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Can someone explain DC Offset to Me?

Posted: Sat Jun 09, 2007 9:36 pm
by Beatport
Can someone explain DC Offset to Me?

And what's considered a siginificant offset? In Logic I'm getting anywhere from .002% to .4% on some stuff. Is that A lot?

Posted: Sat Jun 09, 2007 9:48 pm
by ethios4
http://en.wikipedia.org/wiki/DC_offset
DC offset is usually undesirable. For example, in audio processing, a sound that has DC offset will not be at its loudest possible volume when normalized (because the offset consumes headroom), and this problem can possibly extend to the mix as a whole, since a sound with DC offset and a sound without DC offset will have DC offset when mixed. It may also cause other artifacts depending on what is being done with the signal.
I like to think of it in terms of speaker motion. In theory, the 0 line in audio should correspond to the rest state of the speaker. As the signal goes above the line, the speaker moves forward from center; as the signal goes below the line, the speaker moves back from center (whether positive signal equals the speaker moving forward depends on the phase shift the speaker is wired at). So, if there is a DC offset, the speaker's center of motion is shifted so that it does not correspond to the speaker's rest state. The practical consequence of this is that the signal can't be turned up as much, and may result in speaker damage.

Please correct me if I'm wrong in this...

Posted: Mon Jun 11, 2007 6:53 pm
by M. Bréqs
ethios4 wrote:http://en.wikipedia.org/wiki/DC_offset
DC offset is usually undesirable. For example, in audio processing, a sound that has DC offset will not be at its loudest possible volume when normalized (because the offset consumes headroom), and this problem can possibly extend to the mix as a whole, since a sound with DC offset and a sound without DC offset will have DC offset when mixed. It may also cause other artifacts depending on what is being done with the signal.
I like to think of it in terms of speaker motion. In theory, the 0 line in audio should correspond to the rest state of the speaker. As the signal goes above the line, the speaker moves forward from center; as the signal goes below the line, the speaker moves back from center (whether positive signal equals the speaker moving forward depends on the phase shift the speaker is wired at). So, if there is a DC offset, the speaker's center of motion is shifted so that it does not correspond to the speaker's rest state. The practical consequence of this is that the signal can't be turned up as much, and may result in speaker damage.

Please correct me if I'm wrong in this...
Nailed it. You could also think of it visually. Imagine a .wav (or .aiff) editor. The graphic of the waveform would have it's central point either higher or lower than the actual zero line.

This isn't just a case of a symmetrical wave that is too high or too low. Some wierd waves (complex sawtooth for instance) that are found only in artificial or synthesized sounds have asymmetrical waves. That puts more "activity" on one side of the zero, even though the high point and low point of the wave could be equal values. This could also be considered DC offset, though most analog-modeling synths compensate.

It was common with real analog synths and analog gear when for some wierdo reason the piece of gear would get a static charge; then everything was offset by that slight value. This isn't to be confused with AC 60hz hum, another common problem with the same sort of gear.

Anyways, DC offset is usually bad. I think every wave editor has an option to calculate and remove it. For digital audio with a DC offset, a wave editor is the best choice, and you can remove the offset without changing the character of the sound at all, since you're not changing the relative values between bits, you're simply adding or subtracting a set (and equal) value to them all.

Posted: Mon Jun 11, 2007 7:16 pm
by craw
interesting topic ... I have been wondering about this recently ... but from the other side of the coin as it where ...

I noticed while digitising my record collection that often tracks that I really liked had an asymmetrical waveform, with a large portion of the wave being significantly higher above the centre line on both channels. I have been wondering what these producers have been doing in order to achieve this effect, and what this effect contributes to the sound, and is it something to do with why I like these tracks ...

anyway, nothing to do with the original post, but an interesting insight for me never the less ...

perhaps DC offset can be desirable too??? mabye my ears are just fukt???

I noticed it recently on a carl craig track I was digitising recently ... need to check my waves and check which one ... perhaps his gear is fukt, or does he do that on purpose sometimes???

Posted: Mon Jun 11, 2007 7:30 pm
by M. Bréqs
craw wrote:interesting topic ... I have been wondering about this recently ... but from the other side of the coin as it where ...

I noticed while digitising my record collection that often tracks that I really liked had an asymmetrical waveform, with a large portion of the wave being significantly higher above the centre line on both channels. I have been wondering what these producers have been doing in order to achieve this effect, and what this effect contributes to the sound, and is it something to do with why I like these tracks ...

anyway, nothing to do with the original post, but an interesting insight for me never the less ...

perhaps DC offset can be desirable too??? mabye my ears are just fukt???

I noticed it recently on a carl craig track I was digitising recently ... need to check my waves and check which one ... perhaps his gear is fukt, or does he do that on purpose sometimes???
Could be your turntable; it's an analog signal, that's prone to building up a static charge since there's friction involved.

Posted: Mon Jun 11, 2007 7:58 pm
by laird
Almsot certainly that's your turntable.
Check to make sure its grounded well.

And DC offset has no sound of its own. (unless you count the extra little wear and tear on your speakers over time producing distortion)

It'll just reduce your available headroom... thus your vinyl digitized files might be 15 bits in depth instead of the full 16.

Posted: Mon Jun 11, 2007 8:07 pm
by jasper
I learn something new every day.
Totally awesome

Posted: Mon Jun 11, 2007 8:14 pm
by kettensaege
I've noticed this with nearly all of my bass guitar recordings (yamaha rbx775 with active circuit) Waveform peaks above zero are way larger than those below zero. Anybody got an idea why this happens? I wouldn't say it sounds bad, but then again I haven't had a chance to compare it to other bass guitars in my recording setup.

Posted: Tue Jun 12, 2007 6:03 pm
by gabster
Do we have a DC Offset meter in Live???

Gabi.

Posted: Tue Jun 12, 2007 6:18 pm
by Tone Deft
nope and i'm not sure there is one out there.

To find DC offset, look at the waveform. If it's a one shot, look at the start/end where the volume goes to zero, if it doesn't end on the line marking the center point of the waveform it has a DC offset.

You need to use an audio editor to fix it. I think DC offset is getting a little sensationalised here, it's not good but on a scale from 1 to 10 it's a 6.

I don't think it's been mentioned that if you use a DC offset sample and then a non-DC offset sample you get a slight pop from one to the other as the sharp edge that is the offset is played.