Remove DC offset from audio
Waveform sitting above or below the centre line? fix.fm measures the DC offset on each channel and takes it off. Free, in your browser, nothing uploaded.
Updated
Never uploaded. Everything runs in your browser.
In short
A DC offset is a constant push that sits the whole waveform above or below its centre line. You can't hear it, but it clicks at every cut and wastes headroom; cheap interfaces, old sound cards and some phone recorders cause it. fix.fm measures each channel's offset and takes it off, leaving the sound itself untouched. On our sample the offset went from 2% of full scale to 0%.
What's happening
A DC offset is a constant push added to every sample, so the whole waveform sits a little above or below its centre line. It usually comes from the gear: a cheap interface, an old sound card or some phone recorders.
You can't hear the push itself. You hear a click wherever the audio is cut, faded or starts playing, as the waveform jumps between zero and the offset. It also wastes headroom, so the loud parts clip sooner.
fix.fm measures where each channel sits on average. Its warnings start at an offset of 0.2% of full scale; it calls it moderate at 1% and severe at 3%. The fix takes each channel's own offset off every sample, so the sound itself comes back exactly as it went in.
In Audacity, the same fix is Effect, Volume and Compression, Normalize, with "Remove DC offset" ticked.
How far the DC offset fix goes
A voice 2% off centre read moderate and after the fix fine, at 0%, with the sound unchanged sample by sample. Music with 4% on one side was centred on each side by its own amount, and a tiny offset was left alone. Measured on with the samples described in how we test.
The DC offset fix looks for one thing: a waveform whose middle isn't at zero. The three cases below are a clean reading and a piece of synthesised music, pushed off centre in software, so we know exactly where the centre line was and can check the fixer puts it back there.
One clean reading and one piece of synthesised music, pushed off centre in software, then run through this page's fixer at full amount. Every clip is matched to -23 LUFS. Headphones help to hear the tick at each end.
Clean reading
The reference. The waveform swings evenly either side of its centre line.
Voice pushed 2% off centre
It sounds almost the same, because an offset is silent. Listen to the very start and end: a soft tick as the player jumps to the offset and back.
Voice, after the fix
No tick at the start or end. The voice itself is exactly what went in.
Music, 4% left and -1% right
Synthesised music with a different push on each side. Again, the tick is at the edges.
Music, after the fix
Each side is centred by its own amount. The music is unchanged.
A tiny 0.05% offset
About what decent gear leaves. Nothing to hear and nothing to fix.
A tiny offset, after the fix
Identical to the recording. The check reads it as centred, so the fix leaves it alone.
Public-domain LibriVox voice and music synthesised by fix.fm. The offsets are added in software rather than recorded through faulty gear. So the clean version really is what was there before.
| Case | The check | Against the sound | The sound itself |
|---|---|---|---|
| Clean reading | 0%, fine | none | untouched |
| Voice off centre | 2% moderate to 0% fine | -8 dB to none | unchanged |
| Music, each side off | 4% severe to 0% fine | -8 dB to none | unchanged |
| A tiny offset | 0.05% fine to 0.05% fine | -40 dB to -40 dB | untouched |
The voice sat 2% off centre, which the check reads as moderate. After the fix it reads 0%, fine. The fix takes the same amount off every sample, so nothing about the voice changes: only where it sits.
The music had a different offset on each side, 4% at worst, which reads severe. Each channel is measured and centred on its own, so both end at 0%.
The tiny offset, 0.05%, reads fine and comes back exactly as it went in. Almost every recording has a trace like this, and it does no harm, so fix.fm leaves it.
What this does not prove: these offsets are a fixed push added in software. Some faulty gear makes an offset that drifts slowly during the recording, and taking off one average leaves a little of that drift. A high-pass filter in your editor deals with drift. How we test has the pass marks behind the check.
How it works here
- 01
Finds the centre line
It measures where each channel's waveform sits on average. In a healthy file that's zero; an offset pushes it up or down.
- 02
Puts it back on centre
It takes that same amount off every sample of the channel, so the waveform swings evenly again. Nothing else about the sound changes.
- 03
Stays on your computer
Your file is never uploaded. The DC offset fix runs in this browser tab.
Questions
What is DC offset in audio?
A constant push added to every sample, so the whole waveform sits above or below zero instead of swinging evenly either side of it. It usually comes from the recording gear, not from the sound.
Can you hear DC offset?
Not on its own. You hear it as a click or thump wherever the audio is cut, faded or starts playing, because the waveform jumps from zero to the offset. It also leaves less room before the loud parts clip.
What causes DC offset?
Usually a cheap or faulty audio interface, an old sound card or a phone recorder whose input isn't balanced. A few plug-in effects can add it too. It is rarely in the sound itself.
Will removing DC offset change how my recording sounds?
No. The fix takes the same amount off every sample, so the sound that swings either side of the centre line is exactly what went in. On our samples it changed by less than a tenth of one 16-bit step.
How do I remove DC offset in Audacity?
Select the audio, open Effect, then Volume and Compression, then Normalize. Tick Remove DC offset, untick the peak amplitude box if you don't want the volume changed, and apply. fix.fm does the same thing here, without installing anything.
Is my audio uploaded anywhere?
No. The file is read and fixed inside this browser tab and never leaves your computer.
Related fixes
Often turns up with DC offset- FX-04DistortedClipping and crackle
- FX-09ClicksTicks, pops and crackle
- FX-05Too quietLevel to podcast volume
Working on a song? Fix music checks it for this and every other problem a mix can have, in one pass.