S-video and speaker hum

steveosmith · Joined: Nov 21, 2003 Posts: 4

Hi Gnomies,
My speakers work just fine, until I connect a cable from my s-video output jack to my TV. Then I get a terrible humming sound from the speakers. Very deep, lower than a 60Hz hum. Disconnect s-video cable and hum goes away.

Details:
Dell Latitude laptop
S-video adapter that Dell supplied with it
Cheapie cable connecting plug to TV (could the quality of the cable be the problem?)

Any suggested solutions greatly appreciated!
Regards,
Steve

drwho07 · Joined: Nov 29, 2007 Posts: 1546 Location: Central FL, USA

The cable should be "shielded".

That's about all I can tell you.
Maybe a better quality cable would serve you better.

Good Luck,
Doc Cool

goretsky

Hello,

Are the computer and the speakers on different circuits? If so, then perhaps it is a ground plane issue? Try plugging the computer into an outlet that is on the same circuit as the speakers and see if that makes any difference.

Regards,

Aryeh Goretsky

bern · Joined: Mar 12, 2007 Posts: 1434 Location: ann arbor

steveosmith · Joined: Nov 21, 2003 Posts: 4

I'd heard of the ground issue. Everything's plugged into the same power strip. Just as a test I did unplug the computer and run it on batteries. Same hum.

I should mention that my laptop doesn't have audio output other than the headphone jack. That's what my speakers are plugged into.

Any other thoughts?

Thanks,
Steve.

bern · Joined: Mar 12, 2007 Posts: 1434 Location: ann arbor

Another idea: I know that Dell sometimes sends the audio out the SVideo jack as well as the video. They even sell an adapter that allows you to plug your audio into that adapter. (depends on the model of laptop). Well, if the audio is shuffled into the Svideo connector, and there is something in the cable or the TV set that inadvertently connects the audio and the video signals together, you could easily get a 30 hz buzz.

To try: 1. Different cable; 2. Different TV set; 3. One of the Dell adapters (they might be available at Radio Shack as well, and you could return it if it doesn't work.

Long shot: Is there any electical item also connected to the system that might be plugged into a different outlet box, such as Cable box, Antenna amplifier, network adapter, USB something or other? If so, try unplugging that.

bern · Joined: Mar 12, 2007 Posts: 1434 Location: ann arbor

Another question: what speakers are you talking about? The speakers in the laptop, or is there another amplifier/speakers attached to the system?

steveosmith · Joined: Nov 21, 2003 Posts: 4

I’m using external speakers. And I was suspect that the speakers were the problem. Just to catch up on your earlier post, I did some research and determined that Dell doesn’t - on this model - send the audio out with the video. Also, everything (TV, computer, speakers) are all connected to the same outlet strip.

Anyway, I plugged in headphones in place of the external speakers and didn't get the hum. So instead of plugging in the external speakers, I instead plugged a y-adapter from the headset output jack to the audio inputs on the TV. Sure enough, no hum. So I then plugged my external speakers (using an adapter) into the audio output for the TV. And it works. I'm getting sound through my good speakers with no hum.

So, even though I don't know what the source of the hum was, this is an acceptable workaround. Thanks loads for all your help on this!

Regards,
Steve.

bern · Joined: Mar 12, 2007 Posts: 1434 Location: ann arbor

Congratulations on acceptably solving your problem!

But I am now a bit confused. May I draw some diagrams:
In each case "Speakers" means powered speakers plugged into the wall.
I

Computer headphone Jack: Speakers..\ HUM
Computer SVideo..............: TV set...... /

Computer Headphone Jack: Speakers...\ No HUM
No Video................................................ /

Computer Headphone Jack: Earphones..\ No HUM
Computer SVideo................: TV Set...... /

Computer Headphone Jack: TV : Speakers..\ No Hum
Computer SVideo................: TV set............/

Looking at that diagram has given me another idea. Is your TV hooked up to a cable box, or an Antenna? Is either one of them grounded? (antennas often are, and the cable box may be through the cable). It is possible that the ground plug on the TV power cord only grounds the chassis, not the circuitry. But a ground on the antenna or cable box would ground the circuitry, and that is how a ground loop might get established.

I think it is still a ground loop problem, only now you have minimized the loop by plugging the speakers into the TV set directly.

Significant ground loops can set up a very persistent hum, but rarely carry enough current to be real shock hazard.