Samsung DVD-V940K Training Manual page 74

Circuit Operating Descriptions
7-3 Servo
(1) Outline
The servo system is divided into three loops. The cylinder servo controls the rotation of video heads, the capstan
servo controls the tape speed, and the tension. In addition it's necessary to control cylinder motor, especially
during trick play in 4H'D models. The tension servo maintains the tape tension constant: it keeps the compression
strength of tape against the video heads at the optimum level so that a stable RF signal is produced during
recording and playback. The tension servo operation is entirely mechanical. The cylinder servo loop controls the
phase and speed of the cylinder motor. The speed is kept at a constant 1500 RPM and the
phase determines the mechanical position relative to the vertical Sync signal. The capstan servo loop controls the
phase and speed of the capstan motor so that the video head can trace the video track correctly.
It keeps tape speed constant according to the mode (SP, LP)during playback and recording.
MOTOR
CYLINDER
(VIDEO
HEAD)
(4H' D)
CAPSTAN
(4H' D)
7-16
Table 7-3 : Servo System Signal
SYSTEM MODE
REC
PHASE
PB
SPEED COMMON
SPEED& TRICK PLAY
PHASE (STILL. SLOW)
REC
PHASE
PB
SPEED
COMMON
SPEED& TRICK PLAY
PHASE (STILL. SLOW)
REFERENCE
SIGNAL
V-SYNC
REF25Hz
8MHz
MICOM CONTROL CYLINDER SPEED
TO MATCH H-SYNC SPEED
DIVIDED CFG PULSE
CTL 25Hz
8MHz
MICOM CONTROL CAPSTAN DRIVE
SIGNAL WITH STEP SLOW AND CAP C.L
COMPARISON SIGNAL
SW 25Hz
CYLINDER FG(500Hz)
REF 25Hz
CAPSTAN FG