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CIRCUIT DESCRIPTION
The detector output of the FM tuner (with
the de-emphasis and attenuator circuits re
moved) is fed to the high impedance input stage
(OC44) of the decoder. The low impedance
emitter output of this stage drives a diode ring
demodulator (4
x
IS44) via a 55Kc/s low-pass
filter to remove any 67Kc/s sub-carrier signals
present. The values of the low-pass filter are
also chosen to effect overall phase compensation
of the composite signal.
The diode ring demodulator is switched by the
balanced output of a 38Kc/s oscillator (OC81).
The left and right hand channels are recovered
directly, but as harmonics of the difference sig
nal sub-carrier are removed during transmission
the difference signal is reduced to 2/T( times the
sum signal. The equality of the sum and
difference signals is restored by subtracting
1-2/T( times the sum signal from each channel,
the necessary reversed phase signal being taken
from the input stage collector circuit.
A tuned circuit in the input stage collector
feeds the 19Kc/s pilot tone to the pilot tone
amplifier (OC8l). The push-pull output of this
stage is rectified (2
x 1
S920) in order to bias the
diode signal switches (4
x
IS44) to the "Stereo
On" position. The 38Kc/s rectifier current
pulses are used to synchronise the locally
generated 38Kc/s switching signal with the pilot
tone.
The main supply voltage, stabilised by Zener
diodes (2
x
IS2075), is supplied from the con
trol unit when the RADIO button is pressed.
This supply also biasses the diode signal
switches to MONO. If the STEREO button on
the control unit is pressed the "Blue HT"
supply switches the mono signal off, and the
pilot tone amplifier on. If a pilot tone is present
the decoder is thus switched to produce a stereo
output.
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