When recording, the video signal is applied to b o t h A- and
B-heads. In playback, the picked-up signals coming from
A - and B-heads are alternately switched to m a k e one con
tinuous signal channel as shown in Fig. 10. T o m a k e one
track surely contain one full field information, the tape
wraps the head d r u m a little longer t h a n 1/2 t u r n . W h e n
connecting the two signal, switching noise is generated. To
avoid this, t h e connection is performed a b o u t 7 H before
V-sync appears or at the b o t t o m of the screen where the
A- and B-tracks overlap each other. The heads rotates at
l,800rpm to produce 30 frames (1500rpm, 25 frames in
P A L ) of pictures a second.
V i e w e d f r o m video head
Overlap
Overlap
Noise
• 1 V -
CH-1 input
CH-2 input
Switching pulses
CH-1 output
CH-2 output
Added
1 frame
1 frame
Fig. 10 Connecting CH1 and CH2 signals
with a Switching Circuit
1 frame
Fig. 11 Field and Frame
34
Overlap
Track B
Track A
S w i t c h i n g points
Control track
Audio track
V-sync
Switching point
1H
Field A
(track A)
Field B
(track B)
