Feedback/distortion in audio: How to remove?

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Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by altofiddler » Thu Feb 21, 2008 1:45 am

I have audio files from a (private) concert that contain feedback. One of the performers with a wireless microphone moved too close to the speakers or to someone else's microphone. I would like to filter out the unpleasant part of the sound while keeping the correct note, but I do not know how to do this. I do know that feedback is a mix of tones and harmonics that sounds terrible to the human ear. How do I find and isolate offending frequencies? As an entirely self-taught audio editor, I have no idea how to use filters like Equalizer. Nor do I understand the charts that appear when I use Analyze. I would appreciate any advice anyone can give.
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by kozikowski » Thu Feb 21, 2008 4:17 am

The graphic equalizer isn't that hard to deal with. The blue line on the screen is a series of rubber bands and you can click and move the curves up and down and make new ones as needed Here's an illustration of a curve that makes voices a little more harmonious.

http://kozco.com/tech/audacity/voiceEq.jpg

You can tell where to put that dip by doing an analysis.

Put some feedback performance on the timeline. Select some of it.

Analyze > Plot Spectrum.

Do this on a computer with a really big screen. Pull the spectrum window out as wide and tall as you can. It will give you more and more detailed information as you go. Feedback, contrary to your assumption, is not a festival of frequencies. It's the room ringing like a bell at one frequency. The analysis should tell you where that frequency is. It should look like something that would give you a flat if you drove over it.

Mess with the equalizer to get rid of that one frequency.

There is one more interesting thing you might try. Everybody thinks Noise Removal is just for air conditioner trash or background hiss, but you can train it to eliminate anything.

Find a portion of the performance where there's feedback but nobody singing. This will be a little rough.

Effect > Noise Removal

"Train" the tool what feedback sounds like and it will try to eliminate it from the whole performance. The trick is not to include any voice in that training, or it will try to eliminate the voice, too.

There several reasons why none of this is going to work, but I haven't heard the show, so we'll watch and see how you do. Let us know.

Koz
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by RobinH » Thu Feb 21, 2008 2:01 pm

Help! I am a new audacity user and I am stuck. I have been transferring my vinyl from a ion turntable to my mac osx and now I have been trying to fix the music in audacity. Everytime a import the audio to audacity from my Itunes music it plays back very fast...screeching!!!
All I want to do is get rid of the scratching sounds. I was able to do it last time I used the program and now I can not get it to playback correctly????
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by waxcylinder » Thu Feb 21, 2008 4:02 pm

You are almost certainly trying to import a file type that Audacity does not support (my guess is Apple's AAC format) . Audacity Supports WAV, MP3, OGG and FLAC in 1.3.

Audacity has a nasty habit of just trying to play non-supported files as a short screech, and not giving you a sensible error message about it.

If this is the case you have two basic options:

1. find and use some converter software to convert your file to a type that Audacity supports

2. play the file in any music software that supports that particular file format and record it in Audacity.

You may need to do option 2 as the copy protection DRM may get in the way of 1.

WC
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by altofiddler » Thu Feb 21, 2008 7:00 pm

Koz, thanks for the reply. So, with your voice equalizer, you have reduced the volume of frequencies at about 3000 hz? That one frequency can make voices less harmonious, so you mute it?
I plotted the spectrum of my feedback and I got a chart that looks like the right-hand side of a bell curve, but with serrations. How do I determine the offending frequency from this chart?
I have tried the Noise Removal filter, but it filters out ALL the sound at that point. The main performer, a violinist using a wireless instrument microphone stepped too close to the other equipment on stage, and the resulting feedback drowned out the other instruments who were just playing rhythm.
Last night I tried splicing in another section of music where the violinist was playing the same musical phrase, but I could not figure out how to paste it into the exact right place. I created a second audio track, pasted that bit in, moved it around, and found the place where it sounds right when both track are playing simultaneously. But I can't figure out how to paste it exactly straight up into the main track. Is there a way to mark a track for a "snap-to" placement? Or, when I export the whole song to MP3 and AIFF, will the tracks automatically blend? (I keep trying to use tracks in Audacity like layers in Photoshop.)
Thanks.
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by altofiddler » Thu Feb 21, 2008 7:11 pm

To RobinH: Yes, that screech is Audacity trying to play an iTunes format tune. There are several free audio converters for Macs. Import the tune into a converter first, then import it into Audacity. When you are ready to export it back into MP3, do it TWICE. For some reason, occasionally the first export to MP3 results in a file with 0 bytes. iTunes will then convert the MP3 back into its proprietary MP4 format.

I use Audacity to slow down tunes so that I can learn them on my own instrument while listening to my iPod.
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by kozikowski » Thu Feb 21, 2008 7:28 pm

<<<your voice equalizer, you have reduced the volume of frequencies at about 3000 hz? That one frequency can make voices less harmonious, so you mute it?>>>

Right around 3000 is the place where fingernails go when you rake them down a blackboard. That's also the major peak in the Fletcher-Munsen loudness curve. It's the high point in telephone communications audio frequencies. Many magic things happen at 3K.

Yes, that's one reason what you're doing isn't going to work. Once you eliminate the howl, there's nothing left back there. There is a capture effect with audio systems. Once you overload them, they go non-linear on you and really messy thing happen.

Is there any way you can post a WAV with maybe ten seconds of feedback coming and going? I'm curious maybe I can get rid of it in another program.

Koz
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by altofiddler » Fri Feb 22, 2008 10:28 pm

I have tried to upload a WAV and a MP3 file, but this bulliten board does not allow that action. Is there another site I could upload to? Thanks.
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by kozikowski » Fri Feb 22, 2008 11:22 pm

This is where you're supposed to say, "I found a free upload site called "mmmmmmmm.com" and I put a clip up there for you."

No, huh?

My web host is supposed to offer this, but it's never worked particularly well and since they moved to the New! and Improved! servers, it doesn't work at all. Yet another reason I wanted to get my own host machine working in my Server Room (bedroom #3).

Somebody posted ideas about this. Search for "Upload Music" or "Post Music" or something like that. You will immediately run into a forum problem. The forum will not search for patterns, so you will get every "Post" hit and every "Music" hit.

But it's here somewhere.

Koz
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Re: Feedback/distortion in audio: How to remove?

Permanent link to this post Posted by Mil1ion » Sat May 31, 2008 7:30 pm

kozikowski wrote:The graphic equalizer isn't that hard to deal with. The blue line on the screen is a series of rubber bands and you can click and move the curves up and down and make new ones as needed Here's an illustration of a curve that makes voices a little more harmonious.

http://kozco.com/tech/audacity/voiceEq.jpg

Koz



Regarding the EQ feature, there are 12 names for one to click the dot.

What is the reason for these 12 possibles as the EQ range changes on each one?

Which one does one use to get rid of "too close to a speaker" feedback that sounds like hetrodyne (sp) ?

D
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