Correcting the DC Offset
A musical waveform, as with any other waveform, contains spectral components of varying frequencies. Part of this spectrum is known as the direct current component; basically a 0Hz signal. Ideally, there should be zero spectral activity as the signal approaches 0Hz, for several reasons, which are beyond the scope of this tutorial. Simply speaking, if there is a portion of the signal power contained at close to DC frequencies, it can adversely affect the sound quality of your recording and, at very high levels, can cause damage to your speakers or amplifiers. While it’s highly unlikely that you should worry about equipment damage, sound quality degradation as a result of DC offset problems is actually quite common, is very simple to fix, and can give your recording just a little more crispness and clarity. DC components as part of your musical waveform is often called an offset because it evidences itself as the overall waveform not being centered around 0 amplitude, but by either being above or below the horizontal axis in a simple waveform plot. If it is large enough, you will be able to observe an “offset” as the wav sits above or below the 0-amplitude centerline
Note: If you are going to be remastering a recording, DC offset correction should be the first step. Always apply this before any other steps, including fade-ins and fade-outs, as well as more complicated procedures.
The easiest and most effective way of correcting the DC offset is to use Cool Edit Pro. As with many of my tutorials, I recommend using the "Open Append" command and run the correction on an entire set all at once. This will ensure that each track is shifted together. Otherwise, it's possible that the shift will be greater on one track than on an adjacent one, and the resulting change in amplitude between the tracks may cause a small click to appear. To ensure your recording stays perfectly smooth between the tracks, and to save yourself time, apply the DC offset correction on the whole set. For a quick instructions on using Open Append,
click here.
Note: Only apply this on recordings from a single source. If two separate sources with difference DC offset values are fixed at once, your correction will not be as effective and may actually be worse.
First, analyze the WAV to see if correction is necessary. There are two ways to do this:
1. Select the entire WAV (<Ctrl>+A), then go to Analyze > Statistics. Look at the value for "DC Offset". If it is not zero, this recording could benefit from dc offset correction.
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2. If the DC offset is large enough, zooming in on a quiet portion of the audio will reveal that the DC offset is high enough to cause the waveform to ride above or below the red centerline.
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If your recording needs to have the DC offset corrected, simply select the entire WAV (<Ctrl>+A). Go to Effects > Amplitude. A window will pop up with a set of preset templates to process the amplitude of your waveform. Select the "Center Wave" option, and press OK.
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It's that simple. You can see that the wav is now centered about the red centerline. Compare the plot below to the one above, and notice the huge difference. Your ears will thank you.
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