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5-3-5.
LP Emphasis Circuit (PB)
The demodulated output is subjected to noise rejection by
the sample and hold circuit and fed to the operational
amplifier for LP emphasis via R/P switch.
The operational amplifier serves merely as a buffer during
the playback, and the constant is an open circuit during the
emphasis, performing the de-emphasis operation. The LP
emphasis output is output from Pin @® via R/P switch and
fed once again to Pin @®.
Since
in the PAL system
Pin
@ is fixed to High,
LP
emphasis
is' not applied to it. Pins
@® and
can be
checked
for modulated
input during the recording
and
demodulated output during the playback.
LP emphasis
output (PB)
Demodulated
LA7454W
iad
{PAL is fixed
to SP)
Fig. 5-10.
LP emphasis circuit (PB)
5-3-6.
Mute Circuit and LPF Circuit
The audio signal fed to Pin @ is passed through the mute
circuit and the LPF circuit via R/P switch and output from
Pin @.
In the mute circuit, the audio, signal is muted by the. control ;
signal sent to Pin.@). from the SYSCON during the special
playback
and: the. mute.
signal. during.
generation
of a
dropout of longer than 500 sec.
In the LPF, spurious band above 30 kHz is eliminated, to
prevent erratic operation. of the next-stage noise reduction
circuit from occurring.
5-3-7.
Noise Reduction Circuit (PB)
The audio signal output from Pin @ is fed once again from
Pin @ and then fed to the MOA
of the noise reduction
circuit via R/P switch.
The MOA during the playback becomes a mere buffer, and
the emphasis circuit and the VCA circuit previously in the
feedback loop during the recording serve as open loop
circuits. Therefore, the de-emphasis and extension just the
reverse of those during the recording are made.
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LA7T454W
EMPHESIS-2
LPF output (PB)
EMPHASIS-)
Noise reduction
output (PB)
Fig. 5-11,
Noise reduction circuit (PB)
5-3-8.
Output Circuit
The noise reduction output is fed once again to the IC from
Pin @ , amplified to ~7 dBs by the fine amplifier via R/P
switch, and then output from Pin @. The line amplifier has
the capability to drive the earphone directly. The line output
and the earphone
output can be driven from the same
amplifier.
A shunt switch is built in Pin @®, and shorting is done to the
ground by the mute signal of Pin @ , utilizing the output
resistors of the line. By so doing, the shock sound is held
when the power is ON and OFF,
Noise
reduction
output{(PB)
SYSCON mute
Fig. 5-12,
Output circuit
5-10
The
line amplifier has large amplitude
level and large
power consumption during the earphone drive. The power
is supplied to it from Pin @, which is something different
from other circuits.
> Earphone out
Line output
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