9 .
S e r v o S y s t e m ( N T S C )
Precise servo system for capstan and d r u m m o t o r is re
quired to d r a w a specified pattern on t h e tape in record
ing and t o trace it correctly in playback.
T h e revolution of the D r u m or Head-disc should always
b e at 1800rpm and t h a t of Capstan should be at a speci
fied value depending on the t a p e velocity m o d e . A 30Hz
control (CTL) signal is recorded as a reference for play
b a c k . The C a p s t a n , therefore, should move the tape at a
velocity t o keep the reproduced control signal at 30Hz at
a n y tape m o d e . A-head should meet the beginning of A-
track when C T L level rises. Fig. 28 and 29 are simplified
diagrams of D r u m a n d C a p s t a n servo blocks.
Drum servo system:
In recording, the signals generated by Drum-frequency-
generator (DFG) is speed-detected, a n d the error signal,
or difference signal, is negatively fed back(NFB) to the
D r u m - m o t o r . The signal generated by Drum-pulse-
generator ( D P G ) is compared with a reference signal (V-
sync/2) m a d e from the video input signal, a n d the error
signal is also NFBed t o the m o t o r .
In playback, the internally oscillated 3.58MHz signal is
used instead of the video input signal for making the
reference.
Drum FG, PG
Speed Det.
SWP
generator
Video Input
V
sync.
Sep.
Phase
Comp
Motor
Drive
Drum Motor
RF switching Pulse
3.58MHz
X'tal OSC
Divider
PB
Fig. 28 Block Diagram of Drum Servo Circuit
Capstan servo system:
In recording, the c a p s t a n - F G ( C F G ) signal generated in
Capstan-motor is speed-detected and is NFBed to the
C a p s t a n - m o t o r . The C F G signal is compared with the V-
s y n c / 2 reference signal, and t h e error signal is NFBed to
the m o t o r . Some models use the reference signal m a d e
from the 3.58MHz signal instead of the V-sync/2. The dot
ted block of Fig. 27 are peculiar to the types indicated.
In playback, the reproduced C T L signal is phase-compared
with the reference signal m a d e from the 3.58MHz signal,
and the error signal is NFBed to the m o t o r . The C F G sig
nal is speed-detected and NFBed as above. As a result, the
F G a n d C T L signals are phase-compared with the help of
the reference signal m a d e from the 3.58MHz signal, and
the error signal controls the Capstan-motor t o m a k e the
movement of H e a d s and tracks inphase.
42
REC
