LFE: From the Horses Mouth
I finally got a response from my friends at Dolby regarding the boosting of the LFE channel during the playback of movies vs. music. A product manager replied and included a copy of the ITU-R-REC-BS.775 recommendations. The title of the paper is “Multichannel stereophonic sound system with and without accompanying picture”. Sounds impressive and it is quite thorough. However, the relevant parts of the recommendation are contained in Annex 7 and Appendix 1 to Annex 7.
For those just getting up to speed on this issue. I received a question from a reader about the alignment tones on my Blu-ray calibration and demonstration disc. The individual wanted to know if I had lowered the LFE channel to make sure that the +10 boost that would be applied by the AVR wouldn’t result in my low end being 10 dB too hot. I told him that I didn’t reduce or attenuate the LFE signal. I understood that to be an issue for movies (the LFE is mostly effects and not musical tones). Well, I think both of us were right…here’s the response from the Dolby product manager:
“You are correct in your final assessment. You may have been reading the attached ITU standard, which makes it pretty clear once you get to the appendices at the end. For films, the LFE really must be boosted in the AVR and always will be so in the production environment. One can think of this as the encoded track being “pulled down” by 10 dB, or that it simply allows for 10 dB more headroom. Same difference, so to speak. But yes, for music, there have been differing approaches where in some cases, the mix is done with 0 dB gain and in other cases, +10 on the bass signal. Generally, I think mixers should anticipate +10.”
In the real world, all signals coming from the LFE channel are being amplified by 10 dB. The music exclusion isn’t a reality. So my reader was correct as well. The low end of my DVD-Video programs have been 10 dB too hot since 2000 when I first started producing these titles. However, I’ve never had anyone contact me with any complaints. The reason is that subwoofers usually have their own amplifiers and most people don’t calibrate their systems. They simply turn the sub channel up to down to suit their personal taste. My wife has actually unplugged our sub because she never knows whether we’re going to get too much “boom” channel in our media room.
But what do the relevant sections of the ITU document have to contribute to the discussion. I’ll be sharing that information over the next few days.
6 thoughts on “LFE: From the Horses Mouth”
Hello Mr. Waldrep…I am one who, with limited knowledge of the process, attempts to calibrate my sound system. I do so primarily for movies. Although there have been numerous articles written about “Subwoofer Setup,” the one that I continue to use is from an article written by Tom Nousaine, in the May 2000 Sound & Vision (pp. 87-89). For my environment the current system configuration is ideal (small house, small listening room). I do from time to time adjust the sub depending on the source material. I will say that with your sample that came with my OPPO BDP-83 no adjustments are necessary.
Nice article, keep up the good work.
Priece Rich, Jr.
Hello Michigan…my home state. Thanks for your input. I’m going to be exploring this further…glad the disc is performing well.
Humm, the comment about most not calibrating confuses me. Back when I had my high end separate system I would calibrate it with my Shack sound meter. Now that I downsized to a Pioneer AVR I basically depend on the built in MCACC system to handle setup. I believe most users having AVR’s are using and depending on that systems auto-calibrate system to get things right. So is this saying that all of us have subs that are 10db out of calibration when playing one of your blu-rays? Might this also be true of other companies blu-rays?
According to the ITU document, DVD-Video discs will always have the LFE boosted 10 dB. It states that DVD-Audio and SACD discs would not be boosted. The real question is whether the manufacturers of AV Receivers actually implement the spec. I think it’s safe to assume that the LFE channel on my DVD-Audio titles is too hot by 10 dB.
2L boosts its Blu-rays by 10dB on the subwoofer channel, but they don’t boost the SACD subwoofer. They disclose this in the fine print at the end of their liner note booklets.
It’s funny that your wife unplugged the sub. I unplugged mine about two years ago and haven’t missed it one bit — who needs to rattle the foundations?
So Morten’s Blu-rays would have at least 10 dB too much sound….maybe 20 dB too much depending on the AVR or conversion.