DC offset confusion

R

Rusty K

New member
Even after reading the help files I'm very confused about DC offset and whether or not to correct for it.

I can obviously see an offset in many of my tracks. Do I have a problem that needs correction or don't I?

I have for a long time wondered why some of my tracks seems hotter on one side or the other. I just chalked it up to my amp channels not being the same or even wondering if one ear was damaged a bit. It always seems to be "left". Does that correspond to an above zero line DC offset cause that's what I usually see in my tracks.

In other words does my soundcard offset the same way in each track recorded.......or do I even have a clue as to this subject matter?

I'm also not sure how to accomplish the DC offset correction in CE. I've gone to effects/ amplify /general and DC bias adjust is checked. I tried "center wave" preset and previewed and could hear no difference and I saw no difference in how the wave form looked when I applied the processing.......I'm lost!


Rusty K
 
update

hello again,

On a similar thread I was following in "Mixing and Mastering", it was suggested that to see if I actually had a DC Offset prob. that I should record "silence"/ my hardware and then use my software analizer to tell me is there is an offset. I've recorded "my hardware" I can see the vu meters moving a bit but there is no visible wave form and relating to my previous post in this thread I'm not really sure how to use my CE analizer.

Help!

Thanks,
Rusty K
 
Not sure if you have your tearms right, it's a confusing subject as it deals with that wierd netherworld of electrical voltage vs. audio dbv.

Not that I have that deep an understanding...but, it sounds like you are talking about balance not DC offset. DC offset has no "left" or "right", it is kinda what it sounds like. A wave that is skewed to either + or -. In other words it's amplitude is either top heavy or bottom heavy when viewed in the CEP editor.

It usually happens when certain peices of hardware are pushed a little bit. they impart the bias as part of their processing. The only real problem with it is that your overall dynamic headroom goes down.
 
Stuntmixer,

Yes.....I now realize DC offset has nothing to do with panning etc. but it's still hard to solve a problem one can't hear.....or maybe it's not a problem.

So I haven't found anyone that knows very much about it at all. The headroom thing doesn't worry me nearly as much as Cool Edit help files saying that some wave transforms (I take that to mean plugins/processing) may not work properly if there is a DC offset problem.

I guess I'll go to the main Cool Edit forum for help.

Thanks,
Rusty
 
You can't hear it, but you should correct it, and it's easy to correct. The reason you should correct it is that the files look dumb when they're all hanging down like weeping willows.

Okay, there's a better reason. If you don't correct the offset on a track, it's introduced into the entire mix on mixdown, and your headroom is ultimately compromised. Plus, anywhere on the track that was edited, (like on those rare occasions when you don't play the entire rhythm guitar track perfectly for all 4 minutes of the song all the way through on the very first take) is going to include a very audible "POP" into the track on the parts that go from unrecorded material to recorded material (which has the bias). You can actually see it pretty clearly when you mix a track like a comp'd vocal down to a single file...everytime there's some silence in the track that was in between parts that you mixed down, square waves. Hell, even if you don't edit, there's still gonna' be a noticeable pop at the very beginning and end of the track.

Bottom line...fix it. In the amplitude section, click on the "Center" preset like you did, make sure that the output is "0" which it should be if you click the preset, and also make sure that you check the "DC BIAS" box, along with "ABSOLUTE"... It doesn't surprise me that you didn't hear a difference when you previewed it, but it does surprise me that you didn't SEE a difference after you did it, provided that you had the entire file highlighted/selected when you ran it.

Hope this helps...or, at least makes me look smart...I mean, that's what it's all about, really.

Oh, while I'm being smugly "know it all," lemme' add that you're not going nuts on thinking that you're hearing stuff kinda' left heavy...probably most noticeably on bass and kick, right? Both Centered, but sounds like it's coming heavy from the left ON HEADPHONES, right? It's a psychoacoustic phenomenon based on the way our brains process sound (you don't hear it louder in the left, but you process it in your brain FIRST in the left, and the microsecond of delay in your skull between the left and right ear "hearing" the sound gives the effect that you're hearing). This is why you should usually run stereo delays and choruses from left to right, btw :D

Hey man, I know a lot about baseball too. lmao.
 
