[mythtv-users] Load detection on composite video out
Brian Wood
beww at beww.org
Sun Oct 4 16:05:35 UTC 2009
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
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