Figure 13-3.–Basic principles of a motion-video camera.
Tape Format Systems
The classification of a videotape recorder VTR by
tape width was particularly important in the earlier days
when the quality of the videotape recording was directly
related to the tape format. The old standard used to be,
the wider the tape, the higher the quality of the
recording. Anything smaller than the l-inch videotape
was considered small format and inferior in quality.
Today, 1/2-inch Betacam SP can provide equal or
superior quality compared to the large-format, l-inch
machines. The Hi8 video camera (8mm) is superior to
the 1/2-inch VHS cameras. Today, “small format” is
used mainly to describe small, highly portable television
equipment, such as small camcorders. Like all
state-of-the-art electronic equipment, smaller no longer
implies inferior quality.
The quality of the tape itself has much to do with
the quality of the picture. No matter how sophisticated
the video hardware, the resulting picture is only as good
as the videotape being used.
Videotape is a ribbon of polyester film base coated
with magnetic iron-oxide particles. The surface of the
tape, or emulsion side, that faces the video recorder
heads is highly polished to maximize tape-to-head
contact and to minimize wear on the heads.
Head clogging results when oxide comes off the
tape and gets caught in the head gaps of the recorder. If
the tape clogs the video recording heads, you cannot
play back or record. Normally, the heads will clog after
recording or playing back half a dozen or so tapes. You
should have the heads cleaned according to the
manufacturer’s recommendations or according to
Planned Maintenance System (PMS) requirements.
Videotape dropout occurs when a piece of magnetic
oxide or coating on the tape flakes off or is rough,
causing a “hole” or line of missing information in the
picture when it is viewed on the monitor. Dropout
appears on the TV screen as little black or white lines,
darting across the picture. The main causes of dropout
are dirty heads or imperfections in the tape. Once
dropout occurs, it cannot be replaced or corrected on the
tape.
There are no black-and-white or color videotapes.
Any videotape will record either black and white or
color. Black and white or color depends solely on
whether the camera and monitor are black and white or
color.
THE VIDEO CAMERA
Refer to figure 13-3 to help clarify how a video
camera operates. In the video camera, an image (a) is
gathered by the camera lens (b), and focused on the face
of the camera pickup tube (photocathode) or a
solid-state imaging device (c). The face or screen of the
photocathode is covered with thousands of light
sensitive dots. As light from a particular part of the scene
falls on each dot, the dot becomes electrically charged.
A charge pattern is built up proportionally to the
brightness of the scene. An electron beam in the pickup
tube emits a steady beam of electron particles. This
electron beam scans the charged pattern on the
photocathode and reads over it in a series of lines. The
scanning beam neutralizes each picture element or dot
and produces varying electrical currents (the video
signal). These currents are proportional to the charge
pattern which are proportional to the light transmitted
through the lens.
The current or video signal (picture) is amplified (d)
and then recorded on tape by rotating heads (e) and then
converted back to visible screen images in the
viewfinder (f).
As each dot on the tube screen is scanned, the dot
gives up its information and is wiped clean so the tube
screen can respond to any new light it receives.
13-6
