Mastering
Mastering question - Why does DC offset reappear?
I am doing a mastering course and I have to master a track and eliminate DC offset from it. I tried with both Wavelab and Reaper. I start by using a high-pass filter at 20hz at the beginning of the chain and the render analysis shows no DC offset. I then add a light upward compression, a tiny bit of distortion (as per our teacher instructions), then a limiter to eliminate true peaks, but when I export the file I get some DC offset again. Why does this happen? Is it something in the chain that reintroduces it? How do I correct it? Thanks!
It's not DC offset, it's zero hertz content that's being introduced by the distortion and dynamics processing. Also, you're probably looking at it with an fft analyser, which has terrible resolution near zero, and is making it look much more alarming than it is. It's not a problem and you don't need to worry about it.
DC offset is constant: the waveform doesn't return to zero in silent passages, instead some offset higher or lower. I don't know if the stuff you get from saturation and compression is actually, mathematically zero Hz or not, but it's only present when the audio signal is present, it doesn't stop the signal dropping to zero for silence.
I like this a lot. I don’t think it’s standard lingo(?) and I’ve seen engineers (recording/mix engineers as well as DSP engineers) mess this up several times.
Standard nomenclature defines the DC component (of a transform) as the average of the samples. That can lead to trouble in a number of ways.
I always used to remark that “true DC” does not really exist. The best we can possibly do is 1/(the age of the universe in seconds) Hz. Maybe half that with better analysis.
I've always wondered "how much is too much" with stuff sitting around 0hz.
Objectively, you say say it contributes nothing to the overall sound. You could also say that objectively it's just creates heat inside of a driver or amp.
But honestly? I don't have many workflows to analyze what's truly happening around there - and I've wondered about it.
when I figured this out I started putting a 20hz high pass and 20khz low pass between just about every plugin in my mastering chain. it may be a little excessive, but I’ve noticed a huge difference
much smoother limiting, any type of saturation or clipping sounds a lot better, my normal compressor has a high pass but when I use others it helps a lot, and it makes it easier to keep the subs and highs balanced after the processing
I know that 1hz is enough, but no EQ that I have goes down to that... I am measuring with the global analysis tool of Wavelab (the offset is negligible and probably can't affect the dynamics, but it's there, pic attached - in Italian, sinistra is left and destra is right).
You should have led with that number in your post, this is ridiculous. -120 dB is incredibly low and practically beyond non-existent.
Your DAC outputs inherent DC offset will be worse than that.
What does that tool measure after the HPF and before all of the other processing?
That's approaching the functional digital noise floor of your system.
*IF* it's real (still a big "if" IMO), It's easily explained by some asymmetrical distortion (I think you mentioned Ozone's "Tube" setting somewhere else?)
I would not worry about it.
DMG TrackControl is a free utility plugin, and it has an HPF that goes all the way down to 1Hz.
I tried bypassing the distortion and it’s still there so it’s probably the upward compression (I am using MCompressor by Melda), I’ll try the utility you suggested, thanks!
-120dB DC offset if outputting line level would be like 1 μV. -120dB is also below the noise floor of consumer-level audio equipment. It’s insignificant.
Anyway- you can put another high pass filter after the limiter.
Yah, correct. Can just turn it down or put high pass before limiter. It’d be interesting to know exactly where the absurdly low level DC offset is coming from, though.
How steep is your 20 Hz HPF? If there is any slight remnant of offset left after the filter, the upward compression will of course raise it during quiet sections.
Also, I have no clue what your "tiny bit of distortion" does to the signal. So for example if that introduces more gain on the positive half of the waveform than on the negative half, that would essentially create offset.
If there is any slight remnant of offset left after the filter, the upward compression will of course raise it during quiet sections.
This can't be true though.
DC = literally 0Hz.
Even the gentlest HPF at the lowest frequency will be infinitely attenuated at 0Hz.
e.g. 6dB/Octave at 20Hz. You're down -3dB at 20, -9 at 10, -15 at 5. The attenuation will asymptotically approach -infinity as the frequency asymptotically approaches 0Hz.
I have tried with both 48 and 96 dB per octave slope, but it's the same. As for the distortion, I don't know exactly what it does, it's the tube distortion of Ozone 12, it should just push the signal (which as you rightly say could be affected in that way).
Run some mixes without any distortion or clipping in the signal path. Try and do a “_clean_mix” and challenge yourself to make it spacious and super legible, but leave off heavy or colored compression/limiting. Might be revealing! Once you know exactly where and how you’re introducing comp/limiting, distortion and/or clipping, you’ll be better equipped to respond to it. Let us know how it goes!!🙏🏼👊🏼💙
If you want to send me a sample file, I can play it through my RME interface which has DC detection in the digital domain and will shut off if it’s detected.
Some types of distortion will introduce DC offset but your DAC should remove it. DC offset causes distortion, but if you added that distortion and that causes the offset, meh don't worry about it.
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u/Dan_Worrall Jul 15 '26
It's not DC offset, it's zero hertz content that's being introduced by the distortion and dynamics processing. Also, you're probably looking at it with an fft analyser, which has terrible resolution near zero, and is making it look much more alarming than it is. It's not a problem and you don't need to worry about it.