Sony DTC-A8 / PCM-2600 / PCM-2800 Service Manual ▷ View online
— 67 —
SECTION 6
SERVO CIRCUIT
6-1. Outline of Servo Circuit
Fig. 6-1. Outline of Servo Circuit
As shown in the outline in
Fig 6-1
, the servo block is composed of the capstan servo which controls the tape running
speed so that it becomes constant using the capstan FG (CFG) signal, the ATF servo which controls so that the head
scans the designated track correctly using the ATF signal from the playback track, the reel servo which controls the high
speed tape running to the constant liner speed during FF/REW and search using the reel FG (TFG, SFG) signals, and the
drum servo which controls the number of rotations of the drum to the constant speed using the FG and PG (DFG, DPG)
signals from the drum, etc.
A software servo adopting the servo processor (mechanism microprocessor, IC502 CXP87352) which incorporates the
built-in special hardware is used for these servos.
The AGC PWM signal in the diagram prevents incorrect ATF servo operations caused by the difference in the recording
current of the played back tape by performing automatic adjustments (ATF AGC) of the RF amplifier gain.
scans the designated track correctly using the ATF signal from the playback track, the reel servo which controls the high
speed tape running to the constant liner speed during FF/REW and search using the reel FG (TFG, SFG) signals, and the
drum servo which controls the number of rotations of the drum to the constant speed using the FG and PG (DFG, DPG)
signals from the drum, etc.
A software servo adopting the servo processor (mechanism microprocessor, IC502 CXP87352) which incorporates the
built-in special hardware is used for these servos.
The AGC PWM signal in the diagram prevents incorrect ATF servo operations caused by the difference in the recording
current of the played back tape by performing automatic adjustments (ATF AGC) of the RF amplifier gain.
RF AMP
CXA 1364R
IC1
FG
FG,PG
MECHA MICON (SERVO PROCESS)
IC502
84
CPWM
CAPSTAN
SERVO
OPERATION
CFG
69
78
PBDT
ATF
SYNC DET
TRACKING
ERROR DET
66
A/D
CONVERTER
ATF
PWM
OUT
PWM
OUT
80
AGC PWM
PWM
OUT
PWM
OUT
REEL
SERVO
OPERATION
81
82
TPWM
SPWM
67
68
TFG
SFG
PWM
OUT
DRUM
SERVO
OPERATION
83
70
71
DREF
72
DPWM
79
SWP
21
DREF
DAT DSP
CXD 2605Q
IC503
39
SWP
SWP
CREATION
DFG
DPG
CAPSTAN DRIVER
DRUM DRIVER
T/S REEL DRIVER
— 68 —
This unit uses a head drum with a diameter of 30 mm.
Table 6-1
shows the specifications of drum rotation and tape speed in each mode.
In the LP mode, the drum is rotated at 1000 rpm during the recording mode and at twice the rotation of 2000 rpm during
the playback mode as shown in the table. As a result, the same head playback output as the SP mode is obtained.
Because the transmission rate during LP mode playback is the same as that of the SP mode, there is not need to change
the constant of the playback EQ (equalizer) and PLL circuit for each mode.
Although the head will not be on-track accurately according to the recording track pattern during playback, because it
will have the chance to read the same track twice, the playback data of the playback data without error after error
correction will be selected to generate the audio playback signal.
In the LP mode, because the drum rotation is changed between the recording mode and playback mode, the relative
speed is different. For this reason, during playback, the trace angle of the head in respect to the recording track pattern
on the tape becomes slightly small. Therefore even if after-recording of the sub codes is performed at the tape speed, the
original sub code area will not be written properly. For this reason, during the after-recording of the sub codes in the LP
mode, the tape speed is set to the same speed as the SP mode (8.15 mm/s) to record with the recording track pattern
corresponding to the head trace angle.
the playback mode as shown in the table. As a result, the same head playback output as the SP mode is obtained.
Because the transmission rate during LP mode playback is the same as that of the SP mode, there is not need to change
the constant of the playback EQ (equalizer) and PLL circuit for each mode.
