DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

If you use CTI, you run the risk of introducing excessive
emphasis on edges. Also, CTI operates only on the horizontal
dimension: Excessive CTI is liable to become
visible owing to perceptible (or even objectionable)
differences between the horizontal and vertical characteristics
of the image. CTI works well on cartoons, and
on certain other types of images. However, it should be
used cautiously.
Scan-velocity modulation (SVM)
CRT displays are subject to limitations of how fast the
electron beam can change from one state to another
(for example, from on to off). These limitations are
imposed by limited bandwidth of the video amplifiers
and by the electrical capacitance that the CRT’s cathode
or grid presents to the driving circuitry. These limitations
are reflected as limited spatial resolution of the
reproduced image: An edge is reproduced across a horizontal
dimension wider than desired.
One way to reduce the dimension of an edge involves
making the electron beam position responsive to
changes in beam intensity. If intensity is increasing
rapidly as the beam goes left-to-right, the beam can be
accelerated. If intensity is decreasing rapidly, the beam
can be decelerated so that it dwells longer on areas of
the screen that require high intensity. This process is
scan-velocity modulation (SVM). Luma (or some comparable
quantity) is processed through a high-pass filter;
the result is amplified and applied to the horizontal
deflection circuit of the CRT.
Note that all of the red, green, and blue beams are
deflected together: The technique is effective only on
black-to-white or white-to-black transitions. In
a magenta-to-green transition, both red and blue are
negative-going, even though luma is increasing: In this
example, SVM has unintended effects on red and blue.
Mixing and keying
Mixing video signals together to create a transition, or
a layered effect – for example, to mix or wipe – is called
compositing. In America, a piece of equipment that
performs such effects is a production switcher. In
CHAPTER 27 VIDEO SIGNAL PROCESSING 333