DC offset

Getting Stuff Done How do I .... ?
#1

Hello,

i am new to DSP. How can i remove DC-offsets from my track or recordings in ardour (on ardour 4.6.). Are there some plugins in LADSPA or CALF for this?

#2

In LADSPA there is a DC offset remover (i saw it now). When i add it to effects i see no changes in the wave forms

#3

The waveform will not change, you will need to confirm it a different way. The waveform is of the source, not with all processing applied, as the processing could change at any moment and invalidate the waveform in that case.

Honestly a high pass filter will work for most people for DC offset removal for the record.

      Seablade
#4

Thanks,
so just adding a highpass filter with a 20 hz cutoff frequency will does the dc-removal?

#5
adding a highpass filter with a 20 hz cutoff frequency will do the dc-removal?
Yes.
#6

Also are you sure there is really a DC offset? Many instruments and vocal sounds have asymmetrical waveforms whose positive and negative peaks aren’t the same.

#7

@anahata good point! No i am not sure, there are any way some very asymmetricl peak-points in the waves. How can i know if a recording or a track has dc-offset?

#8

There is a DC component in the audio if the zero point (0 Volt level) is not at the zero point of the sample data.

You can see this if you load the audiofile into Audacity, find a place in the audio where there is no sound or the sound is very quiet. Now zoom in the audio to see how the waveform moves across the black “zero” - line. When there is no sound the waveform should be exactly on top of the black “zero” line of Audacitys waveform display. If there is DC in the audio then the waveform zero point has been moved away from the black line either up or down, depending on if there is positive or negative DC voltage in the sample data. In this case silent audio with DC voltage shows itself as a blue waveform data line that sits besides the black sample data zero line.

DC van be introduced in audio in a analog processing chain or by a damaged AD - converter circuit.

#9

AFAIK some wind instruments could DC offset naturally, if the (condenser) mic is close enough to the pipe’s exit or the whistle (i.e. wooden recorder). It’s because the stram of air moves constantly in one direction. In such case, and any other I guess, highpass filter or a good plugin does the job.

#10

You can also remove DC with Audacity. The Normalize plugin has a DC removal option. On the menu click on Effect / Normalize. On the Normalize - plugin window tag the “Remove DC Offset” Option. If you don’t want the audio volume to be changed (Normalized), then untag the “Normalize Maximum Amplitude to” - option.

#11

You wouldn’t get a DC offset from a microphone on a wind instrument, nor from a change in barometric pressure. All microphones have some kind of high pass filter: a dynamic mic cannot generate DC, and a condenser mic always has some kind of high pass mechanism, including the phantom power blocking capacitors at the preamp input.
If there is a DC offset in your digitised audio, it’s come from a preamp, the ADC or other analog processing.