Although the head will not be on-track accurately according to the recording track pattern during playback, because it
will have the chance to read the same track twice, the playback data of the playback data without error after error
correction will be selected to generate the audio playback signal.
In the LP mode, because the drum rotation is changed between the recording mode and playback mode, the relative
speed is different. For this reason, during playback, the trace angle of the head in respect to the recording track pattern
on the tape becomes slightly small. Therefore even if after-recording of the sub codes is performed at the tape speed, the
original sub code area will not be written properly. For this reason, during the after-recording of the sub codes in the LP
mode, the tape speed is set to the same speed as the SP mode (8.15 mm/s) to record with the recording track pattern
corresponding to the head trace angle.
Table 6-1. Specifications When Diameter 30 mm Drum is Used
Transmission Rate
(Mbps)
9.408
9.408
9.408
4.704
9.408
9.408
Tape Speed
(mm/s)
8.150
8.150
8.150
4.075
4.075
8.150
Capstan Rotation
(rpm)
77.8
77.8
77.8
38.9
38.9
77.8
Drum Rotation
(rpm)
2000
2000
2000
1000
2000
2000
SP
LP
Mode/
Recording
Playback
Sub code after-recording
Recording
Playback
Sub code after-recording
SP : Standard Play Mode
LP : Long Play Mode
Note)The shaft diameter of the capstan motor used by this unit is 2 mm.
LP : Long Play Mode
Note)The shaft diameter of the capstan motor used by this unit is 2 mm.
— 69 —
6-2. Sensor Amplifier (CX20115A) IC3
The CX20115A is a FG, PG sensor amplifier for the capstan and drum systems capable of operating at a single 5V
power supply.
power supply.
Fig. 6-2.
shows the block diagram and pin configuration.
Fig. 6-3. PG Level Shift Circuit
Fig. 6-2. CX20115A Block Diagram and Pin Configuration
Description of operations
• Preamplifier
The preamplifiers in the both capstan and drum systems amplify FG and PG output voltages which are several mVp-
p to several 10 mVp-p (fine) to several 100 mVp-p to several Vp-p which is best suited for performing waveform-
shaping. The output of this preamplifier is added to the comparator with Schmidt.
p to several 10 mVp-p (fine) to several 100 mVp-p to several Vp-p which is best suited for performing waveform-
shaping. The output of this preamplifier is added to the comparator with Schmidt.
• Limiter amplifier
The limiter amplifier is used for absorbing the AM deviation of the FG signal. The output is subjected to zero cross
detection by the comparator and waveform shaping.
detection by the comparator and waveform shaping.
• Drum system
As several 10 mV outputs can be obtained from the drum FG signal, the signal is directly applied to the limiter
amplifier which also serves as the preamplifier. The output carries out zero cross detection and is output from Pin 3
after waveform shaping. Because the drum PG signal has an output level which is below 1 mV, to slice the output
using the comparator regardless of the direct current offset of the preamplifier output, the slice level is synthesized
from the output. This is the level shift (
amplifier which also serves as the preamplifier. The output carries out zero cross detection and is output from Pin 3
after waveform shaping. Because the drum PG signal has an output level which is below 1 mV, to slice the output
using the comparator regardless of the direct current offset of the preamplifier output, the slice level is synthesized
from the output. This is the level shift (
Fig. 6-3.
).
16
15
1
CAP FG
AMP
11
12
14
LIMITER
COMPARATOR
CAP FG
COMPARATOR
10
CAPSTAN
FG x 2
LOGIC
LOGIC
9 CAP FG OUT2
13
8
GND
3
LEVEL
SHIFT
7
6
PG COMPARATOR
BUFFER
DRUM FG
AMP
2
4
PG AMP
DRUM FG COMPARATOR
NC
CAP FG IN2
DRUM COM
DRUM FG IN
DRUM PG IN
PNCLO
DRUM
PG OUT
Vcc
CAP FG OUT1
CAP
LIM IN
CAP
AMP OUT
CAP
FG IN1
DRUM
FG OUT
+
–
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