why an lfe channel if full range signals are sent to all 5 speakers? (1 Viewer)

[KonradN]

Why do we need a dedicated lfe just for the low frequencies (

 

[BrianWoerndle]

Two reasons really,
First:
It gives sound mixers a 6th dedicated channel. It is an effects channel (low frequence effects. It is meant for explosions and such. They decide if they want to send a sound to the fronts, or to the sub.
Second:
There are very few full range speakers (capable of 20-20k at reference), and the sound mixers know that so they send low information to the sub.

 

[Harold_C]

The LFE channel was implemented by Dolby because the main channel speakers and amplifiers in most movie theaters cannot come close to reproducing low bass effects.

By using a separate low bass effects (LFE) channel, you can dedicate an appropriately sized subwoofer to the task without destroying the main speakers.

The five main channels still have "normal" bass recorded in them, because Dolby Digital 5.1 material needs to sound decent on systems that do not have an LFE channel. In effect, the LFE is used for "extra-large, heavy-duty" bass.

Now, turning to the home. There are not more than a handful of full-range speakers that can reproduce bass as effectively as a powered subwoofer. So the best configuration 99.9% of the time is to configure all of your five main speakers as SMALL -- regardless of their physical size. This means that each of these speakers plays full-range above 80 to 100 Hz. All of the "normal" bass below 80 to 100Hz from these five channels is redirected to the powered subwoofer which handles the "normal" bass plus the LFE channel.

An alternative in a very few, very expensive, very robust systems would be to set all five main speakers as LARGE and let each one handle its own channel's "normal" bass all the way down to 20 Hz. The expensive powered subwoofer is then used ONLY for the LFE channel on Dolby Digital and DTS digital recordings. It is not used on any other type of material and not even used on all Dolby Digital recordings since not all of them contain much if any LFE information. This is obviously not a particularly cost effective configuration since main speakers and amplifiers that can perform as well as a powered subwoofer below 80 Hz are few and far between.

Mixed systems with some LARGE speaker settings (full range)and some SMALL speaker settings (high-pass) have some serious disadvantages and this would not really be the best way to go unless the LARGE speakers have built-in powered subwoofers.

 

[RichardH]

In addition, the LFE channel has 10 dB more headroom. It's encoded 10 dB below what it's supposed to be played back at on the DVD, then upon playback, the Dolby Digital decoding chip adds the 10 dB back. This allows for some major explosive bass without having to lose that 10 dB of dynamic range with respect to the other channels.

This alone makes the LFE channel worthwhile.

 

[Harold_C]

Actually, the reason the LFE channel is played back by the decoder at 10 dB higher than any single main channel is because it has to produce sufficient bass output to match the output of the three main channels (plus the surrounds). The 10dB "boost" is designed to do that.

The LFE channel is proportionally louder than any single main channel. But it is not louder than the sum total of all the main channels.

 

[KonradN]

I guess my question is why have an lfe channel if you can just send full range signals to all the main channels and then have your receiver or pre/pro do any necessary bass managment.

 

[Carl Johnson]

Including a seperate discrete channel for the sub just leaves one more option for setting up the system. As you noted some people send full range sound to the mains so in that case using a sub would be impossible without the .1 channel.

 

[Steve_Ma]

A great link that should be mandatory reading for all of us.
Link Removed

 

[Jon D]

Heavy bass takes up bandwidth, possibly sacrificing sound quality, so by relocating heavy bass to the LFE channel, it can free up the main channels for the higher information. Plus what Richard said about the channel being 10 Db higher. It allows for louder bass than the main channels. Would the bass in Titan A.E. or TPM been as massive without the LFE channel? I highly doubt it.

 

[Matt Meyer]

Hey Jon,
This is way OT but.. you should change your address to correctly read "MN,where it's 45 degrees one day, and 0 the next with 7" of snow AND then 2 weeks later it is 91 degrees and then 2 days later it will be 50 degrees."
Only in MN,
Matt

 

[James Bergeron]

Just wanted to say... sometimes the sound engineers do not even put explosions in the .1

"Daylight" for instance I watched with my speakers on "large" and well the tunnel scene was not very impressive, a switch to "small" and voila! All the bass was in the l/r!

 

[Harold_C]

I guess my question is why have an lfe channel if you can just send full range signals to all the main channels and then have your receiver or pre/pro do any necessary bass managment.

You can do that. In fact, that is how DTS handles the LFE channel.

However, remember that Dolby Digital is a multi-purpose format. It is not just designed to be used in high-quality systems, but in one-box systems and even plugged directly into the TV. If you dump the full LFE signal into some of these lesser systems, you'll blow them through the roof.

The separate LFE channel provides the opportunity to complete ignore the LFE signals when, for example, downmixing to stereo for playback through a TV.

A properly mixed 5.1 channel recording will sound perfectly fine without the LFE channel. With the LFE channel, the mix can have "extra-supersize" bass effects.

 

[Richard_s]

Steve_Ma

Great reference in you above post thanks. definitely required reading.

Glad to see someone else from MA posting here...