Double termination of video inputs?

[Franc Zabkar]

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?

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

- Franc Zabkar

 

[Brian Goldsmith.]

"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

 

[Franc Zabkar]

"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?

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.

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

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

- Franc Zabkar