LFE - Low Frequency Enhancement

LFE, the abbreviation which has been defined to date as ìLow Frequency Effectsì and when used in the context of a ì5.1î-channel description is designated the ì.1î LFE or Low Frequency Effects channel is now being termed ìLow Frequency Enhancementî by Tomlinson Holman, who originally proposed the name ì5.1î in a SMPTE meeting of a short-lived subcommittee called Digital Sound On Film in October 1987. The emphasis is on ìEnhancementî rather than ìEffects,î which refers to special effects such as explosions in movies contained in the frequency bandwidth 5Hz to 120Hz. ìEnhancementî used in this context provides for more headroom below 120Hz, where the ear is less sensitive and ìneedsî more SPL to sound equally loud as midrange sound. The emphasis on ìEnhancementî also is in reference to ìbass managementî in which playback of 5.1 recordings can be monitored so as to reproduce both the very low frequency content of the main channels, as well as the LFE channel, over one or more subwoofers as a monaural signal.Holman states the case for Low Frequency Enhancement as follows: ìWhen the 5.1 channel system was adopted for Digital Television a question arose from the set manufacturers: what to do with the 0.1 channel? Surely not all televisions were going to be equipped with subwoofers, yet it was important that the most sophisticated home theatres have the channel available to them. After all, it was going to be available from DVD [and LaserDisc], so why not from Digital TV? A change in the definition of how the 0.1 channel is thought of occurred at that time. The name ëLow Frequency Enhancementí (LFE) was chosen to describe what had been called the Baby Boom or 0.1 channel up until that time. The LFE name was meant to alert program providers that reproduction of the 0.1 channel content over television is optional on the part of the end-user set. In theatrical release, having digital playback in a theatre ensures that there will be a subwoofer present to reporduce the channel. Since this condition is not necessarily true for television, the nature of the program content that is to be recordded in the channel changes. To quote from ATSC Standard A/54, ëDecoding of the LFE channel is receiver optional. The LFE channel provides non-essential low-frequency effects enhancement, but at levels up to 10dB higher than the other audio channels. Reproduction of this channel is not essential to enjoyment of the program, and can be perlious if the reproduction equipment cannot handle high levels of low-freqency sound energy. Typical receivers may thus only decode and provide five audio channels from the selected main audo service, not six (counting the 0.1 as one).íîThus, DTV broadcasters must be cautious and insure that essential story-telling sound centent is not recorded only in the LFE channel. Such low-frequency sound content should be present in some combination of the five main channels, or else it may be lost to many viewers. If the deep bass content of a movieís sound effects is restricted to the LFE channel, then the master will need remixing for television broadcast.