b. Small cassette
Metal tape has made the cassette small as shown in Fig. 38.
T h e volume is 1/4 ~ 1/6 of t h a t 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 m o t o r revolution. Tape
guides have been removed from cassette shells and installed
in the player mechanism to avoid the effect of warped
shells. It h a s a large window for easy tape threading. The
t a p e protecting lids close like eye lids.
(a) Exterier view
Features of 8 m m 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
c. 4F Automatic Track Finding (ATF)
Narrow tracks made by high density recording m a k e ac
curate tracking difficult. T o solve the problem, 8 m m VCR
records 4F pilot signal on the diagonal video tracks together
with video signal instead of C T L signal. This has m a d e
the tracking VR unnecessary.
The pilot signal frequency shifts t o 6
. 5 f H ,
7.5fH, lO.SFA
and 9
. 5 f H
in t u r n track by track. With this order the fre
quency difference between the center-track signal and the
cross-talk signal from a n adjacent track is fu (16kHz) or
3fH (46kHz). Video heads pick u p 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 o u t p u t of the c o m p a r a
tor, or
4 F
tracking error signal, back to the capstan m o t o r
negatively. In Fig. 39(a), if the t a p e slows d o w n , the head
derail to t h e left and t h e 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 ) . T h e n
the level difference between the 16kHz signal a n d 46kHz
signal is applied to the capstan m o t o r to t u r n it fast.
H e r e remains a problem. In this m e t h o d 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-
u p pilot signals in the order of fi, f 4 , 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 t h a t between the applied signal a n d the left
signal always becomes 46kHz. This time the picked-up cen
ter pilot signal is neglected. T h e n t h e side of t a p e velocity
error can be detected. The mixing is m a d e in the tracking
servo circuit in playback.
W h e n the pilot signals picked u p by the solid-line-head are
compared,
The level o f [fi ~ (f4)] signal > the level o f [f3 ~ (f4)] signal
(46kHz) (16kHz)
The circuit judges that the head is deviated to the left and
puts out a signal to m a k e the m o t o r 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) (16kHz)
T h e circuit still judges that the head is deviated to the left.
48
Lid
Sensor-beam
w i n d o w
Changer
grip
(b) Interior view
Fig. 3 8 8mm Cassette Tape
Window area
62.5
95
15
Rearlabel area
Erase-prevention nail
Reel lock
Erase-prevention
hole
Wide opening
Tape-sensor-
beam path
T reel'
Sreel
