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 multip lexing for the S-Video sign als. It
operates in the same way as the co mposite multiplexer
(Sheet 3 Video board) except two multiplexers are used for
each signal to carry th e independent Lum a and Chro ma
signals.
IC800 and IC807 carry the main feed Luma and Chroma
respectively. IC801 and IC806 carry the VCR record loop
Chroma and Luma respect ively. 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 VC R
connector.
Control of the Multiplexer is via serial t o paralle l shif t
registers IC804, IC805 and IC808 these also have the
cont rol lines for the SCART control signals, sync on green
and the Mute signals .

Zone 2 OSD

Refer to circuit diagram L922 sheet 10

Thi s cont ains the OSD sync stri pper and OSD ins er t
circuits for Zon e 2 . It works in a very similar way to the
circuits on sheets 4 and 8 combined together. As Zone 2
onl y op er ates in com posite the ci r cuit is withou t the extra
multiplexing for S-Video and Com ponent signals.
The Zone 2 video comes from the video input s heet 3
bu ffer it is terminat ed on this sh eet then fed to both the
OSD chip IC900 and the OSD insert multiplexer (Pixel
Switch)IC902. Characters are generated by the OSD and
this sig nal is pr esent on the se co nd i n put to the pixe l
switch. Both of the si gnal are sync tip clamped by
transistors Q900, Q901, this means that their DC levels at
th e 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
swit ch to change to the output fro m 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 boar d
(sheet 3) and out via the Zone 2 connector.
The vertical sync output of the sync separator IC901 is
used to determine if the incomi ng video is PAL or NT SC
so that the correct clock can be provided to the OSD chip.
! Vertical sync 50Hz = PAL 17.73447MHz
! Vertical sync 60Hz = NTSC 14.31818MHz