RE: TV Charater Overlay - food for thought

[Keith Doxey <ukha@xxxxxxxxxxx>]

Quoting Des Gibbons <des@xxxxxxx>:

> Wayne,
>
>
> The problem with on screen display, is that nobody I know watches TV
using a
> composite signal.
>

Unless they are watching Terrestrial through the TV's tuner but most TV now
would be from Tivo/Digibox/Cable/Freeview so valid point

> There is no easy way of interlacing OSD onto a s-video or RGB signal,
(for
> easy read cheap :)
>

Not quite true. No easy way for RGB but for S-Video you can switch the Luma
(Y)
signal through the OSD device and get a monochrome overlay. The colour of
the
picture may slip slightly to left or right whilst doing this but it may not
be
noticable.

Y IN ------ OSD ------ Y OUT
C IN ----------------- C OUT


For Colour overlay on S-Video you can use a couple of S-Video/Composite
adapters
to drop down to Composite whilst overlaying the message and split it back
to
S-Video afterwards but there would be noticable softening of the image
whilst
doing this. eg

NORMAL VIEWING OSD CIRCUITRY BYPASSED
/---------------------\
Y IN---------/                       \----- Y OUT

Adapter-OSD-Adapter

C IN---------\                       /----- C OUT
\---------------------/


OVERLAY MODE S-Video-Composite-OSD-Composite-S-Video
---------------------
Y IN---------\                       /----- Y OUT
\                     /
Adapter-OSD-Adapter
/                     \
C IN---------/                       \----- C OUT
---------------------


As I say, it is possible but whether the results would be acceptable depend
on
the individual. By far the best chip for this was the STV5730A which could
do
RGB overlay but that is being discontinued with no replacement :-(

Keith

www.diyha.co.uk
www.kat5.tv


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