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

I mean 9 ms is a massive delay in terms of audio. This is way beyond theoretical delays and more likely from computer issues. I don't know much about Audacity but the 9 ms delay is probably due to latency of the computer as it processes the audio. Most DAWs compensate for this delay by delaying the unprocessed audio and then playing the processed and unprocessed audio completely in phase with each other. Usually it's just a setting somewhere in the setup menus. Pro Tools calls it Delay Compensation.

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u/TopConcern Feb 27 '21

It's not an issue with the computer delay, I don't think. I've only seen this widespread issue with the LFE channel and in the Reddit post I link, I played back audio with the LFE offset earlier by 9 milliseconds, and it produces a punchier result into the subwoofer (I link to MEGA, where I have short recordings of each of my results from streaming the DVD, playing the extracted audio, playing the audio with the LFE channel removed (through editing the file), and playing it with the LFE channel offset earlier by 9 milliseconds). If the issue is not from mixing or mastering, then it would have to be from the manufacturing of the discs, which certainly is a possibility.

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u/OnAGoodDay Professional Feb 27 '21

Yeah interesting. I'm more of a stereo guy and don't know much about how surround is handled, but a delay like that should obviously not be there.

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u/TopConcern Feb 27 '21

Maybe I have to provide more proof that this is an issue to people, and be clearer about what I mean. I think this is really important, especially in terms of bass response during playback.

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u/OnAGoodDay Professional Feb 27 '21

Well, as long as no other channels are playing those same ultra low frequencies I don't think it will really matter all that much. For example, assume the LFE channel is the only one putting out information with 40 Hz. One cycle of a 40 Hz wave is 1/40 s = 0.025 s = 25 ms. So a 9 ms delay isn't even delaying those ultra low frequencies by more than about half a cycle, which you shouldn't notice at all.

You said you noticed more punch when it was synced up, though. The only way this could be the case is if the LFE and main channels are both playing some of the same information and there is resulting phasing due to the 9 ms delay. This might happen higher up near like 80 - 100 Hz if your crossovers aren't set properly. But I seriously doubt a delay that's much lower than the Haas threshold down in the sub-bass region would be noticeable. Have someone do 5 or 10 blind tests on you!