[mythtv-users] Load detection on composite video out

[Brian Wood]

On Sunday 04 October 2009 09:50:26 Yan Seiner wrote:
> I've been struggling with getting my mobo going using the composite
> out.  The bios apparently uses load detection on the port to determine
> if something is hooked up to it or not.
>
> If I hook up a TV, it works fine.  If I hook up my amplified video
> splitter, it fails to detect anything.  So based on a guess that load
> detection is based on current draw, I took my ohmmeter and metered the
> TV and the amp.  The TV meters to about 75 ohms, which at 1V gives me 13
> ma. The amp meters to 22K ohms, which gives me 0.05 ma, probably not
> enough to trigger the load detection circuitry.  (I know this is grossly
> simplifying things, as the load is not purely resistive.  But it's the
> best I can do.)
>
> So.... Can I just add a resistor in parallel, something like 100 or 75
> ohm, to the splitter amp across the input?  I just want to bump up the
> current draw so that the mobo will detect the thing and turn on the
> output at boot.
>
> --Yan

I'm surprised that your amplifier, designed for a composite video input, does 
not present a 75 ohm load at the input.

Yoy're right that the load is not purely resistive, but most video DAs 
(distribution amplifiers) I've seen use a resistive network on the input, 
usually feeding a high impedance op-amp after that. There's also some 
equalization as well, but the load is often just a 75 ohm 1/4 watt resistor.

With most amps that use BNC connectors, a BNC "Tee" is used, with one side of 
the T terminated with a standard 75 ohm terminator.

So yes, I think you can get away with just loading the amp's input with 75 
ohms of resistance, though you might see some change in the EQ curve.

I'd certainly do it.

-- 
Brian Wood
beww at beww.org