Newsgroups: sci.electronics.misc
Subject: Re: Bypassing a TV demodulator
References: <[email protected]> 
"scada"  writes:

> > An opto-isolator would prevent any problems with the "hot chassis". The
> > LED would be connected to his video source, and the phototransistor to
> > the TV set, with only light passing between them.
> >
> Would an opto-Isolator do it? They are great for digital, however the
> baseband Luminance signal is all analog.
> This would be trickey getting the LED to run linear in that region I would
> think! Not to say anything about the
> required bandwidth, around 6 MHZ? Fiber optics would work, but I don't know
> if there is a fast optic-Isolator out there that would!

Optoisolators can used in linear region and they can be used as 
video isolators. There are commercial video isolation devices which 
use optocouplers for video isolation and they work 
(I have used then in some occasions).

Some suitable optocouplers for video isolators could be 
HCPL-4562 and HCNW4562 optocouplers. Agilent used to
provide even a circuit diagram for such circuit in their
datasheet which used to reside at 
http://www.semiconductor.agilent.com/isolator/hcpl4562.html
it is no longer there :-(
Quick seach on Google did not reveal the new location
of this document, but site search at 
http://www.semiconductor.agilent.com/cgi-bin/morpheus/home/home.jsp?pSection=Optocoupler
with keyword "HCPL-4562" seemd to give somethign which looked interresting.

For example this too long URL brings you to the component datasheet:
http://www.semiconductor.agilent.com/cgi-bin/morpheus/networkProducts/dataSheetAbstract.jsp?BV_SessionID=@@@@0971354918.1032175992@@@@&BV_EngineID=cadcfedkgmjibemgcgjcfijdio.0&Section=Optocoupler&sProdNum=HCPL-4562&mktSeg=Optocoupler

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/