Pioneer tuning fork Service Manual page 50

b. Small cassette
Metal tape has made the cassette small as shown in Fig. 38.
The volume is 1/4 ~ 1/6 of that of 1/2" cassette tapes.
The tapes of 13/xm and 10/an thick play 90min and 120min
( P A L : 60min and 90min) respectively. The playing time
can be doubled by shifting capstan motor revolution. Tape
guides have been removed from cassette shells and installed
in the player mechanism to avoid the effect of warped
shells. It has a large window for easy tape threading. The
tape protecting lids close like eye lids.
Sensor-beam
w i n d o w
15
Changer
grip
(a) Exterier view
c. 4F Automatic Track Finding
Narrow tracks made by high density recording m a k e ac­
curate tracking difficult. T o solve the problem, 8mm VCR
records 4F pilot signal on the diagonal video tracks together
with video signal instead of C T L signal. This has made
the tracking VR unnecessary.
The pilot signal frequency shifts to 6 . 5 f H , 7.5fH, lO.SFA
and 9 . 5 f H in turn track by track. With this order the fre­
quency difference between the center-track signal and the
cross-talk signal from an adjacent track is fu (16kHz) or
3fH (46kHz). Video heads pick up three of the 4F pilot
signals recorded on the center and the two side tracks, and
the tracking servo circuit compares the levels of the
frequency-difference signals between those of the center
and adjacent tracks and feeds the output of the compara­
tor, or 4 F tracking error signal, back to the capstan motor
negatively. In Fig. 39(a), if the tape slows down, the head
derail to the left and the level of 16kHz A f A ( f i ~ f 2 ) sig­
nal becomes higher t h a n that of 46kHz A f B ( f 2 ~ f 3 ) . Then
the level difference between the 16kHz signal and 46kHz
signal is applied to the capstan motor to turn it fast.
48
Lid
Window area
62.5
95
Rearlabel area
Erase-prevention nail
Fig. 3 8 8mm Cassette Tape
(ATF)
Features of 8mm video cassette
a. N o tape guide
b . Double-wall tape protector
c. Slack-preventing reel lock
d. Switchable erase-prevention t a b
e. Misload-safe framework
f. Tape-type sensor
g. Optical tape-end sensor
h. Indents for auto-changer arms
Wide opening
Sreel
Reel lock
Erase-prevention
hole
(b) Interior view
Here remains a problem. In this method the frequency of
the right and left difference signals shift to or from 16kHz
or 46kHz every time a video head moves to the next track
and it becomes difficult to find out whether the tape is fast
or slow.
W h e n an extra pilot signal is mixed with the three picked-
up pilot signals in the order of fi, f4, f 3 , fi and fi as
shown in Fig. (a), the difference signal or beat between
the applied signal and the right pilot signal always becomes
16kHz, and that between the applied signal and the left
signal always becomes 46kHz. This time the picked-up cen­
ter pilot signal is neglected. Then the side of tape velocity
error can be detected. The mixing is made in the tracking
servo circuit in playback.
When the pilot signals picked up by the solid-line-head are
compared,
The level o f [fi ~ (f4)] signal > the level o f [f3 ~ (f4)] signal
(46kHz)
The circuit judges that the head is deviated to the left and
puts out a signal to make the motor fast.
W h e n the head moves to the next track (dotted-line-head),
The level o f [f2 ~ (f3)] signal > the level o f [f4 ~ (f3)] signal
(46kHz)
The circuit still judges that the head is deviated to the left.
Tape-sensor-
beam path
T reel'
(16kHz)
(16kHz)