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.