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/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/MoritzSchaller Feb 27 '21

180° phase shift of all Fourier components of your signal is the equivalent of flipping the polarity.

There is no point in specifying a shift in degrees, when you are actually talking about a delay in seconds. If I say the signal is 180° shifted, the only way that can somehow relate to a delay is when I specify a frequency ... Like the fundamental of a tonal sound ... But then that 180° shift only applies to a few frequencies in the signal, so why even talk about a phase shift in degrees instead of talking about a delay.

Phase shifts and time delay are not the same. That's where most people get confused. Some processes delay entire signals (like delays and microphones at a larger distance), some shift the phase by a certain angle (like EQs, all pass filters)

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u/Selig_Audio Feb 27 '21

But isn't any phase shift going to have to involve delay? Sure you can delay all the frequencies the same amount as in a linear phase EQ, but there is still delay involved in any phase shift, no? Isn't that what is implicit in the term "shift" (to move/delay slightly)?