6
attenuation of the residual video signals. With this system a high performance hi-fi stereo
sound processing can be achieved. The AM sound demodulator is realised by a multiplier. The
modulated sound IF signal is multiplied in phase with the limited SIF signal. The demodulator
output signal is supplied to the output via a low-pass filter for attenuation of the carrier
harmonics. The AM signal is supplied to the output (pin 44) via the volume control. It is possible
to get the AM output signal (not controlled on amplitude) on the QSS intercarrier output. The
selection is made by means of the AM bit in subaddress 29H. Another possibility is that pin 35
is transferred to external audio input pin and pin 32 to (non-controlled) AM output pin. This can
be realised by means of the setting the control bits CMB0 and CMB1 in subaddress 22H.
2A.4: FM DEMODULATOR AND AUDIO AMPLIFIER (MONO VERSIONS):-
The FM
demodulator is realised as narrow-band PLL with external loop filter, which provides the
necessary selectivity without using an external band-pass filter. To obtain a good selectivity a
linear phase detector and constant input signal amplitude are required. For this reason the
intercarrier signal is internally supplied to the demodulator via a gain controlled amplifier and
AGC circuit. The nominal frequency of the demodulator is tuned to the required frequency
(4.5/5.5/6.0/6.5 MHz) by means of a calibration circuit which uses the clock frequency of the
Controller/Teletext decoder as a reference. The setting to the wanted frequency is realised by
means of the control bits FMA and FMB in control byte 29H. When required an external sound
band-pass filter can be inserted in front of the narrow-band PLL. In that case pin 32 has to be
switched to sound IF input by means of the bits SIF (subaddress 21H) and CMB0/CMB1
(subaddress 22H). When the sound IF input is selected the subcarrier output or AVL function
are not available. From the output status bytes it can be read whether the PLL frequency is
inside or outside the window and whether the PLL is in lock or not. With this information it is
possible to make an automatic search system for the incoming sound frequency. This can be
realised by means of a software loop which switches the demodulator to the various
frequencies and then select the frequency on which a lock condition has been found. The
deemphasis output signal amplitude is independent of the TV standard and has the same value
for a frequency deviation of 25 kHz at the 4.5 MHz standard and for a deviation of 50 Khz for
the other standards. The audio control circuit contains an audio switch and volume control. In
the mono intercarrier sound versions, the Automatic Volume Levelling (AVL) function can be
activated. The pin to which the external capacitor has to be connected depends on the IC
version. For the 90 degree types the capacitor is connected to the EW output pin (pin 20). For
