Pioneer tuning fork Service Manual page 35

2 . 3 Tape T r a c k Pattern
Fig. 8 shows the track patterns of 1/2" V H S and (3 types.
Both 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
CM
Guard
track
CTL
head
7 9 . 2 4 4
M i n i m u m distance
between video/control
heads
3. Helical Scanning S y s t e m
The video signal to be recorded is in the frequency range
from D C to 4.2MHz (NTSC), 200 times as broad 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"
VCR and 8mm VCR heads are approximately 0.5/rni and
0.25/mi respectively. With a stationary audio head, a sig­
nal is recorded on a track parallelly to the tape direction.
To record video signal with an A C R , the tape velocity
should be 9.5m/sec, too 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- /
Tape guide
Video tape
(a) Front view
Audio track (ch 1)
Audio track (ch 2)
E E
Video track (ch 1)
Video track (ch 2)
Direction
Video field center
(a) VHS
Fig. 8 Track Patterns of 1/2" VC Tapes
Viewed from the Drum
Upper drum
Tape guide
Direction
Lower drum
Fig. 9 Dram and Tape
points of video tracks at the b o t t o m edge and diagonal vid­
eo tracks between them.
M i n i m u m distance
between video/control
heads
6 8 . 0
CTL head
Switching point
(b) (3
ning system has been developed. By scanning the tape
diagonally with revolving two heads, the relative tape-to-
head velocity can be made high enough keeping absolute
tape velocity low. The heads lay down diagonal tracks at
an angle of around 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 made with 262.5 lines, NTSC) and
the following B-track contains the next interlacing field,
and successive two fields compose a complete TV picture
(525 lines NTSC).
Rotary head drum
Video head A
(b) Top view
Audio track (ch 1)
Direction
Audio track (ch 2)
Track B
0 . 0 5 8 5
CTL track
Unit: m m
Video head B
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