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Limiter output
LA7454W
Loop filter
Fig. 5-7.
FM demodulator
diagram
Other filter
Demodulated output
Spike noise
y
elie Spsec
Hold pulse
RF SW
Fig. 5-8.
Sample and hold circuit waveform
5-3-4.
Dropout Detection and Mute Detection Circuit
The FM audio signal extracted by the BPF is level detected
by the dropout detection circuit (DO DET), but at the time
of dropout generation, the DO hold signal is sent to the S/H
circuit for holding. The pulsewidth of the DO hold signal is
detected by the mute detection circuit (MUTE DET). After a
certain period of time (500 sec), the mute signal is output.
The judgment time is determined by the capacitor of Pin
®.
c2001 Ss
"
BPF output
PG doubler output
——~———
—-~--+-
OFF
DELAY
r
DO DET output
aa] ||
a
I
i
$
en
500u
MUTE DET output
uJ
150 msec
'$$
|
Off delay output
1
The mute signal once turned on is held for a certain period
of time (about 150 msec) by the off delay circuit even if the
DO mute signal is turned off. The off delay circuit is so
designed that nine pieces of shift registers serve to make
the hold pulses at the point of head switchover clocks,
thereby causing the power off of the DO mute signal to be
delayed.
A function such as this prevents an audio signa! from being
sent out when the dropout is consciously generated over a
telatively long period
of time,
just as
in the case
o
playback of clogged one side head. The timing chart for
dropout compensation is shown in Fig. 5-9.
LA7454W
Fig. 5-9.
Dropout detection and mute detection circuit
5-8
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