Could a bad hdmi cable be the cause of 60hz speaker hum?

Question

I was at a friend's house trying to help him get his computer/tv/stereo connected in his new apartment. We had some difficulty getting the tv to acknowledge the video signal. The hdmi source option was intermittantly available depending on the position of the cable. So it's a bad cable and needs to be replaced.

But we wanted to do as much diagnosis as possible in case anything else had been damaged or lost during the move. So we carefully placed the cable and backed away slowly and selected the source.... (Wait, wouldn't that be a great opening for a spooky story? Sorry, back to reality.)... Then testing the computer audio through the receiver, there is a very bad 60-cycle hum coming through the speakers. It's still there if we switch to the internal tv speakers and disconnect the receiver.

If the audio is running from the mini-headphone-out (I kept wanting to switch it to line-out, but it didn't ever help) to RCA-ins on the tv, could the bad hdmi cable be introducing this hum even though no audio (in use) is going through that cable? Or does this implicate the stereo-mini-to-rca cable as well? Or is it likely to be something else entirely?

Now, don't let me forget to mention that there's hum in the air already. The apartment is on the side of the building with the A/C pen for 2 apartment buildings (about 20 of them). When we turned everything off, we could distinctly hear the same buzz (maybe a little more pink) from outside. And I could also hear a little bit from the refrigerator in the galley kitchen about 10 feet away from the living room.

So, any ideas, etheric masters?

Answer 1

From your description I don't think it's possible that it could be the source of the noise.

When you switch your receiver to select from a particular input channel, it would likely completely ignore or isolate any activity on any other input sources and they simply would not get into your mix at all. The fact that you hear it on different things suggests that the noise is coming from a common source like you eluded to.

The easiest way to tell would just be to unplug the HDMI cable all together and see if you still get noise on RCA inputs if you really wanted to test the theory. If it does magically get rid of your noise then I suppose we could start forming some theories about any power being transfered over the cable, but I would have doubts there because I've never heard of any significant power sourcing being done over HDMI.

HDMI carries digital audio (numbers with checksums), so it's more or less immune to noise that is not strong enough to flip bits or make them ambiguous to interpret on the receiving end. If the data stream becomes corrupted, you would hear your audio drop out intermittently. In any case, all of that is irrelevant because you're not even using HDMI. It just happens to be plugged in and you've selected an analog source.

One thing you could try is to incrementally move your speakers closer to the AC and see if the noise gets stronger, or have your unit's electrical wiring checked out by an electrician.