DC Offset question

In a previous post to me you wrote:

If I use a high-pass filter it will be set to very low frequencies (10 or 20Hz at most) just to remove any possible DC offset and really low sub rumble that I don’t want. 

I use Izotope’s OZONE and it has the option of using the DC offset when mixing stereo mixes down to 16 bit.  My questions are:

what does the DC offset do;

how is that different from dithering, and;

is there any downside to using the DC offset option just to be on the safe side?

As always, thanks so much.

DC Offset means that some constant value has somehow crept into your audio signal, thus offsetting the waveforms one way or another from being symmetrical around the zero line.  Most audio waveforms are fairly symmetrical, and thus should be centered around the zero line of the y-axis (when looking at a typical waveform plot where the x-axis is time, and the y-axis is amplitude).  Imagine a perfect sine wave, that is normally perfectly symmetrical around the y-axis.  If some steady value was added or subtracted to the sine wave, it would shift the sine way above or below the y-axis by that amount.

DC Offset usually originates in the analog side of audio circuits, in amplifiers, or even A/D converters, although it can also creep in some digital processes as well.  Small amounts of DC offset are usually not anything to worry about, but if they become big enough they can rob you of headroom in your signal, and they can also greatly effect digital processing (especially certain types of plug-ins), or cause digital ticks and pops.

Craig Anderton wrote a great article describing DC offset and its effects, and ways to remove it, much better than I can describe:

http://www.harmonycentral.com/docs/DOC-1082

Like Craig mentions in his article, it’s not a bad idea to filter out all content from your audio from around 20Hz on down with a steep filter as that audio can’t be reproduced by speakers anyway, and it’s just eating up unnecessary dynamic range.  Using a steep high-pass filter like that will also get rid of any DC offset.  If your program has a specific DC offset removal feature, using it will not harm anything… it will analyze your file and try to figure out how much DC offset there is and then it will put things back the way they should be.  It’s not much different than simply using a steep high-pass filter.

Dither is something totally different.  You always want to apply dither when going from higher bit depths to lower bit depths (24 bits to 16 bits, for example).  Dither can effectively increase your dynamic range, as well as helping to eliminate, or minimize, the effects or distortion caused by truncation through the process of randomizing the quantization error.

Bob Katz is the man when it comes to explaining all things digital!  Check out his in-depth article on digital audio and dither here:

http://www.digido.com/articles-and-demos12/13-bob-katz/16-dither.html

Steve

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