7.1.2 Y-carrier Ih/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
out 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 to 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 VHS (EP mode only)
shifts Y-carrier by fH/2(7.9kHz) every other track. The
beat frequency becomes an odd multiple of f n / 2 and the
interferring noise becomes invisible with visual integration
effect. Refer to p-13, Carrier Interleaving.
7 . 2 C-signal cross-talk suppression
Phase Inversion and Phase Shift
A n electric method for suppressing the C-signal crosstalk
is taken in addition to the azimuth recording method be
cause the latter is not effective to low-frequency converted
C-signal. N T S C 0 and 8mm VCRs employ Phase Inver
sion system and VHS and P A L 8mm VCRs employ Phase
Shift system.
The fact that 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 method records information by inverting signal
phase every other horizontal period (H) on A-tracks and /
Audio track
C-signal on track-A is*«
inverted every other H.
Control track
Phase of cross talk from-
track B
Signal phase on track A-
1H delay
40
Field A
1H
Track A
Playback signal
phase
Phase inverter
Cross-talk
cancelled
Fig. 25 Phase Inversion System
(a) H-sync aligned
(b) H-sync not aligned
Fig. 23 H-sync Alignment
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 on every other H
of A-track is reinverted and returned to the original.
Phase reinversion is unnecessary on 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
and added to the nondelayed signal.
d) Then the crosstalk signal is cancelled and the necessary
signal only is taken out because the cross-talk compo
nent of the delayed signal is 180° out of phase from the
nondelayed.
Field B
Audio track
Head B
Control track
1 H
Track B
1H delay
Phase of cross talk from
track A
Signal phase on track B
Delayed
track B signal