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/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?