximately a 2:1 ratio.
Decibels add while the ratios multiply, so 26 dB is a 20:1 ratio, and 14 dB is a 5:1 ratio. There are several
special cases of the dB scale, where the reference is implied. Thus, dBm refers to power relative to 1
milliwatt, and dBu refers to voltage relative to .775V RMS. The original unit of measure was the Bel (10
times bigger), named after Alexander Graham Bell.