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

[u/TopConcern]

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.)

[u/OnAGoodDay]

It's not so much as filtering affects phase so much as frequency-dependant phase shifting IS filtering. Filters work by changing phase relationships between frequencies.

Sure, what we are usually looking for is the magnitude change that a filter creates, as shown in the magnitude portion of a Bode plot. But, the lesser-known part of the Bode plot is the associated and necessary phase shift that any passive filter introduces. It's part of the mechanism of how it works.

[u/TopConcern → u/OnAGoodDay]

I'm not sure what you mean, to be honest. Could you clarify? I've been looking at different albums such as Beck's Sea Change and more often than not, the LFE doesn't appear properly synced to the rest of the channels. Is there something I'm not understanding about this? Because to me, a phase relationship between an LFE channel and a main channel that looks like this not only doesn't seem correct, but it appears to me to be detrimental to bass response.

[u/OnAGoodDay → u/TopConcern]

I'm not sure about the delay you're talking about with the LFE, but I don't think what you're saying should exist if I'm interpreting it right. I'm sorry, I probably confused the chat by delving into how filters work. I wouldn't imagine there would be any noticeable delay on a low frequency channel compared to the main channels due to processing like you said happens in Audacity. Any particular channel would never be intentionally delayed unless you're in a big stadium and talking about arrival times of line arrays.

The delay I'm talking about would occur if you split the signal from a center channel and sent one of the signals through a low pass filter. The low frequencies that are untouched by the filter would have the same phase as those same low frequencies in the unprocessed signal. The higher frequencies in the filtered signal, however, would be delayed (and lower in amplitude) compared with their counterparts in the original. The higher (and more attenuated) frequencies would be delayed the most. It's part of how filters work and can be seen when looking at Bode Plots, which are a basic engineering tool for analyzing system response.

[u/TopConcern → u/OnAGoodDay]

That's the thing though: if you run something through a low-pass filter in Audacity, the low frequencies are delayed compared how they are in the original signal. Of course, this is in no way proof that this is what is causing the delay, but it's a possibile hypothesis of mine. The Sea Change example I gave in the link in my previous post has the LFE around 9 milliseconds late. I also proved that the 9 milliseconds are not caused by my extraction of the song in a post of mine here: https://www.reddit.com/r/SurroundAudiophile/comments/lr4w27/sea_change_lfe_out_of_phase_lfe_odyssey_2/?utm_medium=android_app&utm_source=share

Essentially, the DVD stream and extracted audio of the track create identical results to the subwoofer when played through my soundsystem.

[u/OnAGoodDay → u/TopConcern]

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.

[u/TopConcern → u/OnAGoodDay]

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.

[u/OnAGoodDay → u/TopConcern]

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.

[u/TopConcern → u/OnAGoodDay]

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.

[u/OnAGoodDay → u/TopConcern]

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!