c) The returned signal is delayed by 1H and is added to
the nondelayed signal. Then t h e cross-talk c o m p o n e n t
is cancelled because the cross-talk c o m p o n e n t in the
delayed signal becomes 180° o u t of phase from that of
the nondelayed.
The P I and PS are performed b y a subconverter which
generates a local carrier for converting frequency. (Fig. 12)
Phase returned
counterclockwise
Fig. 2 6 Phase Shift System
Output
8.
Other N o i s e S u p p r e s s i o n
8 . 1 1H Correlative Noise Cancellor
The Comb-filter eliminates r a n d o m noise a n d the beat
caused by the fn/2 shifting. It suppresses vertical image
lines on the screen from flickering by minimizing 1H cor
relative noise, or the signal components unsimilar to those
of the adjacent scanning lines. lH-delayed signal is sub
tracted from the current 1H signal, and noise components
only are taken out and negatively fed back t o the current
signal. Then the noise components only are cancelled.
8 . 2 Other Noise Cancellor
High frequency noise can not completely be eliminated by
preemphasis a n d deemphasis.
This circuit suppresses the noise appearing in the monot- /
Input
onous portion of pictures. In the high-pass-filter ( H P F )
in Fig. 27, the sharp edges of Y-signal are differentiated
and allowed to pass with high frequency noise components.
Their amplitude is clipped in Limiter, and the input signal
is subtracted by the limited high frequency signal. The cut
off frequency of H P F and the limiting level of the input
signal is determined taking picture quality and S / N into
consideration. Then the noise components are cancelled.
8 . 3 Narrow-track Video Head
T o avoid scanning t w o tracks and picking up crosstalk
some VCRs employ another pair of short-gap (narrow-
track-width) video heads in addition t o the standard pair
and m a k e the former specialize in slow-speed m o d e .
Output
Fig. 27 Noise Cancellor
41
7.2.2 Phase Shift (PS) (NTSC)
a) In recording the signal on A-tracks is shifted counter
clockwise by 90° ( 1 / 4 fH) in every H period and t h a t
o n B-tracks clockwise by 9 0 ° .
b) In playback the phase is restored; the phase of A-tracks
is shifted by 90° clockwise and that of B-tracks coun
terclockwise H by H .
Track B : The phase turns clockwise.
Track A : Signal phase turns countercolckwise.
Color signal
phase
Signal phase on track A
Cross-talk phase
Phase returned
clockwise
Output
1H DL
Phase shifter
Signal phase on track B
Cross-talk phase
IH-DL
1H DL
Phase shifter
H.P.F.
Limiter
