DC offset

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Revision as of 17:29, 30 October 2012 by Edgar (talk | contribs) (show no mercy <grin>!)
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ToDo-2 Peter 30Oct12: Initial draft created; ready for review and merciless editing.
See the Talk page for background.

Background

DC offset is an offsetting of a signal from zero. On the Audacity waveform it would mean that the waveform in default view appears not to be centred on the 0.0 horizontal line.

Left and right channels of a recording displaying serious DC offset

The cause is almost always a fixed voltage offset before the analogue signal is converted to digital values (often from a faulty sound card). This voltage is known as a DC offset, and is normally so small as to not be noticeable, but with defective or poor quality hardware it may become big enough to to be a problem. This can also happen if another piece of equipment in the audio chain is defective and producing a large voltage with is being fed through to the computer.

  • A sound that has DC offset will not be at its loudest possible volume when normalized (because the offset consumes headroom). 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.
  • DC offset can cause inaudible low level distortion (which becomes audible when other filters are applied or if the sound is compressed upon export).
  • It can cause audible clicks at the start and end of tracks when played back (even without editing).

Removing DC offset

Ed 30Oct12:

Removing any DC offset is desirable because its presence can cause clicks when playing or editing and it limits the headroom available for normalization so restricts the loudness that can be achieved.

remove as it duplicates previous para.

Ideally DC offset should be removed immediately after importing or recording and before any processing is done.

  • Correct any DC offset (if it is present) by running Effect > Normalize..., with the option checked "Remove any DC offset (center on 0 vertically)". Uncheck the "Normalize maximum amplitude..." box unless you want to run Normalize as well (see here for what Normalize does and when to use it).
  • An alternative method is to use dcRemove in the plugin.org.uk suite of LADPSA plug-ins for Windows, Mac or Linux. This removes the DC (0 Hz) component from the audio, using a high pass filter.
Ed 30Oct12: I do not see the need tor a hint box in the following:
Newer Windows PCs may have a DC offset cancellation feature when recording from the built-in sound inputs. To check or enable:
  1. Right-click over the speaker icon in the System Tray > Recording Devices, or click Start > Control Panel > Hardware and Sound > Sound, then the "Recording" tab;
  2. Right-click over empty space, show disabled and disconnected devices, then right-click over each device and enable it;
  3. Right-click or select each device, choose "Properties" then look in the "Enhancements" tab;
  4. If there is no "Enhancements" tab, look in the sound device's own control panel in "Hardware and Sound".


Limitations

Ed 30Oct12 most of the following is duplicitous; move the "asym" stuff to highlight it:

For most users of Audacity for audio processing:

  • DC offset is not audible (so may not be corrected by the user) but reduces headroom for subsequent processing .
  • Differences in DC offset cause unwanted clicks where audio is tacked together.
Where a waveform is inherently asymmetrical, such as in recordings of brass instruments, Audacity's DC removal method (making the average positive and negative sample values equal) may actually create an offset.
Warning icon Note carefully: Removing offset after the event does not reinstate the original loss of headroom, so you should ideally correct the DC problem at source. Where a waveform is inherently asymmetrical, such as in recordings of brass instruments, Audacity's DC removal method (making the average positive and negative sample values equal) may actually create an offset.