Barco GRAPHICS 1209S, Service Manual

Page: 77 / 378

Date: 10/02/97 R762745
QUAD Comb_Filter R762745
10
CIRCUIT DESCRIPTION " D COMB FILTER " (R762745).
Technical description
General. The decoder section of this board with comb filtering uses the decoder chips TDA4650
/ TDA4665 / TDA 4565.
The implementation of the adaptive digital comb filter for PAL and NTSC 3.58 (not for
SECAM and NTSC 4.43) has changed the input switching circuit and obviously two ICs
for the digital comb filtering have been added. The first IC generates sync and sampling
pulses. The second IC is performing the adaptive digital comb filtering.
We'll concentrate mainly on this section in the next pages.
Filter Switching The decoder IC TDA4650 The VID/ LUM input signal is passing an emittor follower in
order to supply the provided filters.
The multistandard Decoder chip IC 11 is checking sequentially the information on the
backporch of the horizontal sync.
As soon the right system is identified, the appropriate output PAL/SECAM/NTSC4.43
or NTSC3.58 is put at a high level. The filters for the Luma signal are an all pass, a
rejector around 4.43 MHz and a Luma Comb filter (via Q41). The filters for the Chroma
signal are a high Pass 2.5MHz, a Chroma Comb filter (via Q40) and a SECAM BELL
filter (via Q10).
The switching outputs of TDA4650 are supplied to a GAL (IC14), where the decoder
mode will be recognized. The programmed GAL IC14 takes care, using the two outputs
SwBit0 and SwBit1 as address inputs for the IC18, that the correct signal on the 4-
channel Analog Multi-/Demultiplexer IC18 is switched through.
The filters are switched in the following way:
Filter Switching
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