See this image, it's a composite video signal on my scope.

Scope shot

As can be seen the dc offset of the signal is varying with time; it also varies with brightness in the scene.

I am plugging the signal from a composite video signal directly into my scope, however the cable is also spliced and goes to my computer's TV card. Without the TV card attached I do not see this. Now I would like to be able to debug my signal using the TV card, but it seems to be interfering (maybe it's loading the signal too much) - what could be causing this? I have another card I'm going to try but I'm not hopeful. Apart from buffering the signal using expensive video buffer amps, what can I try?

  • \$\begingroup\$ Charging of caps? \$\endgroup\$
    – Kellenjb
    Oct 25 '10 at 20:33
  • \$\begingroup\$ But what cap, and why does it do it every cycle? \$\endgroup\$
    – Thomas O
    Oct 25 '10 at 20:37
  • 1
    \$\begingroup\$ can you put the scope in a low impedance input mode? try it that way. most RF circuits expect a termination resistance like 50 or 75 ohms. (i think 75 ohms is the common value for video). if the scope doesn't have it built in, you could put a non-inductive resistor across the signal. the video card input ought to have that kind of impedance but you never know. \$\endgroup\$
    – JustJeff
    Oct 25 '10 at 22:57
  • \$\begingroup\$ I don't have a 75 ohm termination option on the scope, but I do have two 150 ohm resistors in parallel. I'll try it. \$\endgroup\$
    – Thomas O
    Oct 25 '10 at 22:59
  • \$\begingroup\$ Hmm I removed my resistors (as originally present) and it still swings, though the output voltage is much higher and there is some kind of weird noise on the signal. \$\endgroup\$
    – Thomas O
    Oct 25 '10 at 23:04

I'm guessing that the source is AC coupled ( and shouldn't be)

you've got dc offset shifting with picture brightness. Video should be DC coupled and terminated into 75ohms. If that sounds like something that your wouldn't string across a room, congratulations; it was never designed to. I remember reading that in-studio distribution was never raw analog video. So check the source path for a cap.

Unless it's just a really bad ground connection (high resistance) ;and the ground is elevating with brightness, which is consistent with that. So check the return path resistance too.

Analog video; it's frustrating because it was never intended to go very far as a baseband signal.

  • \$\begingroup\$ Many video devices are AC coupled on the inputs and outputs; it might be nice if that weren't the case, but in practice unless one knows one will only be accepting signals from devices with DC-coupled inputs, one has to accept AC-coupled signals. Further, in many cases, if one will have to deal with AC-coupled signals, it's often helpful to AC-couple one's own input stage. See electronics.stackexchange.com/questions/11334/… for more info. \$\endgroup\$
    – supercat
    Apr 26 '11 at 16:21

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