I have a decoder circuit that includes TW9900 video decoder. The decoder doesn't work properly. There is a matching and AC coupling circuit on analog inputs of the decoder. To finding the problem, i scoped the analog signals on PCB. Why i see a negative offset signal at the input of coupling capacitor? Does it normal? Is there any problem that anybody can see?

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Yellow one shows input of AC coupling capacitor.
Blue one shows output of AC coupling capacitor.

enter image description here Note: Input of coupling also means of output of the camera.

Zoomed figure of decoder input(after AC coupling)
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Camera output directly, not connected to PCB
enter image description here

  • \$\begingroup\$ what were you expecting? The whole point of AC coupling is that offsets can be chosen however the receiving end likes them. \$\endgroup\$ Nov 2, 2019 at 14:50
  • \$\begingroup\$ The receiving end accepts signals between 0.5V and 1.5V range. Why the input signal has a negative offset? Although this also camera output. \$\endgroup\$ Nov 2, 2019 at 14:55
  • \$\begingroup\$ Why shouldn't it. Potential is relative. Also, as you've noticed, this is AC coupling, so the offset can't matter. \$\endgroup\$ Nov 2, 2019 at 14:55
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    \$\begingroup\$ The signal appears to be inverted; I can see the HSYNC repetitive pulses going positive (they are normally negative). \$\endgroup\$ Nov 2, 2019 at 15:33
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    \$\begingroup\$ Judging from the (apparently inverted) HSYNC height, it looks like the source is not series terminated in 75 ohms (HSYNC is normally 0.3V in depth and yours is about 0.6V). Figure out those things first. Many video decoders are somewhat picky about signal polarity and amplitude. \$\endgroup\$ Nov 2, 2019 at 16:00

2 Answers 2


I found the solution thanks to this comment "The signal appears to be inverted; I can see the HSYNC repetitive pulses going positive". I didn't look at the signal carefully at first. In fact the problem was somewhere else. It was a reverse connected video connector.

So thanks the points you've helped


I would like to address the DC offset although from your answer it appears that wasn't your problem.

AC coupled video has a DC offset which is determined by the average brightness of the image. A technique called clamping is used to re-bias the signal such that the sync tip is considered a reference point of -0.3V.

Most video decoders have clamping amplifiers built in, however for rapidly multiplexing between different video sources, external clamping amplifiers can be used on a per-channel basis.

When I left CCTV 10 years ago video decoders had become that inexpensive that people had started using one per channel and hence external clamping was no longer required.


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