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參數(shù)資料
| 型號： | LA7578 |
| 文件頁數(shù)： | 10/18頁 |
| 文件大?。?/td> | 252K |
| 代理商： | LA7578 |
Fig. 8
Normal video signal
Horizontal sync
norrowed by ghost
Fig. 9
Video Signal
Sync rises
Fig. 10
Fig. 11
Pins 3 and 13 (IF AGC pins)
Pins 3 and 13 are the IF AGC filter pins. The signal for which
the peak was detected by the AGC detector is smoothed by pin
13 (1st AGC) and pin 3 (2nd AGC) to generate the AGC
voltage. A signal which has passed through an audio trap is
used for the video signal that is input to this IF AGC detector.
(1) These are typical AGC filter constants.
Single time
constant
Double
time
constant
330 pF
2.2 k
0.47 μF
0.068 μF
820 k
Double
time
constant
330 pF
1.8 k
0.1 μF
0.047 μF
820 k
Pin 3
C1
R1
C2
C3
R2
330 pF
Pin 13
0.47 μF
820 k
(2) Mute (IS-15 switch)
The black noise canceller can be cut by setting pin 13 to
1 V or less. When doing so, the AGC voltage must be
applied to pin 3 from an external source to drive the AGC
system. ( IS-15 supported)
(3) Ghost problems
In a signal that produces ghosts, an interfering signal with
a different phase overlaps with the desired signal, altering
the video signal as shown in Fig. 9. The width of the sync
signal narrows, making it impossible to maintain the IF
AGC charge/discharge current ratio. If the phase change of
the ghost interference wave increases, the symptom that
appears is that the vertical sync portion rises as shown in
Fig. 10. The countermeasure for alleviating this symptom
is to insert a resistor (820 k
to 1 M
) between pin 13
and GND.
Pin 4 (RF AGC VR)
Pin 4 is the RF AGC adjusting pin.
This pin sets the RF AGC operation point for the tuner.
(1) Mute
By setting this pin to 0.5 V or less, the FM output and the
video output can be muted simultaneously.
LA7578N
No.4037-10/18
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