Pioneer SX-750 Service Manual page 13

Multiplex Decoder
Demodulation is performed by switching detection.
A single IC (HA1196) composes the circuit, which
is divided into 3 sections. Fig. 3 shows the HA1196
block diagtam (see circuit diagram on page 65).
1. Switching signal generator
A PPL (phase locked loop) system is employed.
76kHz is generated by a VCO (voltage controlled
oscillator: oscillator in which the frequency is
varied by a control voltage) and converted into
19kHz by a frequency divider. This signal and
the pilot (19kHz) of the received signal are
applied to a phase comparator, which converts
the phase differences of the two signals into a
voltage. The voltage is then fed back to the VCO.
The oscillator signal phase becomes locked to
the pilot signal by this loop (PLL) and a 38kHz
signal synchronised to the pilot signal is obtained
and employed as the switching signal.
2. Automatic stereo detector
With the PLL locked to the pilot signal, the pilot
signal and a 19kHz signal of the same phase are
produced. A voltage is then obtained at the
phase comparator that is proportional to the
pilot signal amplitude. As it increases, the lamp
lights and the switch becomes on. The switching
signal is applied to the demodulator.
3. Demodulator
This is a switching circuit employing two dif-
ferential amplifiers (Fig. a). Q1 and Q2 are
altemately switched on and off by the switch-
Com0osit
siFel
ing signal. The composite signal is amplified at
Q3, switched and demodulated. Q6 and QB are
Ioosely coupled at their emitters by R1
- R3.
QG is driven in reverse phase to Q3. This is
switched at Q4 and Q5, and by composing with
Q1 and Q2 at the collector, crosstalk becomes
cancelled.
Adequate current flow is required to QB and
Q6 to improve distortion figures at this point.
However, if the base bias voltage is raised, the
voltage component at the collector becomes
reduced and clipping occurs (power supply
voltage is limited by IC voltage endurance).
For this reason, current from an external source
is inserted at Q3 and Q6 collectors to become
11 and Ir. The same current amounts are ob-
tained as 13 and Ia from the emitters. Q3 and
Q6 therefore operate with adequate current,
and distortion
at this stage becomes remark-
ablly improved. A feedback amplifi.er amplifies
the demodulated output.
Y c c
S w i t c h i n g
s i g n a l ( 3 8 t l | r ) - *
Com0osite
signal
F i g . 4 E q u i v a l e n t C i r c u i t o f D e m o d u l a t o r
A f a m ! .
Phasc
conlaratol
1 4
F i g . 3 B l o c k D i a g r a m o f H A 1 1 9 6