Daewoo DUB-2850GB Service Manual page 43

5.2.5 CVBS and Y/C input signal selection
5.2.5.1 ALL VERSIONS
The ICs have 3 inputs for external CVBS signals. All CVBS inputs can be used as Y input for
the insertion of Y/C signals. However, the CVBS(Y)2 input has to be combined with the C3
input. It is possible to add and extra CVBS(Y/C) input via the pins which are intended to be
used for YUV interface (or RGB/YPrPb input). The selection of this additional CVBS(Y/C) input
is made via the YC bit.
The function of the IFVO/SVO/CVBSI pin is determined by the SVO1/SVO0 bits. When used as
output a selection can be made between the IF video output signal or the selected CVBS signal
(monitor out). This pin can also be used as additional CVBS input. This signal is inserted in
front of the group delay / sound trap circuit. It is also possible to use the group delay and sound
trap circuit for the CVBS2 signal (via the CV2 bit).
For the CVBS(Y/C) inputs the circuit can detect whether a CVBS or Y/C signal is present on the
input. The result can be read from the status register (YCD bit in subaddress 03H) and this
information can be used to put the input switch in the right position (by means of the INA-IND
bits in subaddress 38H). The Y/C detector is only active for the CVBS(Y)3/C3, CVBS(Y)4/C4
and CVBS(Y)x/Cx inputs. It is not active for the CVBS(Y)2/C3 input.
The video ident circuit can be connected to all video input signals. This ident circuit is
independent of the synchronisation and can be used to switch the synchronisation and can be
used to switch the presence of a video signal (via the VID bit). In this way a very stable OSD
can be realised. The result of the video ident circuit can be read from the output bit SID
(subaddress 00).
5.2.6 Synchronisation circuit
The IC contains separator circuits for the horizontal and vertical sync pulses. To obtain an
accurate timing of the displayed picture the input signal of the sync separator is not derived
from the various CVBS/Y or RGB/YPrPb inputs but from the YOUT pin. For this reason the
YOUT pin must be capacitively coupled to the YSYNC pin. The delay between the various
inputs and the YOUT signal can have rather large differences (e.g. comb filter active or not). By
choosing the YOUT signal as input signal for the sync separator these delays have no effect on
the picture position. Only for RGB signals without sync on green the input of the sync separator
has to be connected to one of the CVBS inputs. This selection is made by means of the SYS
bit.
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 the clock signal coming from the
reference oscillator of the TCG -Controller.
To obtain a stable On-Screen-Display (OSD) under all conditions it is important that the first
control loop is switched off or set to low gain when no signal is available at the input. The input
signal condition is detected by the video identification circuit. The video identification circuit can
automatically switch first control loop to a low gain when no input signal is available. This mode
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CP-780 Service Manual