the system switches automatically to a gated black level AGC. Because a black level clamp pulse is
required for this way of operation the circuit will only switch to black level AGC in the internal mode.
The circuit contains a video identification circuit, which is independent of the synchronisation circuit.
Therefore search tuning is possible when the display section of the receiver is used as a monitor.
However, this Ident circuit cannot be made as sensitive as the slower sync Ident circuit (SL) and we use
both Ident outputs to obtain a reliable search system. The Ident output is supplied to the tuning system
via the I 2 C-bus. The input of the identification circuit is connected to pin 24, the internal CVBS input.
This has the advantage that the Ident circuit can also be made operative when a scrambled signal is
received (descrambler connected between the IF video output (pin 16) and pin 24). A second advantage
is that the Ident circuit can be used when the IF amplifier is not used The video Ident circuit can also be
used to identify the selected CBVS or Y/C signal. The switching between the 2 modes can be realised
with the VIM bit. The IC contains a group delay correction circuit, which can be switched between the
BG and a flat group delay response characteristic. This has the advantage that in multi-standard
receivers no compromise has to be made for the choice of the SAW filter. Also the sound trap is
integrated within the IC .The centre frequency of the trap can be switched via the I 2 C-bus. For mono-
FM versions it is possible to obtain a demodulated IF video signal which has not passed the sound trap
so that an external stereo decoder can be driven. This function is selected by means of the ICO bit (sub-
address 28H). The signal is available on pin 27 (audio output pin when ICO = 0). The S/N ratio of the
selected video signal can be read via the bits SN1/SN0 in sub-address 03H.
2.2.
Video Switches
The circuit has an input for the internal CVBS signal and 2 inputs for external CVBS or Y signals. The
circuit has only 1 chroma input so that it is not possible to apply 2 separate Y/C inputs. The switch
configuration is given in Fig. A. The selection of the various sources is made via the I 2 C-bus. The
QFP-64 version has 2 independently switchable outputs. The CVBS1O output is identical to the
selected signal that is supplied to the internal video processing circuit and can therefore be used as
source signal for a teletext decoder. Both CVBS outputs have an amplitude of 2.0 VP-P . The CVBS2O
output can for instance be used as drive signal for a PIP decoder. If the Y/C-3 signal is selected for one
of the outputs the luminance and chrominance signals are added so that a CVBS signal is obtained
again.
2.3.
Sound Circuit
The sound IF amplifier is similar to the vision IF amplifier and has a gain control range of about 66 dB.
The AGC circuit is related to the SIF carrier levels (average level of AM or FM carriers) and ensures a
constant signal amplitude of the AM demodulator and the QSS mixer. A multiplier realises the single
reference QSS mixer. In this multiplier the SIF signal is converted to the intercarrier frequency by
mixing it with the regenerated picture carrier from the VCO. The mixer output signal is supplied to the
output via a high-pass filter for attenuation of the residual video signals. With this system a high
performance hi-fi stereo sound processing can be achieved. To optimise the performance of the
demodulator the offset can be compensated by means of an I 2 C-bus setting. 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 27) 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 sub-address 29H. Another possibility is that
pin 11 can be used as external audio input pin and pin 49 can be used as (non-controlled) AM output
pin. This can be realised by means of the setting the control bits CMB0 and CMB1 in sub-address 22H.
2.4.
Synchronisation circuit
The sync separator is preceded by a controlled amplifier, which adjusts the sync pulse amplitude to a
fixed level. These pulses are fed to the slicing stage, which is operating at 50% of the amplitude. The
separated sync pulses are fed to the first phase detector and to the coincidence detector. This
coincidence detector is used to detect whether the line oscillator is synchronised with the incoming
signal and can also be used for transmitter identification. This circuit can be made less sensitive by
means of the STM bit. This mode can be used during search tuning to avoid that the tuning system will
stop at very weak input signals. The first PLL has a very high statically steepness so that the phase of
the picture is independent of the line frequency. The horizontal drive signal is generated by an internal
VCO, which is running at a frequency of 25 MHz. This oscillator is stabilised to that frequency by
using the 12 MHz frequency of the crystal oscillator as a reference. The time-constant of the first loop
