[FFmpeg-user] LFE channel + 10dB

Wiebe Cazemier wiebe at halfgaar.net
Sat Apr 13 12:35:10 CEST 2013


Until now, I always had the LFE channel mixed in with the fronts 50%/50%. The mixing is done in my home-made multi-channel preamp/crossover. However, since I calibrated my subwoofer to be flat to 20 Hz with the rest of the audio bandwidth (it was too loud before), bass from films seems weak. That's when I learned the true method of reproducing the LFE channel. The channel is meant to be played back 10 dB louder than the other channels (see [2] and [3], so that one channel can have all the bass from the other speakers, and to be able to have more 'bang'. For example, [1] says:

This 10dB boost is achieved by calibration in the monitoring amplifiers at the studio; a full-scale LFE signal on the tape is set up to play 10dB louder than a full-scale signal on any other channel.


One important point is that the 10dB boost to the LFE is performed at the final amplification stage. The potential extra volume of the LFE channel could not pass through interconnects, analogue recordings or digital recordings without going over the specified limits by a factor of 3. This, indeed, is the whole raison d'être of the LFE channel. It's a channel where by convention everything is recorded 10dB low to make room for louder bangs.

So, I was wondering if ffmpeg (using Mplayer, XMBC, etc) really doesn't apply the 10 dB boost and lets the amplifier do it. [1] has some detailed explanations why it really shouldn't be done in the digital domain. And it makes sense of course, because boosting LFE can't be done; it would mean reducing the rest by 10 dB to avoid clipping.

This gives rise to a few questions:

1) As mentioned, does ffmpeg adhere to this and allow connect 5.1 speakers to apply the boost? And do all those Logitech/etc 5.1/7.1 PC speaker sets do so? [1] mentions that even home cinema receivers don't always get it right.

2) Are there downmixing routines in ffmpeg? The old liba52 did have them, and I saw in the source code that there actually was normalization applied so that mixing channels wouldn't clip them. However, I couldn't find anything about the LFE 10 dB boost, and how the channels were normalized and mixed in with left and right when not having .1 output on the sound card. I also know that when downmixing to 2.0, the AC-3 standard ([2]) requires dynamic compression to accommodate the extra signals, which makes sense, but I don't know of such a standard for downmixing to 4.0.



[1] http://www.avsforum.com/t/748147/lfe-subwoofers-and-interconnects-explained
[2] http://www.atsc.org/cms/index.php/standards/standards/48-atsc-a52-standard
[3] http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/38_LFE.pdf

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