Trouble removing background noise

[Duffield]

Hi, I have been editing for hours, using the great information I've found on this forum and elsewhere. However, I still cannot manage to remove the background noise from a lecture. I've attached two WAV files to this post. The first is room tone. The second is a clip with talking. You'll notice that the background noise alternates between noisier/quieter/noisier/quieter in regular intervals. This is not ambient noise; it's electrical hum of some sort. The room was quiet and I was speaking directly into a microphone. But the mic was about 150 feet away from the sound board, with cables running through the walls -- plenty of opportunities to pick up electrical hum I imagine.

I've been using the notch filter and plot spectrum for hours, trying to single out peaks and to remove them. I've had some luck, but only made a small dent in the problem.

Any advice is appreciated. Thanks!

[kozikowski]

Yes, that's pretty stiff hum.

I would be looking for a cable with an open ground or shield or a microphone with a broken connection. What is the setup at the microphone end and which microphone? How is the board wired and what is the actual recorder? Does it go up and down if you fade the microphone down at the mixer? You can get this problem if you try to use an unbalanced microphone in a balanced system. You can also get it between the mixer board and the recorder. It pays handsomely to figure out which you have.

Room Tone sounds like open shield hum alternating with something turning on and off like theater marquee lights. You can ship properly balanced audio between building floors and sometimes between buildings with no serious damage, but you do have to pay attention. Are all your cables like this:

http://kozco.com/tech/audacity/pix/XLR-Cable.jpg

Describe in detail how you are making these recordings. You can never walk out with a clean recording once you screw one up. The best you can do is patch it together and hope nobody notices some of the musical notes are missing.

We had at one time a "power hum" filter since this is a pretty common problem.

Looking.

Koz

[kozikowski]

Scroll down to the Mains DeHummer ZIP file.

http://forum.audacityteam.org/viewtopic ... =10#p36533

Koz

[Duffield]

Thanks Koz. Your questions about the recording setup are a bit over my knowledge level. But I will recruit someone who understands them and will pass on your information.

In terms of the setup, this is in a church sanctuary. There are always two wired mics that are on during a service. The mics are on stands maybe 20 feet away from each other. The cables run through the wall about 100-150 feet to a sound board in the choir loft, about 20 feet above the sanctuary floor.

I'll try the filter you sent and report back. Thanks.

[kozikowski]

When you pot down the two standing microphones in the choir loft, does the hum go away in the sound file? Does it go away with only one?

Yes if you are just assisting, we really do need to chat with the audio type. If you do have damaged cables you might be letting yourself in for hours of extra work in place of just setting your jaw and fixing the broken cable.

You may also have simple bad positioning of the microphones. They're not infinite. If there's a lot of air between the performer and the microphone, you may have just reached the limit of the system.

Koz