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3DQDVRQLF
2.2.3 AUDIO SIGNAL PROCESSING (LA71511M)
The circuitry of AUDIO part is similar to that of the conventional AUDIO part in case of EE and PB mode,
but in REC mode, due to the internal operation of self-alignment, it shows a lot of differences.
1. EE MODE
LA71511M has 3-input VIDEO/AUDIO switching circuitry internally, and its switching is controlled by the
serial data dispatched from MICOM.
But for the suitable operation to these models, only 2 inputs are used. The AUDIO signal divided by the
resistor, R204 and R205, is supplied to the 73rd pin of LA71511M. On the other hand, the signal divided by
R206 and R207 is supplied to the 75th pin of LA71511M. Its level is automatically controlled by ALC and
then the amplified signal through LINE AMP is obtained at the 77th pin.
The signal from C208 is supplied to REC AMP after the divider circuit (R208, R209, R210) and also to the
LINE OUT.
The ALC point can be adjusted by R208 and R209, the adjustment of which is closely related to the REC
level in REC mode.
The ALC time can be adjusted by R203 and C202.
2. PB MODE
The PB signal picked up from AUDIO HEAD is firstly processed in the frequency characteristic compensator
which is composed of R220 and C214 (EP: C214+C215) and then supplied to the 7th pin.
The input signal passes through EQ AMP, and LINE AMP, so its output signal is finally obtained from the
77th pin.
The circuitry and its operation of EQ AMP is identical to the conventional EQ AMP.
3. REC MODE
(1) AUDIO SWITCHING CIRCUIT
When the AUDIO S/W signal at the 80th pin is "H", the point at the AUDIO HEAD of the external
switching circuit is opened electrically (EE/PB=GND), so the COIL starts to oscillated.
R218
5
Bias Coil
5th pin
(2) As long as the voltage difference between VCC and the 5th pin is maintained at 2.0~4.3V, the COIL
oscillates. In other words, the COIL maintains to oscillate only if the voltage at the 5th pin is 0.7~3.0Vp-p.
Especially, when the R/P and FE HEAD impedance is at the center, the voltage at the 5th pin should be
maintained at 1.85Vp-p, and, if not, when the HEAD impedance is MAX or MIN, the voltage at the 5th pin is
liable to deviate the ranges of 0.7~3.0Vp-p.
The higher the FE HEAD impedance is or the lower the R/P HEAD impedance is, the higher the voltage at
the 5th pin increases.
The AC signal is put on the DC voltage at the 5th pin, and it shows the internal AUTO BIASing.
The control signal at the 6th pin controls the TR(Q205) to ON/OFF, by which the AUTO BIASing is
controlled.
Rec: Open
EE: GND
R216
Q203
R217
Q204
External Switching Circuit
Vcc
R215
Q202
46
R214
80
A-H
S/W
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