Notebook + LCD = Ground-Loop Interference, Horizontal scan-line or hum bar on LCD

I started this topic in a different forum (supposedly all things hardware) about a year ago, but no reply, so I'm hoping someone here may know the cause and effects of this.
Mod, please close or move if this is not appropriate. thanks in advance.

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I came across this problem when connecting a Sony notebook to a 22" LCD monitor, in Battery Mode, everything is calm and steady, but as soon as I plug in the AC adapter (3-pin adapter), there are horizontal scan line (or hum bars or ripples) on the LCD screen. I've tried the following:

- Change to different wall-socket, in different location, different building
- Change LCD monitor (Samsung, Asus, LG)
- Change refresh rate
- Use surge protector
- Use stabilizer
- Use different UPS (PK, APC)
- Use a combination of Surge protector, Stabilizer, UPS

Non of the above solve the problem, and the following are further findings:
- Toshiba notebook have the same problem but not as visible and serious as Sony's
- Fujitsu notebook do not seems to have the problem
- Using different wall-socket (different power line) seems to help, but it only reduce the ripples, does not get rid of it.

Finally, tried the following:
- Use a 2-prong (2-pin) power cable (those Euro-type) on the LCD monitor, it WORKS!

But I'm still a little concerns as what effect will this non-grounded power cable (no ground pin) will have... anyone has any idea or alternative solutions? Appreciate feedback. :bow:

I assume this is an analog connection method that you are using. Digital systems should not suffer from this kind of interference. Is there any way you could use DVI or HDMI instead?

If the ground loop exists because a high enough resistance exists between the connectors of each device and that device's ground pin, there is hardly anything you can do. However, if the resistance is due to the cable between them, then replacing the cable with something higher quality may help.

As for your question about safety, it is absolutely imperative that you:
NEVER REMOVE GROUND PINS FROM POWER CABLES AT ANY TIME FOR ANY REASON.


The device may some day be electrified for a number of different reasons. Personal safety is always of the utmost paramount importance.

Pretty much all of the tried-and-true solutions (Opto-electrical isolators/isolation transformers) involve some very expensive components due to the bandwidth of video signals. It would probably be cheaper if you bought a laptop and/or LCD monitor with a digital interface.

yup, you are right. It's indeed a VGA connection.
thanks for the warning and advise, i'll source for a VGA-DVI converter to see if it works. :twothumbs

There are no devices which convert VGA to DVI signals. Also, if such devices do exist, the problem is likely not in the cable itself, but the components which carry the analog signal (and the grounds will still be shared between those). A converter will not help there.

Popsiclestix, thanks for the heads up. Tho changing the notebook is not an option at this moment, I'll try using a CRT monitor to see if we experience the same effect.

about the VGA-DVI, i found a few places that sell them, maybe i'll bring the notebook along to test it out before buying.

If you are seeing devices such as these




They only convert digital computer outputs to be used with analog monitors.

The whole reason why the other conversion is not common is because most modern monitors have a digital input already. Plus the devices which make that conversion is almost as expensive as buying a new monitor itself ($150+ from what I can tell) and it rarely makes sense.

Let me describe what you are trying to do (as I understand it):

1) A digital signal inside your computer is being converted to an analog signal output by your video card

2) Which is then converted back to a digital signal by your converter, then transferred across the cable.

3) An adapter will convert this digital signal back to an analog signal so that it is compatible with your monitor inputs.

4) This analog signal will be internally converted back to a digital signal that you monitor will use to make an image.

There will be a significant amount of losses.

If you can not buy a new laptop, I'd suggest trying a USB video adaptor.

Devices such as this (DVI output) or this (VGA output).

Be warned that these devices aren't exactly high powered and may not be well suited to high resolution full screen video playback.

Popsiclestix said:

If you can not buy a new laptop, I'd suggest trying a USB video adaptor.

Devices such as this (DVI output) or this (VGA output).

Be warned that these devices aren't exactly high powered and may not be well suited to high resolution full screen video playback.

Click to expand...

thank you so much. :thumbsup:
i will source locally for the USB video adapter. that notebook is basically running office application so no video playback is required.

Have you tried a different VGA cable?

I don't know about most of the above stuff, but you can get scan lines from a modified sinewave inverter, (but not a 'pure' sinewave inverter) - where does the AC power come from?