Pioneer tuning fork Service Manual page 43

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 that
on B-tracks clockwise by 90°.
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 A : Signal phase turns countercolckwise.
Color signal
phase
Signal phase on track A
Cross-talk phase
Phase returned
clockwise
1H DL
Output
8. Other N o i s e Suppression
8 . 1 1H Correlative Noise Cancellor
The Comb-filter eliminates random noise and 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 to 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 and deemphasis.
This circuit suppresses the noise appearing in the monot-
Phase shifter
Fig. 26 Phase Shift System
H.P.F.
Input
Fig. 27 Noise Cancellor
c) The returned signal is delayed by 1H and is added to
the nondelayed signal. Then t h e cross-talk component
is cancelled because the cross-talk component in the
delayed signal becomes 180° o u t of phase from that of
the nondelayed.
The PI and PS are performed by a subconverter which
generates a local carrier for converting frequency. (Fig. 12)
Track B : The phase turns clockwise.
Phase shifter
1H DL
onous portion of pictures. In the high-pass-filter (HPF)
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 two tracks and picking up crosstalk
some VCRs employ another pair of short-gap (narrow-
/
track-width) video heads in addition to the standard pair
and m a k e the former specialize in slow-speed mode.
Output
Limiter
Signal phase on track B
Cross-talk phase
Phase returned
counterclockwise
IH-DL
Output
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