Double termination of video inputs?

"Franc Zabkar" < wrote

I'm currently struggling with a new DVD recorder that has what looks like a common mode noise issue. That aside, I notice that its S-video and composite inputs appear to be double terminated. For example, the impedance looking into the composite input measures 38 ohms instead of the expected 75 ohms. I notice that there is a 75 ohm smt resistor near the connector, but that's all. I'm guessing that the video decoder (LSI Logic L2146) may have an additional internal 75 ohm termination. Is this possible, and if so, is there any logical reason for this, or is it a design gaffe?

***** Are you measuring impedance or resistance?

BTW, the recorder's SPST power switch switches the neutral, not the active.

***** Inside any piece of plug connected equipment there is no distinction between active or neutral.All wires (except those coloured green or green/yellow are to be considered "hot",ergo,it makes not a whit of difference where the on/off switch is connected.

Brian Goldsmith

On Sun, 17 Sep 2006 03:35:11 GMT, "Brian Goldsmith." put finger to keyboard and composed:

Resistance. My understanding is that the termination should be purely resistive.

One of the reasons I'm concerned about the double termination is that certain video sources provide 2Vpp via a 75 ohm series resistor. These rely on a properly terminated 75 ohm load to reduce the level to the standard 1Vpp. If such a source sees a 37.5 ohm termination, then the signal is reduced to 0.67Vpp.

So really the on/off switch should be DPST, not SPST?

- Franc Zabkar

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