Audacity Forum

I recorded this using a microphone directly plugged into my audio card. I don't know why the signal appears to have a constant bass-like signal on it... its not even bass, all the time it is like that, check it out by yourself.

Why it happen to be like that? how is that called, if it got a technical name. And how do I solve it? If you know how to.

It's called "DC offset" and it's usually caused by a faulty sound card.
In this case it may be that your sound card can't cope with such a high sample rate. Try setting the "Quality" setting in Audacity Preferences to 44100 Hz, or 48000 Hz. If you are using Vista or Win 7 you should set your sound card to use the same sample rate as is set in Audacity.

Yes, this is DC offset, but you're also right in that it's the ultimate bass signal - you actually have significant information down at 0 Hz. Try stevethefiddle's suggestion regarding the sample rate. However, if it's faulty hardware, you can correct for it a few different ways.

- Select the High-pass filter effect and enter a really low cutoff frequency - about 10 should do it since human hearing only kicks in around 20 Hz or so.
- Select the Remove DC Offset box under the "Normalize" effect. However, this will also change your overall levels since you're normalizing at the same time.

There are probably even more ways to do it, but these are the ones I can think of off the top of my head.

yeknom02 wrote:Select the Remove DC Offset box under the "Normalize" effect. However, this will also change your overall levels since you're normalizing at the same time.

No ...

since it is possible in the Normalize function to select DC Offset without doing any normalization.

Crazy but true: I do wish the developers would separate out the DC offset into its own function. Especcially sind DC-offset removal is probably the first thing you should do post-capture while Normalizing should usually be doe as the last step in the production process, after all other editing.

WC

<<<Crazy but true: I do wish the developers would separate out the DC offset into its own function. Especcially sind DC-offset removal is probably the first thing you should do post-capture while Normalizing should usually be doe as the last step in the production process, after all other editing.>>>

And while you're in there, combine Normalize and Amplify into one tool with clickies to steer the action.

Koz

<<<- Select the High-pass filter effect and enter a really low cutoff frequency - about 10 should do it since human hearing only kicks in around 20 Hz or so.
- Select the Remove DC Offset box under the "Normalize" effect. However, this will also change your overall levels since you're normalizing at the same time.>>>

We cranked through this a while ago on this forum. Turns out you can create significant damage at the beginning and end of a performance with that HiPass trick. Much better to just let the DC Remover in the Normalize tool do its job with the other parts of the tool disabled.

Koz

Thank you all for your help, this was really helpful.

I just have one more question, I notice that the signal starts its normal amplitude starting from around 0.1, starting from where the DC offset level is, lets say. So if the signal max volume is 1.0, it means I am loosing around 10% of the signal while I am recording?

Precisely why DC Offset is so damaging. The amount of offset comes right off the top of your available volume range.

You should not be recording in the top 10% of the volume range anyway, particularly if you're performing a live musical instrument. Live performance should not peak much over 0.5 to 0.75, and it could be argued that those values are not enough. It's insanely easy to create overload damage during a live performance.

I was recording a professional voice a bit ago and I had set everything up so he was in good order and was producing a lovely track. What I didn't notice was over the course of the show, he got progressively louder and louder. I was sucked up in the drama of the performance and caught it just as he was hitting his first red light peak on the sound mixer.

"Mark, can we take that last line again, please?"

Koz