1.3.3 Playback characteristic of video-head
In recording and playback, high frequency signal c o m p o
nents are decreased by spacing loss, core loss, thickness
loss, etc. as seen in Fig. 6. T o minimize the loss the head-
gap width should be 1/3 or shorter t h a n the shortest signal /
wavelength. T o m a k e the playback frequency characteris
tic flat, compensation is made with a resonant circuit con
sisting of head coils and capacitors and resistors in the
input stage of pre-amplifier.
Fig. 6 High Frequency Compensation by Preamplifier
2. Video Tape
2 . 1 Structure
There are several kinds of tapes classified by uses; broad
casting, professional and home uses. Their dimensions and
magnetic materials are different each other. H o m e tapes
are made of
G O 2
and CR-FEIOS. Coated metal tape for
8mm video high density recording is now available, and
vacuum-evaporation tape is under development. The coat
ed tape is basically the same as the audio t a p e . T h e back-
coated tape shown in Fig. 7 has a c a r b o n layer on the re
verse side for minimizing electric resistance and charge and /
for improving winding characteristic. Some have a bind
ing layer between the magnetic material and base. Vacuum-
evaporation tape is m a d e by evaporating a high retentive
material a n d depositing it on a base in a v a c u u m cham
ber. After t h a t a protective layer of 100A thick is formed
for protecting the material against wear and corrosion.
Pollyester of high Y o u n g ' s modulus and good tempera
ture characteristic is popularly employed for the base.
Fig. 7 Cross-section of Video Tapes
2 . 2 Performance of V i d e o Tapes
The following performance is required to video tapes:
a) Low tape-width variation
b) Hard-wearing
c) Less d r o p o u t s
d) O p t i m u m friction coefficient
e) High mechanical strength
Tape
C o -
7
F e
2
0
3
Metal
Evaporation-
deposited
Bs (Gauss)
1800
2 0 0 0
Br (Gauss)
1500
2 3 0 0
He (Oe)
6 8 0
1500
*
Thickness of
magnetic layer
5
2.5
0.15-0.2
Entire
thickness (/^m)
20
13
13
Use
1/2"
8 m m
8 m m
Table 4 Specifications of Tapes
32
Spacing loss
Core loss
Gap loss
Thickness loss
Playbac
k
amplitud
e
+6dB/OCT
5MHz
Frequency
Am
p
gai
n
5MHz
(a) Tape head frequency response
(b) Compensating
Magnetic layer
Coating layer
Base
• Coating
- Protector
- Evaporation-deposited
magnetic layer
-Base
Coating
