3.
The original approach to recording the higher video frequencies was simply to
increase the speed of the tape so that the wavelength became a usable size. Video
recorders that use extremely narrow gap heads and very rapidly moving tape are
called longitudinal recorders.
Longitudinal video recorders have many drawbacks.
Primarily, the picture quality (resolution) is poor because the active read/write
speed is limited by the tape-handling capabilities of the transport including
problems of spooling the tape and avoiding tape overthrow.
Also, an excessive
amount of tape is required.
4.
Another approach to recording of the higher video frequencies consists of
pulling the tape at a practical speed past a rotating head, resulting in an
increase of head-to-tape velocity.
Systems that use the rotating head principle
also apply other techniques to improve the recording of video signals.
a. One of these techniques is to use frequency modulation (FM) for recording
the signal information.
The FM technique solves the problem of bandwidth.
The
largest frequency span that can be accommodated by direct recording is 10 octaves
(15Hz - 15KHz).
Since the video signal has a span greater than 18 octaves (0 -
4mHz), direct recording is not practical.
b. This difficulty is overcome by using a frequency modulator to change the
video information into an FM signal, with a full lower sideband.
c. In one application the video signal is frequency modulated onto a carrier
from 1 to 7mHz wide a span of only 3 octaves.
The reduction in the span of
frequencies is important in the designing of the inductors used in the
record/playback heads.
The use of FM permits the recording of the signal at
5.
The constant level FM can be amplified and limited in playback to reduce the
effect of signal dropout. Signal dropout is due primarily to tape surface defects.
These tape surface defects result in poor contact between the head and tape
resulting in substantial loss of signal (fig 1-4). Another technique is to allow
the head to penetrate or intimately contact the tape.
Figure 1-4.
Effect of surface defects
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