how to join composite sync signal onto component

Hi

I've mentioned this before, I know... but I've now a bit more info so I'm on
the same path again...

The UK TiVo can be persuaded to output component video on the RGB pins of
the scart by sending commands to the video chip using the iic (i-squared-c)
bus.

I want to use this to connect my X1, especially since now it's ceiling
mounted the distance is just too much for the composite video connection it
currently uses.

I have the component adaptor for the X1 so I made up a SCART-RGB (3xRCA)
connector and connected them: however nothing happens (ie the X1 just keeps
searching for a signal).

Some investigation reveals that the reason for this is probably that the
YCbCr output doesn't contain sync, while the X1 expects sync-on-Y.

Now when you connect using RGB from the TiVo the TV uses the composite video
signal for its sync... so it's reasonable to assume that that would be a
good way to do this here. I can persuade the TiVo to output just the sync
signal (ie no picture) on the composite output, so I just need a way of
adding the composite sync to the Y.

I read on a Usenet post about adding sync signals using a fast signal diode
but that would only work if the sync signal is TTL-level (0 or 5v) and it's
not - I _think_ it's -0.3 and -0.7 (I'm not that great with the oscilloscope
though

I know there are RGB-YUV converter boxes I can buy for 80 quid or whatever
but I want to do this on the cheap a) because I'm skint and b) it'd be cool


So is this something that you guys come across a lot and is there a standard
FAQ for it? 'cos I'm drawing a complete blank on the web

Ideally of course there'd be an option on the X1 to have a separate sync
signal, but there's not

So the things I need to know...
- what signal levels would the X1 accept as sync on its Y input?
- how do I get from -300mV/-700mV to those levels?
- how do I add the signals together?

Ideally I'd like to do this using discrete components so I don't need a
separate power source but if necessary I could use powered components.

G

Geoff Winkless wrote:
Now when you connect using RGB from the TiVo the TV uses the
composite video signal for its sync... so it's reasonable to assume
that that would be a good way to do this here. I can persuade the
TiVo to output just the sync signal (ie no picture) on the composite
output, so I just need a way of adding the composite sync to the Y.

It turns out the TiVo can be made to set all its outputs to high-impedance
specifically for the purpose of using wired-AND: so (after doing that) just
joining the signal wires together works just fine.

So I finally have a decent picture on my projector (apart from TiVo's MPEG
artefacts, of course :-))

Geoff