points of video tracks at the b o t t o m edge and diagonal vid
eo tracks between t h e m .
Unit: m m
3.
H e l i c a l S c a n n i n g S y s t e m
The video signal to be recorded is in the frequency range
from D C t o 4 . 2 M H z (NTSC), 200 times as b r o a d as that
of audio signal. The highest recordible signal frequency
depends on the gap-width of recording/playback head and
writing velocity or tape-to-head velocity. The gaps of 1/2"
V C R and 8 m m VCR heads are approximately 0.5/rni and
0.25/mi respectively. W i t h a stationary audio head, a sig
nal is recorded on a track parallelly t o the tape direction.
T o record video signal with an A C R , the t a p e velocity
should be 9.5m/sec, t o o fast to record for a long time. A
120C audio cassette tape can record video signal only for
35 seconds at 9.5m/sec. T o solve the problem, helical scan- /
ning system has been developed. By scanning the tape
diagonally with revolving two h e a d s , the relative tape-to-
head velocity can be m a d e high enough keeping absolute
tape velocity low. T h e heads lay d o w n diagonal tracks at
an angle of a r o u n d 5° to the edge of the tape. All tracks
laid down by the A ( C H l ) - h e a d are called " A ( C H 1 ) -
t r a c k s " and all those laid down by the B(CH2)-head are
called "B(CH2)-tracks". One A-track contains one full TV
field (a coarse picture m a d e with 262.5 lines, NTSC) and
the following B-track contains t h e next interlacing field,
and successive two fields compose a complete TV picture
(525 lines N T S C ) .
Rotary head drum
Tape guide
Upper drum
Tape guide
Video tape
Lower d r u m
Direction
Video head A
Video head B
(a) Front view
(b) Top view
Fig. 9 Dram and Tape
33
2 . 3 Tape T r a c k Pattern
Fig. 8 shows the track patterns of 1/2" V H S a n d (3 types.
B o t h of t h e m have two tracks for analog audio signals at
the t o p edge, a control track for indicating the starting
Audio track (ch 1)
Audio track (ch 2)
Video track (ch 1)
Video track (ch 2)
Video field center
Direction
E E
CM
Guard
track
CTL
head
7 9 . 2 4 4
M i n i m u m distance
b e t w e e n video/control
heads
(a) V H S
(b) (3
Fig. 8 Track Patterns of 1 / 2 " VC Tapes
Viewed from the Drum
CTL head
S w i t c h i n g point
M i n i m u m distance
b e t w e e n video/control
heads
Direction
Audio track (ch 1)
Audio track (ch 2)
CTL track
Track B
0 . 0 5 8 5
6 8 . 0
