Connection troubles...

[jducoeur]

Okay, I'm looking for some ideas here, because I clearly don't understand quite what I'm doing.

We're trying to run some videos from msmemory's laptop to the TV system. (Watching Torchwood.) The video side was easy, once we realized that she has an S-Video output, which I could hook into the DVR. But the audio is being a hassle.

The only audio output from the computer is the headphone out. Okay -- I've got several headphone-to-RCA adaptor cables, so I used that to plug it in. Which works -- but it's got a persistent annoying hum, very much like an ungrounded turntable. It's not the cable doing it: I tried several cables, and they all have the same effect. It's not the DVR: plugging the same cables directly into the receiver has the same hum. And I don't *think* it's the computer, since plugging in real headphones doesn't seem to have the same effect. (Although I haven't conclusively ruled out that possibility yet.)

Am I missing something here? Is there something wrong with the idea of using such an adaptor to go from the computer to the A/V equipment? It's not preventing us from watching, but it's a bloody irritant...

Comments

[dlevey.livejournal.com]

I've run across am odd hum going from the laptop to the stereo. I eventually traced it to a nearby cellphone, with periodic checks to the nearest tower. From what I can tell, the cordless phones (2.4GHz) produce a different but also noticeable hum. I don't know why headphones don't do this also, unless they do but the hum isn't being amplified as it is through the stereo.

[jducoeur]

See the later comments in this thread. Our problem turned out to be asynchrony between the power outlets -- unplugging the laptop fixes the problem...

[metahacker.livejournal.com]

There's a chance you have a ground loop hum situation. If the two pieces of electronics are plugged into different outlets, especially on different sides of a UPS, their grounds may not actually be at the same voltage. This will cause a hum like you describe as the 60hz from the AC power itself bleeds into the audio. The usual solution is to plug the two bits of electronics into the same outlet if possible, but that sounds like it might be tricky.

More info at, e.g.: http://www.channld.com/hum.html

[dsrtao]

This is almost certainly what's going on.

[jducoeur]

And experiments indicate that that was, indeed, exactly the problem. As it happens, since we're trying to run off the laptop with the beefy battery, the solution turns out to be simplicity itself: unplug the laptop while watching the show. Thanks!

[metahacker.livejournal.com]

Ah, the advantages of laptops... This can be a truly annoying problem in other contexts. Glad it worked out!