Arcam AV8 Home Theater System User Manual


 
The S-video C and OSD C signals do not need a DC clamp
as they will float to the same level due to the nature of the
signal.
S-Video Multiplexer
Refer to circuit diagram L922 sheet 9
This contains the multiplexing for the S-Video signals. It
operates in the same way as the composite multiplexer
(Sheet 3 Video board) except two multiplexers are used for
each signal to carry the independent Luma and Chroma
signals.
IC800 and IC807 carry the main feed Luma and Chroma
respectively. IC801 and IC806 carry the VCR record loop
Chroma and Luma respectively. The Main feed is buffered
by IC803 it then goes to the OSD insert sheet, the VCR
output is buffered by IC802 and then output to the VCR
connector.
Control of the Multiplexer is via serial to parallel shift
registers IC804, IC805 and IC808 these also have the
control lines for the SCART control signals, sync on green
and the Mute signals.
Zone 2 OSD
Refer to circuit diagram L922 sheet 10
This contains the OSD sync stripper and OSD insert
circuits for Zone 2. It works in a very similar way to the
circuits on sheets 4 and 8 combined together. As Zone 2
only operates in composite the circuit is without the extra
multiplexing for S-Video and Component signals.
The Zone 2 video comes from the video input sheet 3
buffer it is terminated on this sheet then fed to both the
OSD chip IC900 and the OSD insert multiplexer (Pixel
Switch)IC902. Characters are generated by the OSD and
this signal is present on the second input to the pixel
switch. Both of the signal are sync tip clamped by
transistors Q900, Q901, this means that their DC levels at
the input to the pixel switch should be about the same.
The fast blanking signal from the OSD controls the
position of the pixel switch. Fast blanking is high when
OSD characters are being generated this causes the pixel
switch to change to the output from the OSD chip. The
pixel switch IC902 has a video buffer output and a gain of
two. This buffered output is fed back to the Video board
(sheet 3) and out via the Zone 2 connector.
The vertical sync output of the sync separator IC901 is
used to determine if the incoming video is PAL or NTSC
so that the correct clock can be provided to the OSD chip.
! Vertical sync 50Hz = PAL 17.73447MHz
! Vertical sync 60Hz = NTSC 14.31818MHz