Crossover ONLY at 80hrz or you will miss sound

[robruffo]

This is something few people know.

The LFE channel of virtually all movies (and multi-channel music discs) cuts off at about 80Hrz, AND if you set your crossover lower than that, LFE sound info above your crossover point will simply be rolled off/lost - it won't be sent to speakers other than the sub.

I know how stupid this is (it's REALLY stupid) but it's how virtually all receivers are designed to work, even premium, flagship ones from the likes of Denon, Yamaha and Pioneer.

What I do is use pure-direct with my Sierras for most music (skips bass-management on my system and simply sends all sound to my stereo pair) and keep things at 80hrz for movies with speakers set to small.

[Mag_Neato]

i have to check, but i believe my Onkyo TX-SR703 allows the LFE point to be adjusted.

[Fantom]

The LFE channel is a crazy thing. It's mainly a hold over from the past. In all modern multi-channel formats (Dolby, DTS, TrueHD, DTS-HD:MA, LPCM) ALL of the channels are full range. Yes, even the LFE channel is full range by spec.

This means movies can do whatever they want. Some movies use the LFE channel for bass almost exclusively. Other's just put the full range (including bass) into each channel and let the receiver do the crossover (best solution). Other do a mix of both.

Multi-channel music should *never* use the LFE channel. Multi-channel Dolby encoded music certainly could, but it makes no sense. SACD does not have an LFE channel to begin with, just up to 5 full range channels.

A crossover is not instantaneous, but rather a smooth curve. A woofer crossed at 80Hz will still be doing something at 100Hz or even a bit higher. Likewise, the other speakers need to play frequencies below 80Hz. I am not sure how frequencies are encoded in the crossover range when the LFE is used. Especially if upper frequencies from the LFE are simply rolled off while lower frequencies in the other channels are rolled into the LFE. It just doesn't seem like it would work properly, knowing that the processor will be doing it's own crossover magic to the encoded signal before it hits the speakers.

If anyone knows more about how this works, I'd be happy to learn.


If only the non-LFE channels are used for the sound (LFE completely unused), then a processor could cross at any frequency and everything would be fine. Bass from all channels would get mixed into the LFE. I do not know how this mix should properly be done, but I'm sure there is a spec for it.

Now to the heart of the matter. That is a good question as to what happens to frequencies in the LFE above the crossover point. I suspect this answer will be tied to the LFE+mains crossover question above.

As far as I can tell, encoding without ever using the LFE channel seems to be the best for all involved. And yet the LFE channel is still used and is still present in new formats.

[Fantom]

Ah, I found a helpful article from Dolby.

http://www.dolby.com/assets/pdf/tech_library/38_lfe.pdf

This emphasizes the difference between subwoofer and LFE. They also explain how it was a compatibility hold over from 70mm film days and has some modern use in theater setups.

Dolby says that even if information is in the LFE channel, it is not necessarily reproduced by the subwoofer.

By specification, a crossover should work both ways so to speak. Low frequencies being rolled off from the main channels should be redirected to the woofer. Higher frequencies being rolled off from the LFE channel should be redirected to the main channels.

Sounds that are in the crossover range should likely be encoded to specific channels (allowing the crossover to roll off some of the sound to the woofer). If sounds in the crossover range were encoded to the LFE channel, then the upper frequency portions of these sounds would need to rolled into all 5, 6, or 7 main channels, which would be quite odd.

So a couple things here:
1. A processor or receiver *should* place rolled off LFE frequencies into the other channels (LFE is NOT equal to woofer)
2. A good encoding should use the main channels for most anything in the crossover region since at least part of the sound will not be played by the woofer.

If your receiver or processor does not do #1, then I'd feel cheated. Perhaps this is part of THX certification? Anyone know how to tell this before you buy it?

I could see an encoding using the LFE for frequencies 50Hz and below, with the sound engineers assuming these sounds are all woofer territory. With an 80Hz crossover, you would not hear much at all from the mains at 50Hz. However, with a 60Hz crossover, your main channels will clearly play back part of a 50Hz signal. Of course, if all is done properly, you will not lose any sound. You will just have portions of that 50Hz signal played back by multiple main speakers instead of 1 or more woofers.

[robruffo]

Ah, I found a helpful article from Dolby.

http://www.dolby.com/assets/pdf/tech_library/38_lfe.pdf

This emphasizes the difference between subwoofer and LFE. They also explain how it was a compatibility hold over from 70mm film days and has some modern use in theater setups.

Dolby says that even if information is in the LFE channel, it is not necessarily reproduced by the subwoofer.

By specification, a crossover should work both ways so to speak. Low frequencies being rolled off from the main channels should be redirected to the woofer. Higher frequencies being rolled off from the LFE channel should be redirected to the main channels.

Sounds that are in the crossover range should likely be encoded to specific channels (allowing the crossover to roll off some of the sound to the woofer). If sounds in the crossover range were encoded to the LFE channel, then the upper frequency portions of these sounds would need to rolled into all 5, 6, or 7 main channels, which would be quite odd.

So a couple things here:
1. A processor or receiver *should* place rolled off LFE frequencies into the other channels (LFE is NOT equal to woofer)
2. A good encoding should use the main channels for most anything in the crossover region since at least part of the sound will not be played by the woofer.

If your receiver or processor does not do #1, then I'd feel cheated. Perhaps this is part of THX certification? Anyone know how to tell this before you buy it?

I could see an encoding using the LFE for frequencies 50Hz and below, with the sound engineers assuming these sounds are all woofer territory. With an 80Hz crossover, you would not hear much at all from the mains at 50Hz. However, with a 60Hz crossover, your main channels will clearly play back part of a 50Hz signal. Of course, if all is done properly, you will not lose any sound. You will just have portions of that 50Hz signal played back by multiple main speakers instead of 1 or more woofers.

Yet... What I say above, from experience, is often true.

[Fantom]

Oh, I don't doubt you. Specs are often not followed, especially when it goes unnoticed (and saves money).

Do you know of any good tests for this? I supposed I could make a sweep that is all in the LFE and encode to Dolby.

The thing is, if a processor does not handle the LFE correctly, then there is not even a guarantee that an 80Hz crossover will produce proper results in all cases. However, assuming most LFE channel frequencies are low enough (sub 50Hz), it'd become rather difficult to tell the difference between proper and improper processing with an 80Hz crossover.

Sigh. The things consumers have to put up with. With all the new technologies and minor unspecified differences, you have to spend more time researching a product than the amount of time you work to afford it.