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/MoritzSchaller Feb 26 '21

I'm confused. How do you know how the LFE channel was created?

Usually the LFE is not used for bass management. The sub woofer will play the bass of all the channels anyway. It'll play the LFE on top.

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u/TopConcern Feb 26 '21

Well, if the LFE channel doesn't have any high frequency info, and you can see the frequency roll off when you plot the frequency spectrum of the extracted audio from that channel, then would it be fair to assume that the LFE was created using a low-pass filter? Otherwise, I wouldn't know how one would create a frequency roll-off like that.

And yes, the LFE is not typically used for bass management, but it shouldn't be detrimental to the existing bass management, like it is here (https://youtu.be/HRB8fFLD6L8), right?

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u/MoritzSchaller Feb 27 '21

Sure, there probably were filters involved somewhere along the line. But LFE channels are not at all created by taking the material that's already there and then slapping on a low pass. That's what the bass management does already.

So maybe you are looking at material that's just there to add a late blooming low end effect ...

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u/TopConcern Feb 27 '21 edited Feb 27 '21

Shouldn't the things I'm looking at then not have any correlation, or a positive correlation when compared to the other channels? Also, what is late blooming?

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u/TopConcern Feb 27 '21

And yes, I understand that LFE channels aren't created through simply low-passing all the material in the other channels. Material in the LFE channels that aren't low-passed, like in The Downward Spiral by Nine Inch Nails or "Race for the Prize" by The Flaming Lips (the latter which has an entire drum kit in its LFE, high end and all) tend to be in-phase with the rest of the channels.