To hum or not to hum?

[Peter Koller]

I am currently shooting a splitscreen-scene: I have the actor sitting on one side of the table having a conversation with himself and wanted to play his other character (already shot) from a notebook to make it easier for him.
Everything was ready, camera was rolling and then it happened: I moved the boom over the table (and the notebook) and I got this damn 50Hz hum in my headphones. To make things worse the owner of the notebook is in Germany right now and I don´t have the batteries for it, because I thought I will plug it in anyway.

I had a few troubles with the sound before and it still seems like a black magic box to me:

The whole audio I use is unbalanced. I am using the Sony ECM959A stereo mic (I know, it is not so good for dialogue, but it is all I have at the moment) which is plugged in the XL1 3,5mm mic plug. With the extension the cable is 5 meters long.

At first I was nervous, that I might pick up hum with such a long unbalanced setup, which I did, but the hum disappeared completely when I operated the XL1 with the batteries and not from the power outlet.

I had a monitor connected through the (don´t know the word right now) yellow video-out, which was no problem unless the boom went over the monitor. But later on after I moved the camera to another position I got a terrible hum from the monitor as soon as I connected it no matter where the mic was. Nothing else was changed.

And now that notebook is causing the noise. The funny thing is the hum is almost constant no matter where the mic is positioned with in 1,5 meters of the notebook exept for 2 really tiny spots where the hum suddenly dies.

Changing to the batteries killed the hum caused by the camera, I have no explanation for the monitor. It hums or hums not.

With the notebook I hope it goes away when I run it from the battery, too. But will it?

Is my long unbalanced cable picking up the hum? But then why don´t I get any hum from the 500W lamp right above the table? Can a beachtek or similiar adapter solve this, especially when the camera is operated from the power outlet?

I read Jay Rose´s book "Producing perfect sound" but this issue is still a shakespearean mystery to me... to hum or not to hum?

A confused and humming Peter

[Jeff Donald]

The hum could be caused by bad grounds in the AC. When you run the XL1 from battery and not AC the hum dissapears. You probably can't fix the AC so try to minimize your exposure to it. Keep cable lengths as short as possible. Do not run mic cables close to power cables. If mic cable and power cable must cross do so at a 90 degree angle. Balanced audio will certainly help and so will a Beachtek or Studio 1 adapter. Also, do not coil audio cables, if necessary fold them loosely in long lengths. Coils and loops are more likely to pick up the hum.

Jeff

[Don Palomaki]

The immediate solution is to use battery if that works. A couple other things to try is having the laptop and camcorder power supply plugged into in the same power strip.

If you have a AC power isolation transformer, it might help if you put the XL1 or laptop on it.

The two points of no nosie may be a null caused by phase cencellation of two diffferent noise sources.

If the Sony ECM-959 is like the ECM-MS957, it may be possible to connect it in balanced audio mode. The 957 uses a 5-pin XLR connector on the mic body, two pins for each channel and one for the mic ground.

[Peter Koller]

Thank you for your advice guys!

Running the laptop from its battery eliminated about half of the noise, then I switched to manual audio on the camera and lowered the audio level a little, which killed it so far that no one who does not especially search for the noise should not be able to notice it.

Cheers, Peter

And no, the 959 has only a 3,5mm mini-jack, not an XLR connector.

[Don Palomaki]

AGC, an all too obvious issue. With AGC enabled gain is max if there is no other input signal, any stray hum will be maxed out and cam become much more apparent during otherwise quiet scenes.