Pioneer tuning fork Service Manual page 42

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