Advertisement
7.
SERVO SYSTEM DESCRIPTION
7-1 . OUTLINE
In the VTR, the rotation of the drum motor and the capstan
motor is controlled by the servo circuit.
• I n recording mode, the drum servo controls the rotation of
the drum motor so that the rotational phase of the drum
motor is always synchronised with the phase of the video
signal to be recorded. Thus the vertical sync signal
component of the video signal is recorded at a specific
tape position.
Moreover, the rotation of the capstan motor is controlled to
be at a constant speed by the capstan servo. Therefore,
the tape speed becomes constant and the signals are
recorded onto the tape at the proper pitch.
• In playback mode, the drum servo controls the rotation of
the drum motor so that it is at a constant speed in order to
obtain a playback signal with no time base fluctuations.
The capstan servo controls the capstan so that the video
head correctly traces the tracks which are recorded onto
the tape in order to obtain a playback signal of the
maximum level.
For 8 mm video, a system called ATF has been adopted for
tracking.
Rotation sensor
Sensor amplifier
ATF detection
Speed/phase detection
Servo signal processing
Motor driver
Motor
7-3. DRUM SERVO SYSTEM
7-3-1 . Sensors and Amplifier
A printed coil type detection element is used to detect the
rotation of the drum motor, and the following two types of
coil are provided.
The D FG (Drum Frequency Generator) coils are mounted
along the circumference of the drum at intervals of 30
degrees in the rotational angle to obtain a 1 2-cycle sine
wave from each drum rotation.
A DPG (Drum Phase Generator) sensor coil is mounted at a
specified position along the circumference of the drum to
obtain a 1 -cycle pulse for each drum rotation.
The DPG functions as a marker to show the physical drum
mounting position. The respective waveforms are shown in
Fig. 7-1 .
Drum System
Print coil type sensor (PG/FG)
LB1 885M
LB1 885M
Inverse induction voltage detection-type
brushless motor
- 4-42 -
7-2. CONFIGURATION
For the configuration of the servos, refer to the block
diagram. Both servo systems can be basically divided into
the following blocks.
a) Detection sensors
These include the FG/PG sensor, sensor amplifier, video
head and playback amplifier (ATF).
b) Detection processing block
Includes phase and speed signal generation, ATF error
signal generation and phase compensation circuit
c) Motor drivers
d) Motors
The servo circuits are included on the CP1 circuit board.
The circuits are almost all ICs. The ICs, etc. , which
comprise the various blocks are shown in Table 7-1 .
The CXP80724 is a control microprocessor which was
developed for servo control. Apart from servo control, it is
also responsible for system control and mechanism control.
The mechanism control circuits are also included on the
SV-1 circuit board.
CXP80724
Hall element detection - type
brushless motor
Table 7-1 .
Because the drum motor rotates at 25 rps during normal
operation, the frequencies are as follows.
DFG
1 2 x 25
300 Hz
=
=
DPG
25 Hz
=
The DFG (Pin @ ) and the DPG (Pin @ ) which are output
from LB1 885M (IC381 ) are input to Pins @ and @ of
CXP80724 (IC351 ) respectively.
Capstan System
MR sensor
LB1 857M
XR-1 082
LB1 857M
Advertisement
