T h e features of V C R not found in A C R are as follows:
a) Broad frequency b a n d w i d t h ( D C - 4 . 5 M H z )
b) Helical scanning system for achieving high tape-to-head
relative velocity
c) The frequency b a n d w i d t h of Y-signal compressed by
frequency m o d u l a t i o n ( F M ) .
d) C-carrier down-converted t o achieve easy phase control
High frequency signal is liable to be affected by jitter
which causes color distortion.
e) Guard-bandless double-azimuth scanning system to
achieve high density recording
f) Phase Invert (PI) or Phase Shift (PS) system to reduce
C-signal cross talk
(a) U-loading System
1. Structure o f V C R
VCR consists of a tape transport mechanism, a d r u m scan
ner, video-, audio-, control- and erase-heads, electronic
circuits for processing TV signal and servo control systems.
1 . 1 T A P E TRANSPORT M E C H A N I S M
Fig. 1 shows the M-threading (VHS) and U-threading (13)
mechanisms. T h e tape is pulled out of cassette a n d wraps
a half of the periphery of the tilted d r u m . It travels in 3
dimensions and is twisted a little, while audio cassette tape
travels on a plane. The mechanism requires high accuracy
because it deals with signals of megahertz order. The height
and tilt of the D r u m and tape-guides should be properly
adjusted.
Drum
(b) M-loading System
U-loading system: In this system the twisting a m o u n t is
smaller t h a n that of the M-loading system, and the tape
travels smoothly. Quick shifting of the functions and view
ing pictures in F F a n d R E W m o d e is possible because the
tape is threaded right after loading.
Full-erase-head
M-loading system: The tape length required for threading
is shorter t h a n that of U-loading system although twisting
is larger. Shifting functions takes 2 or 3 seconds because
threading is m a d e after P L A Y is depressed. Video heads
a n d tape wear well because the tape does not touch the
heads in F F and R E W modes. However, Picture Search
is m a d e after threading.
Audio-head
Tape
Pinch-roller
Fig. 1 Tape Transport Mechanism
30
Drum
Loading ring
Guide roller
Capstan
Guide roller
H
S reel
T reel
Tilted post
Guide roller
Tilted post
Guide roller
H
Capstan
S reel
T reel
Capstan
Control-head
Drum
(c) Simplified Tape Path