DC offset? -Sound Forge - PC - MAGIX Community Forums Archive

Subject:DC offset? Posted by: EricFG Date:4/13/2002 4:50:06 PM What exactly is a "DC offset"? I know it's when the waveform doesn't match the center line, but why's it happen? On the CDR that I just re-recorded and equalized for optimum sound I noticed that my average waveform was riding above the centerline. How does that offset affect the actual music? Can I hear the difference? Should I even bother correcting it? What factors lead to DC offset? In my many hours of recording with Sound Forge I've rarely seen this happen. Why today with this one CDR? Thanks so much, Eric Gaumann
Subject:RE: DC offset? Reply by: beetlefan Date:4/14/2002 3:35:53 AM DC offset is a product of the soundcard. An electrical current is used to bias the incoming signal to make it more linear. On better cards this is not a problem but on cheaper cards (Soundblaster, ESS, mainly) it can be bad. Most editing software has a checkbox or some other means whith which you can configure it to add or remove the correct amount when you record so that you get "0". If your card is of quality you can keep the box unchecked, if you wish. I have a Turtle Beach Santa Cruz card and it behaves, so I keep the box unchecked. If you get a click or pop at the beginning or end of tracks this may well be the culprit. This was a problem with early commercially produced CDs as well. I can't say what happened in your particular case, but you can remove the offset in Sound Forge or any other quality editor. It is always important, or at least a very good idea that if you have an offset that you first remove it before doing any processing except de-clipping. DC that is way off will result in reduced dynamic range and a lower signal to noise ratio.
Subject:RE: DC offset? Reply by: Rednroll Date:4/14/2002 1:13:27 PM Please ignore any of the previous information you have just read from beetle fan. An audio signal is an AC signal. Most electronics gear inputs and outputs have what is known as AC-coupling capacitors. What these capacitors do is eccentially remove any DC signal that may get added to your audio signal when traveling from input to output of your external electronic equipment. Capacitors will degregate over time, thus lose some of their coupling intention and in result DC will be added to your audio signal. Most people have used old vintage gear, be it a keyboard or compressor they love, and you will notice that these devices are noisy now. Another way DC can be added to your audio signal is by a mismatch of grounds within your studio set-up. Most Analog gear has seperate grounds within the electronics. 1 for a power ground and 1 for an audio ground. If you did not seperate these grounds, then high current noises from the Power supply would get introduced into your audio, thus making it noisy. Well it's very well posible that the audio grounds used between the different electronics gear within your studio may not be having the same audio ground of zero volts, thus whatever the difference between these 2 devices it will cause some DC offset added to the audio signal. This is why if your studio is not properly grounded you will start to hear things like hums through your monitors, which becomes more evident as you add more gear to your studio. As you have noticed this is visible looking at it in sound forges wave form. What you will hear is a Low Hum added to your audio and the audio will sit above the Zero line in sound forge, thus it will reduce your dynamics range. When you do a DC offset process in Sound Forge it will eccentially do what those AC-coupling capacitors where intentionally meant to do. It will probably do it better since you are now in the digital world and you can make a brick wall filter. You can't make a very good brick wall filter in the analog domain and frequencies like 20Hz AC is very similar to a DC voltage source. So "cheaper sound cards" as beetlefan has mentioned will use cheaper band pass filters that don't resemble much to a good brick wall filter. Well these filters won't remove all of the DC offset and they also may actually reduce some of the level of your lower frequencies. Bettle doesn't seem to have any experience with external gear and only knows about the effects of using a cheaper sound card. That's probably because he can't purchase anything more than a computer and an ok sound card, yet he can point his finger and say "you're using a cheap soundcard"...which by my explanation MAY or MAYNOT be the source of your DC offset, you can actually get DC offset using a very expensive sound card. I believe you originally said you received the audio on a CD, and I'm assuming you probably imported the audio digitally through a rip function using your CDrom. So what you probably have in sound forge has never seen the inputs of your "cheap" Sound Card. Hope this explanation helps, Rednroll and shut-up Beetle you are a clueless wonder to these forums.

Please ignore any of the previous information you have just read from beetle fan.

An audio signal is an AC signal. Most electronics gear inputs and outputs have what is known as AC-coupling capacitors. What these capacitors do is eccentially remove any DC signal that may get added to your audio signal when traveling from input to output of your external electronic equipment. Capacitors will degregate over time, thus lose some of their coupling intention and in result DC will be added to your audio signal. Most people have used old vintage gear, be it a keyboard or compressor they love, and you will notice that these devices are noisy now. Another way DC can be added to your audio signal is by a mismatch of grounds within your studio set-up. Most Analog gear has seperate grounds within the electronics. 1 for a power ground and 1 for an audio ground. If you did not seperate these grounds, then high current noises from the Power supply would get introduced into your audio, thus making it noisy. Well it's very well posible that the audio grounds used between the different electronics gear within your studio may not be having the same audio ground of zero volts, thus whatever the difference between these 2 devices it will cause some DC offset added to the audio signal. This is why if your studio is not properly grounded you will start to hear things like hums through your monitors, which becomes more evident as you add more gear to your studio.

As you have noticed this is visible looking at it in sound forges wave form. What you will hear is a Low Hum added to your audio and the audio will sit above the Zero line in sound forge, thus it will reduce your dynamics range. When you do a DC offset process in Sound Forge it will eccentially do what those AC-coupling capacitors where intentionally meant to do. It will probably do it better since you are now in the digital world and you can make a brick wall filter. You can't make a very good brick wall filter in the analog domain and frequencies like 20Hz AC is very similar to a DC voltage source. So "cheaper sound cards" as beetlefan has mentioned will use cheaper band pass filters that don't resemble much to a good brick wall filter. Well these filters won't remove all of the DC offset and they also may actually reduce some of the level of your lower frequencies. Bettle doesn't seem to have any experience with external gear and only knows about the effects of using a cheaper sound card. That's probably because he can't purchase anything more than a computer and an ok sound card, yet he can point his finger and say "you're using a cheap soundcard"...which by my explanation MAY or MAYNOT be the source of your DC offset, you can actually get DC offset using a very expensive sound card. I believe you originally said you received the audio on a CD, and I'm assuming you probably imported the audio digitally through a rip function using your CDrom. So what you probably have in sound forge has never seen the inputs of your "cheap" Sound Card.

Hope this explanation helps,
Rednroll

and shut-up Beetle you are a clueless wonder to these forums.