Barco GRAPHICS 1209S, Service Manual

Page: 80 / 378

Date: 10/02/97 R762745
QUAD Comb_Filter R762745
13
b) RGB .
The selection of an RGB source simply means that the SYNC-EX signal is passed
through D24 and feeds the SYNC board.
Sync Switching
Decoder - Sharpness.
We 'll limit the discussion to the differences with the former version.
a) Comb filtering in NTSC3.58
As for the NTSC3.58 comb filtering is done by the digital comb filter, the dual baseband
CCD delay IC may be bypassed. This happens with Q13 - Q15 and Q12- Q14 when NT3
is at a high level.
b) Sharpness control.
In this version, sharpness is done with discrete components. A DAC TDA8444 provides
the ajustable voltage: SHARPNESS .
The sharpness control is based on the principle of combining two signals, one with a
' max negative' sharpness together with an amplitude adjusted sharpness signal.
The signal is delayed twice by means of two delay lines of 180ns. A combination of
signals produce on the junction R62, R65 and R68 the max. neg. sharpness signal
whereas on the junction R52, R55 and R61 the sharpness correction signal. The latter
signal is amplitude adjusted in IC12 by the sharpness voltage. The two formed signals
are combined on the junction R73/R78 and this signal is further used as Luma signal.
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