A and B Rolls
The process used to create optical effects, such
as dissolves or wipes in film or videotape. The
A roll contains the outgoing scene and the B roll
contains the incoming scene. The point where the
A and B scenes begin to overlap is the start of
the effect. The length of the overlap is known as
the duration of the effect. See Figure 10 on page
24.
Analog
See Analog vs. Digital Recording Methods on Page
54.
ASCII
American Standard Code for Information Interchange.
A standard for data coding, particularly in computers.
It is a list of encoded letters, numbers, characters,
and other symbols similar to those on a standard
typewriter keyboard. It is pronounced as "AS-KEY."
topˆ
Auto
Assembly
The computer assisted compilation of a series
of edits. Information on each edit’s source,
duration, and nature may be fed into the computer
from punched paper tape or a floppy disk. Auto-assembly
is the fastest method of editing videotape since
all creative decisions have been made and previously
entered into the computer for the auto assembly.
topˆ
Black
Level
The video signal level corresponding to black
areas in a scene. For a composite signal, black
is standardized at +7.5 units as viewed on a waveform
monitor IRE scale. This elevated black level is
often referred as "setup" and serves
as a guard band between video and sync. For component
video, black is at 0 units. See Figure 22. topˆ
Blanking
The process of turning off the electron scanning
beam of a camera or picture tube so it will not
be seen while it repositions itself for the next
scan of a field or line. There are two forms of
blanking pulses in a television signal. The horizontal
(H) blanking pulse cuts off the beam during the
retrace period from the right to left side of
the picture. The vertical (V) pulse cuts off the
beam as it moves from the bottom of the screen
back to the top to start the scan of the next
picture field. Blanking widths are chosen to provide
adequate time for beam retrace in a practical
TV set. Since the width and position of the blanking
pulses (relative to sync) establish the heighth,
width, and position of the picture as viewed on
a monitor, their values must be carefully monitored.
This is particularly important when switching
between camera and/or VT sources to obviate shifts
of the picture area. topˆ
Burst
(Color)
A color reference signal included as part of the
overall composite video signal. Eight to ten cycles
of color sub-carrier (3.579545 MHz, often abbreviated
as 3.58) are inserted before the start of every
horizontal line. It can be seen just following
the H sync pulse in the H blanking interval. It
provides color-synchronizing information for the
color decoding circuits in monitors, receivers
and other TV equipment. These have "color
killer" circuits that disable the color decoded
if the burst is not present so that the signal
is processed as monochrome. The burst must be
precisely timed in relation to H sync. See SCH.
See Figure 23. topˆ
Chrominance
(Chroma)
The color information in a television picture.
Seen on a waveform monitor as the color subcarrier
riding on top of the luminance signal. Low chroma
means that the color picture is pale looking or
washed out. Excessive or high chroma means that
the color is too intense and has a tendency to
bleed into surrounding areas, contaminating nearby
colors or in sever cases, causing video breakup.
See Hue, Saturation. topˆ
Clipping
A form of video distortion. It is seen as a loss
of detail in the black or white areas of the picture.
It may be caused by excessive video levels that
cannot be handled by the television system. To
avoid these problems in transmission or recording
situations where such overloads could cause serious
problems in transmission or recording situations
where such overloads could cause serious problems,
controls are provided on cameras, proc amps, and
recorders that allow an operator to constructively
clip extraneous peaks. Example: A camera is viewing
a scene that contains bright lights and/or specular
reflections. The video operator would adjust the
camera for appropriate skin tone levels and allow
the extraneous super-white peaks to be clipped.
See Compression. topˆ
Color
Bars
An electronically generated standard set of colors
used as a reference for proper equipment setup.
Color bars include the three additive primary
colors (red, green, and blue) and their complements
(cyan, magenta, and yellow) displayed in vertical
rows, plus gray and black. The bars appear left
to right in order of decreasing luminance –
yellow, cyan, green, magenta, red, and blue. Videotape
machines, cameras, telecine chains, and monitors
all use color bars as a reference for proper setup.
The waveform can be examined on a waveform monitor
or vectorscope to verify that the encoding process
was proper and/or that it was not changed by any
subsequent transmission or recording process.
Color bars are readily available and easily interpreted
and so have become a de-facto set standard. There
are several variations of color bars. Full field
bars show the color bars running the full height
of the screen. EIA (Electronic Industries Association)
bars assigns the bottom third of the pattern to
the –I, Q, and black level set signals,
which are useful for setup of camera encoders.
SMPTE bars are the same as EIA, but insert another
set of short color bars above the I and Q bars.
These bars run in the reverse color order of the
regular bars and are a convenient aid for the
setup of color monitors. All versions allow the
overall amplitude of the subcarrier (chrominance)
to be set at 100% or 75% of the standard value.
The 75% value is usually used for recording to
avoid possible overload problems from highly saturated
yellows and cyans. The amplitude of the gray bar
can be set at 100% or 75% IRE units. See Figure
24. topˆ
Color
Frame
The color subcarrier (SC) frequency is chosen
to minimize its appearance in monochrome receivers.
A side effect of this choice is that the SC phase
is not contiguous between frames since the color
frame sequence takes 2 frames to complete. If
color disturbances are to be avoided, seen as
horizontal picture shifts, edits should be made
only between even numbered time code frames or
between odd numbered frames. Editing systems automatically
made these choices. topˆ
Component
Video
A form of video, in which the luminance and chrominance
signals are generated, transmitted and/or recorded
on videotape as two independent signals, usually
on individual video tracks. Unlike composite video,
where the chrominance and luminance signals are
combined as one signal; component video signals
retain maximum bandwidth and avoid mutual interference
through many generations of editing or duplication.
topˆ
Composite
Video
A video signal that combines luminance and chrominance
by using one of several world wide electronic
encoding methods, such as NTSC, PAL, or SECAM.
Encoding a video signal reduces the bandwidth,
and therefore the resolution, since a single channel
is used to carry all the information. Current
practice is to use composite video signals for
most applications, although component video editing
is gradually replacing composite editing. topˆ
Compression,
Amplitude
The lack of gray scale separation in signal levels.
A lack of detail in the white areas of the picture
is called white compression and a lack of detail
in the very dark areas is called black compression.
However, controlled white compression is often
introduced in camera signal processing to show
detail in white areas that might otherwise have
to be clipped. See Clipping. topˆ
Control
Track
A guide pulse acting as an electronic sprocket
hole recorded on the videotape. Control track
pulses are used by servo systems to maintain a
tape speed that allows precise playback head tracking.
Control track pulses are recorded one per television
frame and are used in lieu of time code by very
basic editing systems to locate edit points and
make edits. Missing control track signals may
cause the video signal to break up or mistrack.
This type of problem is analogous to that resulting
from torn sprocket holes on film. topˆ
Cue Track
A secondary audio channel used to record time
code or other non-program information. topˆ
|
