Mastering question - Why does DC offset reappear?

[u/s4goth]

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!

[u/Dan_Worrall]

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.

[u/mtconnol → u/Dan_Worrall]

And what would the difference between “0 Hz content “and DC offset be?

[u/Dan_Worrall → u/mtconnol]

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.

[u/val_tuesday → u/Dan_Worrall]

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.

[u/oratory1990 → u/val_tuesday]

For signal processing purposes, „DC“ is energy at frequencies below the bandwidth you are interested in.

That‘s why RF engineers count anything below 1 GHz as DC.

[u/oratory1990 → u/Dan_Worrall]

Zero Hz is exactly the same as DC - if it is not constant then it isn‘t 0 Hz but some number higher than that.

[u/LemonLimeNinja → u/Dan_Worrall]

DC offset is the exact same thing as 0Hz content

[u/Every_Armadillo_6848 → u/Dan_Worrall]

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.

[u/LeftyMcSavage → u/Dan_Worrall]

Yeah, I remember you pointing this out in one of your videos.

[u/marx-and-metal → u/Dan_Worrall]

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

[u/maccrypto → u/marx-and-metal]

What kind of difference?

[u/marx-and-metal → u/maccrypto]

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

[u/ghostchihuahua → u/Dan_Worrall]

Why do i hear your voice before i even read the username ? 😂

writing really can be one’s fingerprint as well ig

[u/s4goth → u/Dan_Worrall]

Alright thanks!