Bottom line...fix it. In the amplitude section, click on the "Center" preset like you did, make sure that the output is "0" which it should be if you click the preset, and also make sure that you check the "DC BIAS" box, along with "ABSOLUTE"... It doesn't surprise me that you didn't hear a difference when you previewed it, but it does surprise me that you didn't SEE a difference after you did it, provided that you had the entire file highlighted/selected

chrisharris,

Yes I'm still having a problem with the technique of achieving the "fix". If you can clarify it I would really appreciate it.

Mono wave only?....so I have to approach each channel in a stereo wave separately? I ask because I'm working on an old CD of songs recorded in a pro studio that in "waveform statistics" analizes at 0.09% DC offset in the right channel only(negligible right?) but it was there. I'm still not sure how DC offset would affect only one channel in a stereo mix. I would only want to process the right channel, even if both channels had the same dc offset I would still process them seperately as mono files since each channel has a center line....am I getting warm?

It was suggested to me that I try recording "silence" then looking for anything left that would be dc offset. I actually have a pretty good soundcard, LynxOne and a Grace 101 pre so I'm not really sure that I have a problem with dc offset. I'm just really curious about this especially since I work on tracks that didn't originate in my home setting.


Thank you,
RK
 
You'll almost always have different amounts of d/c offset on the L and R channel when it's there, b/c you almost always have different amounts of whatever track is the culprit in the left and right side. Like a mono git track that's panned hard right and has dc offset will only introduce that offset to the right side, b/c it's only in the right in the stereo mix...I usually see one side with slightly more than the other, and it usually corresponds roughly to the percentage of the offending track that's in each half of the mix.

but you correct it in stereo...I do, anyway, even if there's none on the left, b/c all processing introduces some phase shift that I wanna' keep equal between the L and Right. I'm not sure bias adjustment introduces any phase shift, but if it doesn't, then there's absolutely no harm in running a totally centered mix through a bias adjustment, and if it DOES introduce phase shift, keep it equal between the left and the right by doing the whole mix...And yeah, that's negligible for sure...means that you are missing about one hundreth of a decibal of headroom on the right, lol. I HARDLY HEAR THAT AT ALL, lmao.

By the way, the 'no input signal" noise test your talking about is certainly useful to find out whether your soundcard (the A/D converters, more specifically) are introducing the offset, but to my way of thinking, that's a waste of time for you until you figure out how to correct the Bias that is definitely coming from SOMEWHERE, lol. I mean, DC offset is introduced all the time in tiny amounts that aren't obviously visible, and since the offset itself isn't audible, it's kind of a so what for what most homereccor's are doing. But when you get bias/offset severe enough to SEE, it's definitely there, and it's definitely effecting how much you amplitude you can squeeze out of it ultimately, and there are definitely 2 very loud clicks in the track too (at least).

Anyway, if you checked the buttons exactly as I posted, highlighted the entire wave, chose "center" with the absolute option engaged, and it didn't work...then I'm stumped and baffled and have to assume that CEP is not picking up that there's offset there at all, even though you can see it...in which case, I would love to see a screen shot of what one of these waves looks like to make sure we're talking about the same thing.

anyway, if MOST of your analog tracks have visible offset, then your converters or your preamp or something outside the computer is the problem. If it only happens after you process a track with some plugin, then it's a math problem that might be specific to that one particular plugin.

Answer one more thing for me...have you ever taken one of these files and tried to get the data from CEP's analyzer about how much (if any) bias is on the track? I think it's "Get Statistics" under the Analyze tab...it's around there somewhere. If you haven't, you should, b/c if you run the analyzer, and it says there is "0%" of offset, then that would at least explain why CEP isn't fixing it...it doesn't know it's there, so there's nothing to fix, which (again) makes me want to see what one of these looks like . :D

I'm about a long-winded bastard, I know...sorry. Just basically posting to say that other than what I posted yesterday about how to fix it in CEP, I don't know anything, lol. That's as specific as my limited understanding will get you...lol...a preset.
 
chrisharris,

Longwinded hell....I appreciate it!

Yes it could very well be that I have no dc offset for Cool Edit to correct. I've actually been working on another project so I haven't been back to my "self generated" tracks to check all this out yet. You may be assured that I will!

Thanks a million,
Rusty K
 
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