All my Sound Cards Produce Some Form of DC Offset

[jsthomps]

Hi all,
I have be experimenting with Audacity 1.3.x for the last week or so and I discovered something that I did not see until I changed the the meter range from -35dB to to -145dB. I have dc offsets in almost every laptop and sound card that I have (about 5 and counting). They range from from -60dB to -40dB. One USB sound device (Vantec NBA-2OOU) I have is even worse with a DC offset of -24dB. Is this a common issue with cheap audio cards? I know that I can remove it later with "normalize" or high pass filtering but it is quite annoying. Is it time to upgrade to a better sound device? If so, what would the group's recommendations be? I would like to help some neighbors out by digitizing their analog music collections. If that works out well I was thinking of starting a small business offering digitizing services to the community but I want to make sure that I have a least some reasonable quality ADC devices.
Scott

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[bgravato]

Is this a common issue with cheap audio cards?

I'd say it's not uncommon... DC offset is usually quite harmless though and easy to correct.

I just checked on my macbookpro and there's no dc offset neither on the built-in line-in nor on the built-in mic.

Also tested my desktop computer ('normal' pc running Linux with 2 sound cards, built-in card and a dedicated sound card), but couldn't detect any dc offset on either of the cards...

You can find a lot of info on sound cards on this thread: http://forum.audacityteam.org/viewtopic.php?f=27&t=9477

[kozikowski]

Well. Maybe.

The limit of most electronics is -129, so anything below that is effectively invisible.

I know that I can remove it later with "normalize" or high pass filtering

You would think. We did a shoot-out a while ago and the tool in Normalize won. High pass filtering will not remove the damage at the beginning and end of a piece. Normalize-DC will.

I can well imagine that all digital coders have some DC offset. I have three the same model audio devices and one of the three is almost perfect, the other two have very serious offset.

http://us.startech.com/product/ICUSBAUD ... io-Adapter

You can edit and do production with these files all day long as long as you don't try to mix in anything else. That's when you dig yourself a hole.

http://kozco.com/tech/audacity/pix/DCOffset.jpg

http://forum.audacityteam.org/viewtopic ... 40#p135352

I can also imagine that if you paid more than $19 USD that the performance might be better. The Stereo Digitizer in my Macs have no DC offset. None. I cranked the resolution, bit depth and audio gain up as far as they would go and made a recording. The meters claim a noise and offset value of something beyond -96. I can't measure it. I made an edit where I selected a segment and Normalized the DC level. I couldn't tell where the edits were.

So yes it is possible to get a well-behaved digitizer.

I never plugged in that UCA202 I bought. I wonder how that does.

Koz

[steve]

We did a shoot-out a while ago and the tool in Normalize won. High pass filtering will not remove the damage at the beginning and end of a piece. Normalize-DC will.

I thought it was a draw
High-pass filtering will not remove the damage at the beginning of the piece (it does remove DC off-set right to the end of the piece).
DC Off-set with the Normalize tool does not remove DC off-set if the waveform is asymmetric (positive going peaks have a different amplitude to negative going peaks).

If I have space at the beginning of the recording to trim off a bit, then I prefer to use high-pass filtering to correct DC off-set. This method has the added advantage of removing unwanted sub-sonic rumble. High-pass filtering is slower than the Normalize effect.

Is this a common issue with cheap audio cards?

Yes it's quite common, but not universal.
The problem is not always due (entirely) to the hardware. We have seen cases where the sound card drivers have caused DC off-set, and updating the sound card drivers has significantly improved the issue.

On Windows 7 and Vista there is sometimes an "enhancement" setting to correct DC off-set:
"Control Panel > Sounds > Recording tab > [select device] > Properties > Advanced"


The internal sound card on my laptop has DC off-set at about -90 dB (difficult to measure because the sound card is so noisy).
My Behringer UCA 202 USB sound card (using ALSA drivers on Linux) has no DC off-set.

The Behringer UCA 202 is remarkably good for the price. The limitations are that there is no input level control, so for best performance the signal level from the audio source should be adjusted (the signal level from cassette/CD players is usually about right for the UCA 202, but if the signal is too high it will distort. If you're recording from a mixer then this is not a problem as you can adjust the output level of the mixer). The other limitation is that it is limited to 16 bit recording, but 16-bit recording can still be very good as long as the input signal is at a good level.