Clipping/Distortion/Fuzz at Low Recording Volumes

[bob_e_s]

Hi all,

I've had some great help of this forum in the past, and my recording skills have come on leaps and bounds as a result.

I use Audacity to record my own music, playing every instrument and layering up into full 'rock band' type productions. I get a finished sound that is generally more than adequate for my purposes. My recording interface is a Line 6 UX1, which I can plug my guitars or vocal mic into, and then use Line 6 Pod Farm software to get the desired effect (I don't use the effects in Audacity at all).

I had until recently a problem with clipping on my final mixed and exported tracks, which were the fault of recording at too high a volume.
However, on a couple of tracks on my most recent album I've noticed a clipping sound (like a mild, varying distortion) on the finished mixed and exported MP3.
Now, rather than being general across the track the poor sound is limited to one instrument in each track, with the other instruments sounding fine.

One track has a mandolin part, one a clean sounding electric guitar. Now the mandolin part has quite a low recorded volume, so I had to use the gain slider in Audacity to boost the sound to match the rest of the instruments, but the electric guitar doesn't have this problem (in fact, I probably lowered the volume with the gain slider before mixing and exporting).

It's a minor thing, and most people listening wouldn't notice, but it irks me!

My songs are on myspace, so if necessary I'll post links so you can listen for yourself.

Cheers,

Rob

[DVDdoug]

What MP3 settings are you using? MP3 is lossy compression. You might try exporting to WAV first, just to make sure the quality loss has nothing to do with the MP3 format. Using a higher MP3 bitrate (or higher quality setting) will give you better quality (and bigger files).

In the View menu there is an option to Show Clipping. Do you have that checked? Do you see a red waveform where it's clipped?

As you may know, mixing is done by addition (summation). You can easily get clipping if you mix two or more signals that don't clip by themselves.

Perveived loudness is more-related to average levels than peak levels. A mandolin track with 0dB peaks is not going to sound as loud as distorted electric guitar with the same 0dB peaks. Even though your mandolin doesn't seem loud, or doesn't clip by itself, adding it to the mix may cause the peaks in the mix to clip.

Audacity itself won't clip... It uses 32-bit floating-point internally, so it's not limited to 0dBFS (1.0). If you overdrive your ADC during recording, your ADC will clip. If you overdrive your DAC during playback, your DAC will clip.

And, if your peaks are over 0dBFS and you save to an integer format (such as 16-bit WAV), the saved-file will be clipped. This might be what's happening, since you notice the distortion after exporting.

-----------------------------
I've never actually mixed in Audacity, so somebody else may have a much better solution... But, here's my suggestion -

1. Render (export) your mix as 32-bit WAV.*

2. Open (import) the mixed WAV file and use the Amplify effect to set the New Peak Amplitude to 0dB (or slightly below 0dB).

3. Now, after adjusting the levels, render again to your final MP3 format.

If the new waveform with mazimized 0dB peaks doesn't sound loud enough, the solution is dynamic compression (compressor effect... not related to MP3 file compression). Compression (or limiting) can boost the average level without bossting or clipping the peaks.

(If you've got questions about compression & loudness, it's probably best to start a new thread.)


* P.S.
Like I said, I've never done this... I think you need to select custom FFmpeg Export, then wav and pcm_s32le.

[steve]

1. Render (export) your mix as 32-bit WAV.*

2. Open (import) the mixed WAV file and use the Amplify effect to set the New Peak Amplitude to 0dB (or slightly below 0dB).

3. Now, after adjusting the levels, render again to your final MP3 format.

I agree that the most likely cause is that although the individual tracks appear to be OK, the final mix could be going over 0 dB.
An easier way to prevent this (for the benefit of other readers, this only works in recent versions of Audacity 1.3.x):
1) Ctrl+A (or "Edit > Select > All")
2) Tracks > Mix and Render.
3) Effect > Amplify (new peak level set to 0 or less - I usually use -1 dB if the final export is going to be WAV format, or -2 dB if the final export is to be MP3)

Another alternative is:
1) Ctrl+A (or "Edit > Select > All")
2) Ctrl+shift+M (mix to new track)
3) Apply the Amplify effect to the new mixed-down track
4) With the mixed-down track selected use "File > Export Selected".
The advantage of this second method is that you still have the individual tracks should you wish to do more editing/processing.

Another tip is to grab the Meter Toolbar with your mouse and stretch it to the full width of your screen (makes it easier to see), Keep an eye on the Playback Meter when you are adjusting the track levels and check that the Playback Meter stays below 0 dB.

[Gale Andrews]

1. Render (export) your mix as 32-bit WAV.*

2. Open (import) the mixed WAV file and use the Amplify effect to set the New Peak Amplitude to 0dB (or slightly below 0dB).

3. Now, after adjusting the levels, render again to your final MP3 format.

I agree that the most likely cause is that although the individual tracks appear to be OK, the final mix could be going over 0 dB.
An easier way to prevent this (for the benefit of other readers, this only works in recent versions of Audacity 1.3.x):
1) Ctrl+A (or "Edit > Select > All")
2) Tracks > Mix and Render.
3) Effect > Amplify (new peak level set to 0 or less - I usually use -1 dB if the final export is going to be WAV format, or -2 dB if the final export is to be MP3)

Another alternative is:
1) Ctrl+A (or "Edit > Select > All")
2) Ctrl+shift+M (mix to new track)
3) Apply the Amplify effect to the new mixed-down track
4) With the mixed-down track selected use "File > Export Selected".
The advantage of this second method is that you still have the individual tracks should you wish to do more editing/processing.

Just to clarify that in the same way, the above depends on your Quality Preferences being set to the default 32-bit float in the first place. If not, any clipping from the render cannot be removed by Amplify.


Gale

[steve]

Just to clarify that in the same way, the above depends on your Quality Preferences being set to the default 32-bit float in the first place. If not, any clipping from the render cannot be removed by Amplify.

Yes, and although 32-bit is the default Quality setting I should have mentioned that. Also, if any tracks have been imported it is necessary to check that they are 32-bit float (it will say in the box on the left end of the track). Depending on the file type an imported file might not be 32-bit float even if the Audacity Quality setting is 32-bit float. To convert a track to 32-bit float, click on the track name and from the drop down menu select "Set Sample Format > 32-bit float".

[bob_e_s]

Chaps,

Thanks for the responses. I tried exporting to WAV, and as expected it sounded great.
I then imported the WAV into Audacity then exported again into MP3, which saw the return of the fuzzy guitar that wasn't there on the WAV.

I'd obviously like to be able to get the decent sound in an mp3, as the huge filesize of a WAV track isn't very user-friendly.

I've attached a screen shot of the rendered Audacity file, be interested to know if you think this file should be clipping (I've recorded tracks with much higher peaks than this that don't clip, or fuzz in the mp3).



This is the link to my myspace page to the same song in the image above, if listening to it helps!
http://www.myspace.com/568937461/music/ ... 3-85196809

Hope you can help!

Rob

[Gale Andrews]

As DVDDoug asked, what MP3 bit rate are you exporting at? To check. click File > Export and click "Options". Try raising the bit rate or using variable bit rate instead of constant bit rate. Variable bit rate will hopefully allow more bits to be used for the hard-to-encode guitar part and less for the mandolin.


Gale

[bob_e_s]

Ok, I'll try that tonight and let you know how it goes!

Cheers.

[bob_e_s]

Changed the bit rate from constant to variable, seems to have done the trick.

Thanks chaps.

[kozikowski]

Isn't that option there because not every player supports variable?
Koz