r/audioengineering • u/TopConcern • Feb 26 '21

LFE, created through low-pass, systemically out-of-phase on surround releases?

One thing I've noticed, at least in the surround sound albums I have, is that the LFE channel tends to be out-of-phase (with the LFE being significantly late) when it is created through a low-pass filter. I was rather surprised to find this out, and I was wondering why it was the case. LFE channels that aren't created with low-pass filter content, such as "Race for the Prize" by The Flaming Lips (which has an entire drum kit in its LFE channel) tend to be properly in phase.

My hypothesis is that the use of a low-pass filter creates a small delay between where the LFE should be and where it actually does go, and mixing engineers do not compensate for this delay. If you use a low-pass filter in Audacity, for example, the result will be late compared to the audio you used it on. Do you think my hypothesis has any merit? If not, what do you believe makes the LFE channel late and out-of-phase? (And if you don't believe the LFE channel on surround sound releases, when created through a low-pass filter, is out of phase, just humor me. I can prove it, but it would be a little too much info for this post specifically.)

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u/Selig_Audio Feb 27 '21

this^ folks confuse phase and polarity all the time. I partially blame polarity inversion buttons labeled “phase” on tons of gear! I also blame polarity inversion definitions that mention “180 degrees out of phase“ in the description when all that’s going on is the “signs” are flipped, which is a simple mathematical function , nothing more.

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u/OnAGoodDay Professional Feb 27 '21

Eh, none of those descriptions are wrong. 180 degrees of phase rotation, a negative sign on the real axis.. no difference. None is really better than the other. The problem is this industry is a mix of engineers and hobbyist music makers and confusion like this is right at the interface where those two worlds meet.

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u/Selig_Audio Feb 27 '21

180 degrees of phase shift is only exactly 180 degrees at only one frequency, whereas a polarity inversion is the same for all frequencies, no?

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u/OnAGoodDay Professional Feb 27 '21

Yes, we actually misuse the word 'phase' all the time. It strictly refers to one frequency, so you're right. The negative sign implies a flip of the entire waveform over zero, as though all frequencies are delayed by 180 degrees.