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5-2-3.
Input Selection, ALC Circuit
Audio input consists only of input of the built-in microphone
and amplified signal is input from Pin @ of IC201 to the
ALC circuit by the microphone amplifier. (Pin @ is normally
High and microphone amplifier output is selected).
The ALC circuit Keeps excessive input to a certain fixed
value by detecting the output of the ALC amplifier with the
ALC detector and performing feedback control of the ALC
amplifier using DC voltage proportional to the input signal.
The ALC
transient response
(attack time and
recovery
time) is determined by the CR of Pin @.
During microphone input, recovery time is shortened by
strengthening the gain for maintaining of a wide coverage
range of 22 dB by sound erasure.
From Pin @ of ~—t
J1005
MIC AMP output
TEST
{Normally "H") c2o26 me
m
5-2-4.
Mute and Low-pass Filter Circuit
The
output of ALC
is fed to the low-pass filter (LPF)
through the R/P switch and the mute circuit.
The mute circuit mutes the audio signal when the mute
signal is sent to Pin @ from the SYSCON.
The
LPF
is a tertiary active filter built into the IC. It
eliminates the unwanted
band above 30 kHz to prevent
erratic noise reduction at a later stage.
In the PAL system the control signal input to Pin @ is fixed
to High and LP emphasis is not performed.
With this model the voltage of Pin © is fixed at 0.5% the
accuracy of R.
The
demodulation
sensitivity
of the
PLL
demodulator
coincides
with
the
modulation
sensitivity
of the
VCO.
Therefore, if the demodulation sensitivity is adjusted (fixed
at R) at Pin
@ during playback, adjustment of recording
time is performed at the same time.
Since in the PAL system @ is fixed to High, PAL is fixed to
SP.
AUDIO
MUTE
LA7454W
Fig. 5-2.
From ALC to LPF circuit
5-2-5.
Noise Reduction Circult (REC)
The output of the LPF is output from Pin @ and then fed to
Pins @ and @. The output of Pin @ is sent to the line
output and the earphone terminal as a monitor output.
The audio signal of Pin @) is fed to the + side of the MOA
(Main Operational Amplifier) of the noise reduction circuit
via R/P switch.
An emphasis circuit for the somewhat reduced high band is
inserted in the negative feedback loop of the MOA. The
output of the MOA Is therefore subjected to pre-emphasis
what elevates the band to a higher band, Further, as the
feedback amount
is either increased or decreased
in
proportion to the input level by the VCA (Voltage Control
Amplifier), the feedback
amount
of a large input level
increases and the output is held and that of a small input
level decreases
and
the output
is increased.
For this
reason, the dynamic range (a difference between a large
level and a small level) of the audio signal is compressed.
When the higher band component only of the audio signal
is input, the gain of the VCA
is increased
by waiting.
Therefore,
emphasis
2 is canceled,
and the amount
of
emphasis
is held, thereby preventing
the pre-emphasis
from becoming too large. The waiting and the emphasis 2
are primary filters of reverse
characteristics.
These are
mounted in the IC. Because the relative brightness in the
IC is good enough, perfect reverse characteristics can be
realized.
The transient response
of the noise reduction circuit is
determined by the capacitor at Pins @ and @, and the
attach/recovery time is determined by Pin @, and the hold
time by Pin @.
C2016 g
LPF output
peas
O-
Pee
1C201
LA7T454W
Fig. 5-3.
Noise reduction circuit (REC)
5-2-6.
LP Emphasis Circuit (REC)
The output of the noise reduction
circuit is fed to the
operational amplifier for the LP emphasis via R/P switch.
The LP mode of NTSC is only 10 um in track width, which
makes impossible to secure the sufficient signal-to-noise
ratio merely with the emphasis
of the noise
reduction
circuit. The LP emphasis is therefore added.
During the recording operation, pre-emphasis
is applied
because the emphasis time constant is inserted into the
feedback loop of the operational
amplifier. A resistor is
mounted in the IC, and the capacitor is connected to Pin
@. In the PAL system, the control signal input to Pin ®
fixed to High and LP emphasis is not performed.
5-2-7,
Limiter Circuit
The amplitude above a certain level of the output of the LP
emphasis is clipped by the LIM (limiter) via Pins ® and @,
and
prevented
from
being
an
over-modulation
by the
next-state modulator to avoid interference on the video
image.
LP emphasis
Limiter output
output(REC)
Noise reduction output
REC
1c201
LA7454W
(PAL is fixed
to SP.)
Fig. 5-4.
LP emphasis (REC)
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