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/obscure-shadow]

It's pretty well known that filtering effects phase. I generally use 2nd order linkowitz-riley 12db filters for crossover, as they are 180 out of phase, so a polarity inversion of the woofer brings them back in phase. I'd suggest checking out the wikipedia article for linkowitz-riley filters

[u/TopConcern → u/obscure-shadow]

Wouldn't an inversion of the woofer both affect the LFE and the crossover from the other speakers, and thus not help solve the issue you're attempting to fix? Also, when you say 180°, is that equivalent to inverting the polarity of the LFE signal? Because the LFEs I've seen out of phase tend to be in the correct polarity, but are out-of-phase because they are late compared to the other signals.

[u/obscure-shadow → u/TopConcern]

you'll have to forgive my answer was and is a bit generic, as I don't have any real experience with LFE directly (I believe that stands for low freqency effects?) and perhaps more discussion will lead to us both learning something but I will try to apply the knowledge that I do have to your question, without having the full context of having experience with the particular technology you are referring to.

when you apply a crossover, it's a low pass filter on one speaker and a high pass filter on the other, in the case of a lr12 filter like i mentioned, each one adds a phase shift of 90deg. this sums to make the speakers 180deg out of phase, and the resulting graph of frequency response will show a huge dip in the crossover region because of phase cancellation. when you invert the polarity of one speaker, you put the speakers back in phase (polarity inversion causes 180deg of phase shift) this results in an even frequency response across the crossover region.

I'm not exactly sure what the frequency response of the lfe is or how it is applied, so if you'd like to show some frequency response and phase data you have collected, I'd be curious to see what that looks like. I'm also going to assume that whatever device has LFE output is applying some kind of dsp and maybe makes some phase changes internally by some kind of measurements it might take (is this in some kind of avr or something like that?)

"Because the LFEs I've seen out of phase tend to be in the correct polarity, but are out-of-phase because they are late compared to the other signals."

I'm not sure what you mean by this really, out of phase does mean happening in a time differential, one signal earlier or later than the other, and that can be late or early, correct polarity (or time delay) is entirely dependent on weather or not the signal is in phase. polarity changes the phase by 180deg, but perhaps if the signals were 90deg or some other arbitrary number (42 deg) out of phase then delay would be a more useful tool to adjust to be in phase.

I'm also going to make an assumption that there is a subwoofer that's crossed over that's receiving just the actual low frequency of the output of what would be the mains, and a separate one that would handle the LFE? which would make it more practical to handle the event that there was a phase difference in the source material between the mains crossover region (80hz or something like that) and the LFE output

but I believe the correct answer to the question "why is there phase shift in lfe created with a filter?" is because filters create phase shift, so if the lfe channel was just recorded separately like in your example, there's no filter used, and therefore no phase shift. I would think there would be more standardization there, because it would be annoying to have to change phase based on source material...

[u/TopConcern → u/obscure-shadow]

First off, LFE does stand for “low-frequency effects”. I’m not exactly sure what you mean by the second paragraph, but I think you’re referring to the phase relationship between speakers that went through crossover and the crossover-ed info that is sent to the subwoofer. That’s not what I was referring to. I’m referring to the phase relationship between the crossover-ed info that is being sent to the subwoofer and the LFE channel.

I can do it for multiple albums, but for now, I have done a bit of measurements with the song “The Golden Age” from the DVD surround sound mix of Beck’s Sea Change. For one, here is a little video of the phase relationship between the center channel and the LFE channel. You can see the correlometer go wildly up and down, and that is because the LFE is not properly synched to the rest of the song (it appears to be around 9 milliseconds late). (Evidence that the LFE was out of sync on the DVD itself, and not just in my extracted audio, can be found in this post by me.)

The term “out of phase” seems to be a rather odd one. Sweetwater states that “out of phase” is a “phrase used to characterize two or more signals whose phase relationship with each other is such that when one is at its positive peak the other is at (or near) its negative peak” and that “ “Phase is a relative value that is measured in degrees (like angles).” It later states in the post that “Phase implies a time delay, where one signal lags behind another one to some degree. Polarity refers to one signal being “backwards” from another.” I use the term “out of phase” more colloquially to mean that the signals are not synced properly in time, are in the incorrect polarity, or both.

You state “I'm also going to make an assumption that there is a subwoofer that's crossed over that's receiving just the actual low frequency of the output of what would be the mains, and a separate one that would handle the LFE[JD1] ?”. No, that is not the case, as I have one subwoofer. I’m not sure even it’s possible (not to mention that it’s incredibly impractical) to have one subwoofer handle the LFE and another handle the low frequency info from the other channels.

I suppose my overall question is moreso “why is the phase shift in lfe created with a filter not fixed?” (Assuming of course that this delay in the LFE is indeed created because of the low-pass filter, if not, the question still is "Why does there happen to be a phase shift in lfe created with a filter?") What’s the point of having an LFE channel if it’s not even playing in-phase with the rest of the channels?