Pioneer tuning fork Service Manual page 46

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1 1 . Trick P l a y
Trick play such as STILL, SLOW and
S E A R C H is possible by shifting tape speed. Tracking con­
trol becomes difficult when the tape speed varies because
the video heads are forced to trace wrong tracks where a
head becomes off-azimuth to the recorded signal.
There are two problems in trick play.
1) V-sync
W h e n the tape runs fast or slow, the relative tape-to-head
velocity is shifted and the number of scanning lines per
frame becomes smaller or larger than 525. Then the verti­
cal synchronization will be lost. To compensate this in trick
play quasi V-syncs are added next to the real V-syncs.
2) Azimuth
loss
Head-A picks up the signal on track A and head-B picks
u p that of head-B. When the tape speed is shifted, the video
heads trace tracks of wrong azimuth causing noise and
p o o r picture. There are two ways to solve this problem.
One is adjusting the phase of the tape to the heads with
a tracking variable resistor to make the heads trace the /
1 1 . 1 Frame STILL
In Frame Still, the picture vibrates when the image in the
original picture moves or the signal waveform of the field-B
differs from that of field-A because the screen displays
field-A and field-B alternately. The locus of heads and the
amplitude of reproduced R F signal are shown below.
1 1 . 2 Field STILL (Fine Still, Clear Still)
When head A and B trace track center as shown in Fig. 31
(b), the picture will become coarse because they trace only
one field although the picture does not vibrate and become
noiseless.
Tape direction
Tracking
area
(a) Frame still
1 1 . 3 S L O W
Simple SLOW: This just slows down the tape velocity. The
noise appears when a head crosses tracks.
Fine SLOW: This method is a combination of Field Still
and Step Forward and is employed in VHS VCRs. The ve­
locity shifts between Slow and Standard alternately.
44
P I C T U R E
track center and to make the crossing point of the track
boarder away from the center.
In this way the noise still remains in the picture. Another
is adding extra head(s). A n additional h e a d - B ' of o p p o ­
site azimuth to head A installed close to the main head A
will pick up the signal when A derails. A and A ' comple­
ment each other and read a complete field.
4-head system has two pairs of complementary heads. The
track width of the two additional heads of the 4-head VCR
has been m a d e narrow to meet slow mode tracks. There­
fore, when slowly recorded tape is played the additional
heads become the main heads.
1 1 . 4 x 2 and x 3 SPEED
In Fig. (c) A and B heads read imcomplete two fields.
Reproduced
RF signal
envelope
RFSWP
(b) Field still
2-head
3-head
4-head
VH-600
VE-D70
V X - 9 0
Fig. 3 0 Various Types of Head
RF signal
envelope
RF s w i t c h i n g
pulse (RFSWP)
(c) x 2 speed
id) x 3 speed
Fig. 31 Trick Play

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