DC offset

[ajcamletearthlink.net]

I know this is a basic question but....: Can someone explain what DC offset is, why it occurs, what it sounds like, and how to get rid of it?

 

thanks for your patience,

ajc

[Anderton]

Sorry I didn't get to this sooner. I'm including a lot of boring technical details in the beginning for those who are into an explanation of how this phenomenon originates. If you just want to know the bottom line, skip down several paragraphs.

 

DC offset is primarily an analog phenomenon. However, it can also end up in digital systems via means explained later.

 

In a nutshell, if there's no signal coming out of a preamp or other electronic device, the output should be exactly 0. IF the output is something other than 0 (either positive or negative), then that device exhibits a DC offset.

 

With most analog audio electronics, you will not obtain a DC offset because the output is coupled through a capacitor. A capacitor will pass AC (like audio, current that varies between positive and negative) but not DC. However, a leaky or otherwise defective capacitor may allow DC to pass.

 

The other problem occurs with DC-coupled amp systems that have no coupling capacitor. In these circuits, the active amp stage will often have a trimpot to allow trimming the output to exactly 0 in the absence of any input signal. If the trimpot is misset, or if there is no trimpot, then the potential for a DC offset exists due to inaccuracies in preamp component tolerances.

 

DC offset is more likely in high gain circuits. For example, suppose there is one millivolt of offset at the input of a mic pre with a gain of 1000, and nothing is there to trim the offest. 0.001 x 1000 = 1V, a substantial amount of offset!

 

Why does this matter? Suppose the output sits at +1V and the amp signal can reach a maximum voltage swing of +15V and -15V. Now you've reduced the available headroom on the positive side to +14V. You do have more headroom on the negative side (-16V), but as most signals are roughly symmetrical, you don't really gain any benefits.

 

In digital-land, there are two main ways DC offset can get into a signal. One is by recording an analog signal with a DC offset into a DC-coupled system. The other, and more common cause, is inaccuracies in the A/D converter or conversion subsystem that produce a slight voltage offset at the output.

 

There are three main bad effects of DC offset:

 

1. If there is a level change between sections of digital audio, you'll hear a "pop" at any abrupt transition.

 

2. Reduced headroom.

 

3. Effects that depend on a signal that's symmetrical about ground will not work as effectively. For example, a distortion plug-in that clips positive and negative peaks will clip them unevenly if there's DC offset.

 

There are two solutions to solving DC offset problems. First, DC offset correction is a function built into many digital audio editors. This function analyzes the signal, and adds or subtracts the required amount of correction to make sure that 0 really is 0. (I presume it does this by checking for the lowest-valued continuous string of samples, and assuming that should be 0...if it isn't, it performs the necessary transformation to make this the case. Any programmers know if this is how the process works?)

 

If you have a cut you want to put on CD with a DC offset, another option is to simply select a 2 millisecond or so region at the beginning and end and apply a fadein or fadeout. This will create an envelope that either starts or ends at 0, respectively. It won't get rid of the DC offset component within the cut (so you still have the restricted headroom problem), but at least you won't hear a pop between transitions.

 

Sorry if that got a little long-winded...I just wanted to make sure the advanced people had something to chew on too .

[lloydmurphyhotmail.com]

Thanks Anderton!

 

You've taught me something new and useful there.

 

http://www.musicplayer.com/ubb/smile.gif

 

Lloyd.

[Anderton]

An addendum...I mentioned a 2 ms or so fade out/in as a workaround to getting rid of DC offset. But you may need to increase that value if you still hear any kind of pop. You should have to go beyond 7 ms or so, though.