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Subject: Question about audio editing rss

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Jim Cote
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I have an audio file with some low level background sound (not just noise). I'd like to filter out all sections of the file that are below a certain AMPLITUDE, and not based on FREQUENCY. I would think this would be a common need, but I can't seem to find any effect in Audacity, for example, to do this. Thoughts?
 
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Paul DeStefano
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You could use a compressor setting to mark a threshold of occurrance.

Of course, every piece of software calls this something else and hides it somewhere else.
 
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Jim Cote
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I tried the Compressor effect in Audacity, but it only seems to change the HIGH amplitude stuff. All the low level sound stays as it is.
 
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Mark Johnson
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Yeah, I think the Compressor software filter will do the opposite of what you want--suppress the amplitude stuff and bring the lower level audio up. I don't know any way to do what you're after. I assume you already tried the Noise Removal filter? I find it always leaves me with disappointingly metallic sound like everyone's talking through a pipe. But it might be the closest you can get.
 
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Paul DeStefano
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Your compressor doesn't have a setting for HiRange/LoRange?
 
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Jim Cote
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Geosphere wrote:
Your compressor doesn't have a setting for HiRange/LoRange?


It's got Threshold, Ratio, and Attack Time.
 
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Gil Hova
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What you want is an "expander," which is the opposite of a compressor, and is related to a gate.

In case that first sentence made your eyes water...

(1) A compressor, as has already been written, reduces the level of sound ABOVE a given threshold. It's generally used to even out the volume peaks of a piece of audio.

(2) A gate silences a track of audio once it falls BELOW a given threshold. It's generally used to silence a noisy track when there isn't a lot going on.

(3) An expander is what you want. It kind of combines the above two items. It will reduce the level of sound BELOW a given threshold, but it won't completely silence it like a gate. You can choose just how severe to make the cut.

Unfortunately, I don't use Audacity, so I can't tell you where to look. I use Sound Forge in my day job, and in order to run an expander, I can use a "Graphic Dynamics" plug-in that can act as a compressor or an expander, depending on what I need.

One note: You're probably going to want to use the expander to drop out the noise when no one is talking. Ideally, the noise is inaudible when someone is talking, so in theory, if you can get rid of it when no one's talking, it should be inaudible.

Whatever solution you decide on, try to be as gentle as you can. People tend to notice (and be annoyed by) change, not consistency.

In other words, if you have a loud background noise and you gate it, then every time someone speaks, the noise will kick back in. Then the person will stop talking, and the noise will cut out too. Then more talking, and the noise will return.

Because there's a lot of changing sound in that hypothetical track (the noise cutting in and out), people will find it annoying and difficult to listen to.

So if you do run an expander on the track, try not to set it so it's too severe. If you find that the noise seems to come "out of nowhere" and you can still hear it under the person talking, pull back and just attenuate it a bit instead. The noise might still be audible, but you may be able to find a level where it's not so annoying.

If you can tweak it so that the noise is difficult to hear when no one is talking, and it's not audible when someone is talking, then you're good to go.

I hope this all makes sense. I'm going to go to Audacity's site now and see if they have an expander under another name.

Good luck!
 
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Michael Becker
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Adobe has a new program called Soundbooth - I think it's still in beta, but you might be able to download a demo - that has a graphical noise removal option. I've seen it used in removing things like cell phone rings from an audio file. It worked really well for something like that.
 
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Gil Hova
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It looks like Audicity supports VST plugins, but I think most of those are commercial.

It also supports plugins using its own open-source model, but there doesn't seem to be a lot to choose from in that area just yet.

If you can get your mitts on a noise-reduction plugin, that would work for constant, unchanging noise at regular frequencies like tape hiss or some consistent hums, but not for dynamic, changing sounds like background speech or traffic.
 
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Jim Van Verth
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IngredientX wrote:

(1) A compressor, as has already been written, reduces the level of sound ABOVE a given threshold. It's generally used to even out the volume peaks of a piece of audio.

(2) A gate silences a track of audio once it falls BELOW a given threshold. It's generally used to silence a noisy track when there isn't a lot going on.

(3) An expander is what you want. It kind of combines the above two items. It will reduce the level of sound BELOW a given threshold, but it won't completely silence it like a gate. You can choose just how severe to make the cut.


Just to add one more piece of info (and possibly muddle things up), a limiter is to a compressor as a gate is to an expander. That is, a limiter is the ultimate extreme of a compressor, just as a gate is the ultimate extreme of an expander. I mention it only because that term is often connected with the other three. But as IngredientX says, you really want an expander.

