Getting to the Bottom End of the LFE
The ITU specification for using the LFE (Low Frequency Effects or Low Frequency Enhancement) is contained in Annex 7 of the document. This is Bible for both equipment manufacturers, content producers (both movies and music), and facilities. I read it with interest because it appears that my production process has wrongly ignored the attenuation of the LFE channel. I don’t lower the LFE channel by the 10 dB recommended for DVD-Video titles. Have I screwed things up for all of my customers?
Here’s the opening paragraph of Annex 7 “Low frequency effect channel (LFE)”:
“The purpose of this optional channel is to enable delivery of higher levels of low frequency energy, that can be reproduced by those users that are equipped with sufficient low frequency reproduction capability to produce the high level low frequency effects. It was originally devised by the film industry for the digital sound systems.”
The idea is that movies have these low frequency sound effects that shouldn’t burden the main speakers. The document specifically states, “the LFE channel is an option, at the receiver, and thus should only carry the additional enhancement information”.
There you have it…for movies. The LFE channel handles frequencies in the range 20-120 Hz. But how are things different for music programs?
It turns out that there is another ITU recommendation ITU-R BR. 1384 that “specifies that the LFE channel recorded with a level offset of -10 dB for the recording and exchange of multichannel sound program material, and this offset is compensated for in the reproduction system”. This harks back to the question originally posed by one of my readers. He was correct that hardware manufacturers routinely reproduced the LFE channel with a positive offset gain of 10 dB relative to the main channels on playback.
The ITU specification inserts a NOTE at this point in the document. It turns out that the whole 10 dB offset thing applies only to the DVD-Video format. The LFE channel is or should be handled differently by makers of AVRs for DVD-Audio discs and SACDs. Here’s the NOTE:
The film industry encodes the LFE channel such that a positive gain of 10 dB is required on reproduction and the reproduction level for DVD-Video is set to a positive gain of 10 dB relative tot eh main channels. However, the music industry, such a DVD-Audio or Super Audio CD, is currently coding the LFE channel such that zero offset gain is required on reproduction.
From this description, it appears that the manufacturers of AV Receivers should treat the LFE channel for music productions differently. They should have included a setting that would dictate whether the offset is applied or not. Instead, it seems virtually all AVRs boost the LFE channel regardless of whether the bass management system is active or not.
Finally, the ITU document talks about the fact that most domestic systems and the content delivered through them don’t require heavy handed “Low Frequency Effects”. In fact, they change the meaning of the LFE acronym to “low frequency enhancement” to soften the requirement.
“The LFE channel must therefore be considered as, at most, an enhancement, and definitely not as an essential part of the mix. If LFE use is thought advantageous in particular circumstances then it should only be used when there is a full understanding of the way the whole system of LFE, stereo down mix, bass management, and subwoofers is intended to work.
This last bit of information makes me quite happy. I agree that the LFE material on my productions is an “enhancement”. I don’t put much down in the “boom” channel but it is not silent either.
The next section of the ITU-R BS. 775-3 document is titled, “ Usage of the low frequency effects channel (LFE). I’ll parse that section in a future post.