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ed of L301
to L304 and C311
to C320 for elimination of undesirable residual switching
signal and through
a de-emphasis
network
consisting of R325,
R326, C321
and C322, into
the npn-pnp
direct coupled
audio amplifier, where
the signals are amplified
to a required
level for the output from J311
and J313.
From these jacks, the audio signals are led to the
TAPE
OUTPUT
jacks through the function switch.
Figure 1 presents an interna! block di-
agram showing the functions of the PLL basis MPX stereo decoding IC HA1156. The input
stereo composite signal, amplified by the audio amplifier, is delivered to the phase detectors
PD-1 and PD-2. A part of the stereo composite signal is also applied to the stereo decoder section.
The VCO (Voltage Control Oscillator) produces a free run oscillation in the neighborhood of
76KHz with the time constant determined by a capacitor C305 and resistors R311 and R312 set
on the outside of pin (4. The VCO output has its frequency divided into 19KHz through the two
stages of the frequency divider (D!V-1 & DIV-2), and is reverted to the phase detector PD-1,
which contains two input terminals designed to produce an output in proportion to the product
of the two input signals. The signal applied to one of the inputs of PD-1 is the 19KHz square
wave formed through frequency division of the 76KHz
VCO output signal by the two stages of
the frequency divider DIV-1
and DIV-2,
and the 19KHz
pilot signal included
in the stereo
composite signal as a reference signal is applied to the other input. Therefore, the output of
PD-1 which has passed through the low pass filter LPF-1 provides DC output voltage in proportion
to the phase variance between the two inputs. This DC output voltage is amplified by the DC
amplifier, and supplied to the 76KHz VCO as a control voltage. This means that the output
frequency and phase of the VCO have been phase-locked to the input pilot signal. The 38KHz
sub-carrier reproduced
by PLIk: as stated above is delivered through the stereo switch to the
stereo decoder section as a switching signal, thus driving the decoder section. One of the inputs
of PD-2 is given the 19K Hz resulting from the frequency division completed by DIV-1 and DIV-3,
whereas the other input gets the 19K Hz output contained in the composite signal, and the output
is provided with a DC output in proportion to the amplitude of the pilot signal. This DC output
is furnished through LPF-2 to the trigger amplifier which drives the stereo indicator lamp and
stereo switch. Therefore,
insufficient supply of the pilot signal results in failure to light the
stereo indicator and to turn on the stereo switch located in the path of the 38K Hz switching signal,
STEREO
COMPOSITE 6
SIGNAL
{NPUT
PHASE
DETECTOR
(PO-1)
AUDIO
19KHz
AMPLIFIER
STEREO
INDICATOR
LAMP
PAASE
DETECTOR
(PD-2)
DECODER
O
OR
AUDIO OUTPUT
TRIGGER
19KHz
STEREO COMPOSITE
SIGNAL
38KHz SQUARE WAVE
Figure 1. Block Diagram of the HA1156
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