TV RGB input impedance question

[dav]

I tried to measure the resistance between R,G,B and GND on my TV scart socket.
I got about 105ohm on each of them.
I read somewhere input impedance should be 75ohm on each input due to a 75ohm series resistor on each R,G,B.
So why do I measure 105ohm??

Just tried on another of my TVs and I get 160ohm 

[SeanB]

75R termination resistor into an IC input, which is configured to act as a buffer. Or they simply use a direct connection to a buffer which has an internal termination that is only switched in if it detects a signal or a connection from supplying a current out. Could also be they did not care, just put a 150R resistor there and a coupling capacitor, relying on the short cable so the ringing would not be a worry and also on the cable being so poor that it loses all the power reflected back.

[T3sl4co1l]

Anyone know if SCART is different from usual? (i.e. basically all TV related signals are 75 ohm: RF, composite, VGA colors, etc.)

It may be terminated over a frequency range (certainly true of transformer coupled RF ports!), or perhaps, intentionally high to increase peaking which might help account for HF losses in the cable.

Tim

[miguelvp]

According to this is 75 ohm:

http://members.optusnet.com.au/eviltim/vga2arc/vga2arc.htm

SCART video (RGB SCART)
Video amplitude: 0.7Vpp
Video DC offset: no DC offset, AC coupled
Video input impedance: 75
sync signals stripped from Composite video (CVBS)
sync polarity negative

Edit, but look at the Green terminator. Maybe it can carry the Sync on it?

[Richard Crowley]

Edit, but look at the Green terminator. Maybe it can carry the Sync on it?

The green channel looks exactly like the red and blue. All three have the expected 75 ohm termination.
And it looks like they are using the composite video signal (also terminated with 75 ohms) to feed the LM1881 sync separator chip. And using the back-porch output to clamp the R, G, and B video amp circuits. Rather sophisticated for a consumer gadget.
The good old days of analog video!

[miguelvp]

What is C5 doing between the 75 Ohm terminator (Edit: I meant the Ground) and the negative 5V rail? and C4 tied to the positive 10V rail?

Edit: I guess I'll answer it myself. That's on the other side of the signal so the CRT must expect higher voltages because analog CRTs could carry a composite sync on the green channel. So I guess that will raise the voltage to the zone where it needs to be at.

[retrolefty]

I tried to measure the resistance between R,G,B and GND on my TV scart socket.
I got about 105ohm on each of them.
I read somewhere input impedance should be 75ohm on each input due to a 75ohm series resistor on each R,G,B.
So why do I measure 105ohm??

Just tried on another of my TVs and I get 160ohm 

 A DC measurement is not the same thing as an AC impedance measurement.

[dmills]

Edit: I guess I'll answer it myself. That's on the other side of the signal so the CRT must expect higher voltages because analog CRTs could carry a composite sync on the green channel. So I guess that will raise the voltage to the zone where it needs to be at.

Nope, C4, C5, C6 are just power supply decoupling caps, nothing to do with the signal paths as such (Well, they do, obviously, current flows in loops, but you know what I mean).

Regards, Dan.

[miguelvp]

That makes sense, sorry my brain is not working great today, had little sleep (about 2 hours) trying to update my project progress.

After reading your post it's obvious the rails and grounds are common 

[dav]

A DC measurement is not the same thing as an AC impedance measurement.

I know DC is different from AC, but 75R should be termination resistor value.
Should I measure RGB input impedance at 1kHz???
Please someone clarify this!

[dav]

None could explain me?

[Richard Crowley]

The standard output and input impedance for analog (and digital, for that matter) video is 75 ohms.
If you have examples (especially mass-market consumer gear) that doesn't measure 75 ohms, you will have to ask THEM how they made those design decisions.  I don't know how you expect US to answer that question?