7 . 1 . 2 Y-carrier I h / 2 Shifting (NTSC)
T h e horizontal sync pulses are aligned in the track pattern
when recording in /3 1-hour or VHS 2-hour mode as shown
in Fig. 23 (a). With this pattern the cross talk H-sync does
n o t interfere with the main H-sync because they are read
o u t at the same time. In the 13 2-hour and V H S 4-hour,
however, the H-syncs are not aligned. The H-syncs advance
or delay from those of the adjacent tracks by 0.75H or
0.25H as shown in Fig. (b). The H-sync cross talk produces
vertical stripes on the screen as shown in Fig. 24 because
azimuth recording is not so effective t o the deeply record
ed sync siglal which is much lower t h a n Y-signal in fre
quency. T o avoid this trouble
0
and V H S (EP m o d e only)
shifts Y-carrier by fH/2(7.9kHz) every other track. The
beat frequency becomes an o d d multiple of f n / 2 and the
interferring noise becomes invisible with visual integration
effect. Refer t o p-13, Carrier Interleaving.
(b) H-sync not aligned
Fig. 2 3 H-sync Alignment
7 . 2 C-signal cross-talk suppression
P h a s e Inversion and P h a s e Shift
A n electric m e t h o d for suppressing the C-signal crosstalk
is taken in addition to the azimuth recording m e t h o d be
cause the latter is not effective t o low-frequency converted
C-signal. N T S C
0
and 8mm VCRs employ P h a s e Inver
sion system and V H S a n d P A L 8mm V C R s employ P h a s e
Shift system.
T h e fact t h a t the signal waveforms on the two adjacent
lines are almost the same to each other makes the follow
ing noise cancelling process possible.
7.2.1 Phase Inversion (PI) Fig. 2 3 .
a) This m e t h o d records information by inverting signal
phase every other horizontal period (H) on A-tracks and /
Fig. 2 4 Vertical Stripes
leaving the phase uninverted throughout B-tracks. When
playing back, the reproduced signal contains cross-talk
from an adjacent track.
b) In playback the phase of the signal o n every other H
of A-track is reinverted and returned t o the original.
Phase reinversion is unnecessary o n B-track because the
signal on B-tracks has not been inverted when record
ing.
c) The phase-returned signal on A-tracks is delayed by 1H
a n d added to the nondelayed signal.
d) T h e n the crosstalk signal is cancelled and the necessary
signal only is taken out because the cross-talk c o m p o
nent of the delayed signal is 180° out of phase from the
nondelayed.
C-signal on t r a c k - A is*«
inverted every other H.
Field B
Playback signal
phase
Cross-talk
cancelled
Phase of cross talk from
track A
Signal phase on track B
Fig. 25 Phase Inversion System
40
(a) H-sync aligned
Field A
Phase of cross talk from-
track B
Signal phase on track A-
Head B
1 H
Track B
Delayed
track B signal
1H delay
Audio track
Control track
Audio track
Control track
Track A
1H
Phase inverter
1H delay
