In another thread they were discussing various sub-woofer settings. The comments on LFE LPF and crossover settings had me a bit confused. Why are there separate settings and how are they related? A little on-line research cleared the air. (I'll refer to the Denon manual as that is what I own.)
(Feel free to quote any errors below and I'll amend the post accordingly. This is my basic understanding of the topic and hopefully kept the explanation simple enough for beginners to understand.)
LFE, or low frequency effects, is part of the digital surround sound spec, or the 0.1 in the 5.1. There is a good explanation of its development in
this article. I highly recommend this article if you want a deeper understanding of bass management in movie sound tracks.
This is a separate low frequency effects channel and while it's not considered part of the lower frequencies in, say, music, it can contain low frequency content from other channels so it is not an entirely separate channel in the strict sense. If that sounds confusing, read the above noted article. It's primarily the digital channel for your movie rumbles and explosions. When you set the LFE LPF (low pass filter), you are setting the highest bass frequencies that will be sent to your sub for effects. It's labeled as a low pass filter because there is a hard cutoff at 120Hz in the spec, but since most sound engineers roll off the effects at 80Hz, many AVRs default to a LPF setting of 80Hz. Since the LFE channel incorporates sounds from other channels, you can set the LPF to values higher than 120Hz but it is uncommon to use a higher setting.
Going over the Denon manual, setting the subwoofer mode to LFE passes the low frequency signals from all speakers set to "small" to the sub. Setting the mode to LFE+Main sends the low frequency signal from all channels to the sub, regardless of the size setting. The distinction is that if speakers are set to large, they still get the full range signal with the low pass filter also sending low frequencies to the sub (deep bass from both the sub and mains/speakers set to large). Using combinations of large / small and LFE / LFE+Main you can control where the bass effects are sent, although it is most common to set all speakers to small and let the sub handle bass effects (LFE setting). Technically, with all speakers set to small, LFE+Main should yield the same result.
With most AVRs, if the subwoofer channel is enabled, you can set the crossover point for all speakers or individually set the crossover point for each pair of speakers (channel: front, center, surround, back, etc.). The Denon manual states that the low frequencies will be output to the subwoofer if enabled. The LFE LPF and crossover frequencies both default to 80Hz, but potentially they can be different. The crossover setting covers content not delivered to the LFE channel as part of bass effects. The above article implies that there is some overlap here, so there is some magic in the surround spec with multi-channel bass management, but this primarily covers 2 channel content and the rest of the bass in multi-channel content. Offloading the deep bass to the subwoofer has certain benefits but I won't go into details here.
Why the two settings? Many people will likely opt to have the LPF and crossover set to the same frequency in the 80Hz to 100Hz range. While deep base has the property of not sounding directional, those with speakers capable of going very deep might opt to have the LPF higher for movies but prefer a lower crossover point for music to keep most of the sound emanating from the front stage. Others may prefer a high crossover point to clean up the main speakers. Others may use different frequencies to fine tune a system to match a room's acoustics. It's about flexibility and personal preference. The two settings are there because modern AVRs have the subwoofer doing double duty, playing both surround effects and deep bass content.