I believe there are a lot of public domain VST plugins out there -- it's used for Linux sound editors, and those Linux guys like free stuff (both beer and speech).

And don't use the noise reduction that comes with Audacity, unless you like little tinkly electronic noises and echoes in your background.

Jim
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Jim Van Verth
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Jvsquare wrote:

I believe there are a lot of public domain VST plugins out there -- it's used for Linux sound editors, and those Linux guys like free stuff (both beer and speech).


Whoopsie, wrong acronym. The Linux-related plugins are LADSPA plugins and I think the latest Audacity supports them. There are a bunch listed here:

http://ccrma.stanford.edu/planetccrma/software/ladspaworld.h...

I believe the SWH collection has a gate, and the TAP collection has an expander.

Jim
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Jim Cote
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I downloaded the former. I can play with the settings and get it to change the sound, but not in desired ways. Granted, I have no idea what the settings mean. I don't get why this is so hard. I would think what I want would be THE MOST COMMON need for a filter.
 
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Jim Cote
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There appeas to be no "binaries" for the TAP plugins.
 
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Gil Hova
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ekted wrote:
I downloaded the former. I can play with the settings and get it to change the sound, but not in desired ways. Granted, I have no idea what the settings mean. I don't get why this is so hard. I would think what I want would be THE MOST COMMON need for a filter.


Expanders actually aren't used a whole lot in the audio world. They tend to sound artificial, and there's usually no need for them. Most recordings have a low enough noise floor that no expander is necessary.

Gates are more common, but are usually used in a "low-level" practice (to cop a term from programming). When you're recording something like a drum set, you'll often gate some or all of the mics to make sure that only the drum is coming through the track. A drum is a good use for the gate because either it's hit or not; barring things like drum rolls or brushes, a drum's sound is usually over pretty quick.

What that means is that in the case of applying an effect or process to an entire process, gates are rarely used; that's more of a "high-level" finished product. By introducing a gate so late in the stage, you're almost guaranteed coming out with something that, quite honestly, won't sound professional.

There's a true (albeit arrogant) maxim in the sound world that says, "The best way to eliminate noise is to not record it in the first place." Of course, that doesn't help you, but I hope it shows why this isn't as common a task as it may appear.

If you want to try the gate anyway, here's some settings to look for.

THRESHOLD: This is the sound level that the gate will listen for. Anything softer than this, and the gate will "close," and your track will be silent.

ATTACK: This is the speed at which the gate will close. Have you ever been watching TV, and the sound on the channel suddenly drops for no apparent reason? The TV station probably had their compressor's attack set too quickly. You usually want to set this so it's a little slower than most transient sounds (T's or P's).

RELEASE: The gate will swing back open when it encounters a sound that's louder than the threshold. This is the speed at which it will swing back open. Longer releases tend to sound more natural, but it all depends on your track.

Again, I don't recommend a gate for what you're doing, because the drop to silence will be too distracting, and won't sound good. An expander will have one more parameter that will control how many dB to drop the audio level. That should hopefully make your sound more natural.

I hope this helps. Understand that sound may not be surgery or rocket science, but it's not exactly flipping burgers either. Like any craft, it takes time to learn.
 
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Jim Cote
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Ok, some more info. I've discovered the source of the noise I am trying to filter. Apparently the microphone of my headeset (AC-400) is either picking up the tiny bit of sound leaking out from the speakers over the ears, OR there's some signal crosstalk between the input and output of the headset. Any thoughts about this? Either way, if I unplug the speaker cable from the headset, the recording is clean. But I need to be able to hear sound while recording (headset or computer speakers).

Do I need to buy a higher quality headset? If it's the first case, I simply need a headset that completely covers the ears and lets no sound out. In the second case, I might need a headset with separate shielded cables for input and output. Thoughts on this? Recommended brands/models?
 
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Sounds like you need a separate mic and headphones, and headphones that are the Cliff Richard stylee whole ear variety at that. I use Sony MDRV700s and these work very well, though they're a bit expensive.

A separate mic is a good idea, because then you can use a balanced (XLR or TRS) cable to hopefully get a cleaner signal (assuming you have some way of getting balanced signals into your computer).

CRT monitors can also create hum, so try and keep the mic away from the CRT, or use an LCD monitor if possible.

Failing all of that, a noise gate plugin seems like the way to go, though you'll still get noise underneath the spoken parts (just not under the silence).
 
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Gil Hova
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Without hearing your tracks or examining your setup, I'd say that it's much more likely to be the first case. Generally, you'll get a pretty significant bleed when you don't use headphones that cover the entire ear.

EDIT: Have you tried lowering the volume of the headset, or do you find the noise bleeding through at any volume?
 
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