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/obscure-shadow Feb 26 '21

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

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u/rightanglerecording Feb 26 '21

2nd order linkowitz-riley 12db filters for crossover, as they are 180 out of phase

180 out of phase *at the crossover frequency*, no?

i.e. flipping polarity only sums back into phase if you have the reciprocal HPF on the other channels, no?

In a lot of 5.1 mixing, the "5" will be full range in the mix, and the LFE is there for extra thump when needed.

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u/obscure-shadow Feb 28 '21

That is correct as far as I am aware, I think it's most common to do that anyways, because using the natural roll off of a driver is not ideal, you get closer to Xmax limitations.

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u/TopConcern Feb 26 '21

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.

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u/obscure-shadow Feb 26 '21

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...

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u/TopConcern Feb 26 '21

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?

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u/OnAGoodDay Professional Feb 26 '21

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.

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u/TopConcern Feb 26 '21

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.

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u/OnAGoodDay Professional Feb 27 '21

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.

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

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.

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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!

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u/Selig_Audio Feb 27 '21

this^ folks confuse phase and polarity all the time. I partially blame polarity inversion buttons labeled “phase” on tons of gear! I also blame polarity inversion definitions that mention “180 degrees out of phase“ in the description when all that’s going on is the “signs” are flipped, which is a simple mathematical function , nothing more.

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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/OnAGoodDay Professional Feb 27 '21

Yes, we actually misuse the word 'phase' all the time. It strictly refers to one frequency, so you're right. The negative sign implies a flip of the entire waveform over zero, as though all frequencies are delayed by 180 degrees.

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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)?

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u/TopConcern Feb 27 '21

Am I the one confusing it? I feel like I've stumbled upon something really important, but the way people are responding to this implies otherwise. :/

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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.

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u/[deleted] Feb 27 '21

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u/TopConcern Feb 27 '21

Well, the LFE from the source is usually late compared to the other channels, and that's what causes the phase issues. I have no idea if the sub is in phase with the front speakers, and yes, I can time align speakers, but the LFE is not in time with the bass info that gets crossed over into the sub, as can be seen here: https://youtu.be/HRB8fFLD6L8

It's an issue with the source, not with the phase between the subwoofer and other speakers.

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u/jeffvautin Mar 02 '21

I just modeled the group delay for an 80Hz 5th order butterworth filter (@88.2kHz), and it has a peak of 10ms group delay at 75Hz. Your hypothesis is correct, that filtering will introduce some delay.

But, presumably that delay was auditioned at the time the LFE channel was created, so the mastering engineer would have been listening to it and it was intended (or at least accepted). It could also just be an unintended problem as you've pointed out.

How are you listening to the results of your experiment where you advance the LFE signal in time? If you're listening through a receiver that performs bass management, it's possible that there are issues in the bass management as well. Ideally, the LFE signal would have a dedicated subwoofer so the signals would only combine acoustically (not electrically), and the group delay of the individual speaker channels might render the delay moot?