There are two schools of thought about what is OK to put in the LFE channel (the 0.1 channel) in 5.1, 7.1 etc. In this Heads or Tails article, Mike and Alan present their reasons for favouring one over the other.
Mike - Only Effects Should Go In The LFE Channel
I favour only putting effects related content in the LFE channel. After all, it is called the Low-Frequency Effect channel. Beginners often get confused between the role of the LFE channel and bass management, and I think that this is because in lower cost systems the subwoofer speaker is handling two roles, bass management, ie low-frequency audio from the other channels that the channel speakers cannot handle is redirected to the subwoofer. Then in a surround system, the LFE channel content is also routed to the same speaker.
It is important to understand that the LFE is a channel and is not the same as the subwoofer. It’s actually a 120Hz bandwidth limited channel (hence the 0.1 description highlighting that it isn't a full bandwidth channel) that has an additional 10 dB of electrical headroom above the other main channels. Its purpose is to supplement the overall bass content of the program or to ease the burden on the other channels.
The History Of The LFE Channel
Dolby explain that the LFE channel was originally devised for 70mm movie productions to deliver a separate bass signal to one or more additional subwoofers placed behind the movie screen. This allowed deep bass effects, such as large explosions and other LF content, to be added to movie soundtracks without having to upgrade the existing speakers and amplifiers in the three main screen channels. It also meant that the headroom of the 70 mm magnetic audio recordings would not be taxed at low frequencies, which would have detracted from their loudness capability at mid and high frequencies. Finally, no additional frequency crossovers would need to be retrofitted into existing cinema processors to redirect the bass from the main left, centre, and right channels to the subwoofer(s). Taking advantage of the available channel capacity on 70 mm prints to deliver a separate bass effects signal proved to be the most direct, convenient, and economical way to supplement the low-frequency capability of movie soundtracks. To maintain full compatibility with existing theatres, the Dolby Digital film format includes a separate LFE channel. Hence the name LFE: Low-Frequency Effects and not low-frequency bass channel.
For me, because it is an effects channel it should only be used for effects so that in systems that have LFE reproduction the consumers will enjoy it but if it is not there then the mix will hold together.
If you put other content through the LFE channel like music elements then you run the risk that when it is played back through a domestic system with bass management that the LFE channel and the bass management path could be out of phase and so your bass elements will be significantly reduced. Also because the LFE channel is calibrated differently and the crossover frequency may be different it is very difficult to make sure your mix will translate across if you use the LFE channel to enhance other content like music.
Finally, it is recommended that you should never put anything critical to the mix exclusively in the LFE channel. This is because when a Dolby Digital decoder folds down a 5.1 source, it drops the LFE channel completely and so any content routed to the LFE channel won't be reproduced when the fold-down mix is used. So as long as the LFE channel is only used to enhance very low-frequency effects then the loss of them from fold down mix will not be catastrophic.
Alan - LFE Stands For Low Frequency Extension, Not Low Frequency Effects
Which is really my point in a nutshell. I guess it's because in the early days of Dolby Digital I had the honour of working with Industry Legends like the then UK Head of Dolby John Iles. Because I had direct access to the people at the forefront of surround sound development, it opened my eyes to that old saying, "There's more than one way to skin a cat".
There are important things to remember. Firstly, Dolby Pro Logic decoders employed Low Frequency Extension by routing anything below 80Hz in the front channels to the subwoofer. This was because back when 5.1 digital audio was optically encoded into the spaces between 35mm film sprockets, they still utilised the analogue Dolby LtRt "SVA" optical track as a "safety fail-over" in case the fragile digital optical track got damaged. So, the LtRt Pro Logic track had to be "comparable" in spectral response to the digital 5.1. It was for this reason also, that the Dolby Fold-Down (LtRt) settings for making this safety track, reflected the relative gain settings of the speakers in a Dolby licenced dubbing theatres. The Left and Right were at unity, the Centre at -3dB, the surrounds at -3dB and the LFE included, at +6dB.
Compatibility Vs Consistency
Which brings us to the problems you face when trying to deliver a mix that sounds consistent in a "home uncalibrated environment'. Basically, a £300 home cinema kit set up by your mate from the pub. It's much harder to ensure that the playback lineup is correct in a domestic environment than it is in a cinema. Since 5.1 playback has become more available, certain problem areas have been identified. These are usually incorrect centre and surround speaker placement and subwoofer level.
The ITU fold-down specification does not use the LFE track. The UK DPP broadcast delivery specification for 5.1 TV mixes has dialogue diverged across the Left, Centre and Right channels. I don't agree with either, but unfortunately, these are standards that have been set in response to reported playback issues.
This comes back to my first point - the use of the ".1" LFE track as Low Frequency Extension. Any decent cinema mix or playback room has twin subwoofers. Our first 5.1 room at dB Post in Soho had twin 18-inch subwoofers set into a THX wall. It is such a waste not to use these channels for LF Extension when they're available and so capable.
Like Being There
Those of you lucky enough to have heard (and felt) a live orchestra in a good venue will know just how powerful it is. Like a rock concert, you feel as much as you hear. Low Frequency Extension in Music is as much of an "Effect" as say the sonic pulse of an explosive going off or a dinosaur's footsteps. Imagine the power of the iconic "Jaws" score, shaking you in your seat. In the concert mixes I've done, we've used the LFE for kick drums, bass guitars, bass drops, anything that can recreate that "wall of air" you would feel if you were at the concert.
Auto-alignment systems are becoming far more common on even budget home cinema kits. We've had to fight to get the data bandwidth for a ".1" channel - we might as well use it. And if we're talking about history - the first surround system with LFE tracks I heard of was used in the original Disney "Fantasia". They had multitrack surround sound and "vibrator" channels on optical film playback, running in sync with three projectors, giving super widescreen and a fully immersive audio experience. They fought technology to be able to bring us an immersive audio - we should take our inspiration from their innovation and stop trying to just accommodate the lowest common denominator.
What Do You Think?
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