7
the 110 degree types a choice must be made between the AVL function and a sub-carrier
output for comb filter applications. This choice is made via the CBM0 and CMB1bits (in
subaddress 22H). When the AVL is active it automatically stabilises the audio output signal to a
certain level. The signal on the deemphasis pin (28) can be supplied to the SCART connector
via a buffer stage. It is also possible to use this pin as additional audio input. In that case the
internal signal must, of course, be switched off. This can be realised by means of the sound
mute bit (SM in subaddress 29H). When the IF circuit is switched to positive modulation the
internal signal on the deemphasis pin is automatically muted. The audio switch is controlled by
means of the INA/INB bits in subaddress 22H. It is, however, also possible to overrule the
audio switch position by means of the ADS and ADX bits (subaddresses 28H and 21H
respectively).
2A.5: SYNCHRONISATION CIRCUIT :-
The IC contains separator circuits for the horizontal
and vertical sync pulses and a data-slicing circuit which extracts the digital teletext data from
the analog signal. The horizontal drive signal is obtained from an internal VCO which is running
at a frequency of 25 MHz. This oscillator is stabilised to this frequency by using a 12 MHz
signal coming from the reference oscillator of the micro controller/Teletext decoder. The
horizontal drive is switched on and off via the soft start/stop procedure. This function is realised
by means of variation of the T ON of the horizontal drive pulses. In addition the horizontal drive
circuit has a ‘low-power start-up’ function. The vertical synchronisation is realised by means of
a divider circuit. The vertical ramp generator needs an external resistor and capacitor. For the
vertical drive a differential output current is available. The outputs must be DC coupled to the
vertical output stage. In the types which are intended for 90 degree
picture tubes the following
geometry parameters can be adjusted:
•
Horizontal
shift
•
Vertical
amplitude
•
Vertical
slope
•
S-correction
•
Vertical
shift
The types which are intended to be used in combination with 110 degree
picture tubes have an
East-West control circuit in stead of the AVL function.
