[FFmpeg-user] LFE channel + 10dB

Wiebe Cazemier wiebe at halfgaar.net
Thu Apr 18 16:57:27 CEST 2013

----- Original Message -----
> From: "Wiebe Cazemier" <wiebe at halfgaar.net>
> To: "FFmpeg user questions" <ffmpeg-user at ffmpeg.org>
> Sent: Saturday, 13 April, 2013 3:30:54 PM
> Subject: Re: [FFmpeg-user] LFE channel + 10dB
> > Wiebe Cazemier <wiebe <at> halfgaar.net> writes:
> > 
> > > 1) As mentioned, does ffmpeg adhere to this and
> > > allow connect 5.1 speakers to apply the boost?
> > 
> > Please test and tell us!
> This is harder than it sounds, because you don't know if the test
> file already compensates for it. The reference I mentioned says:
> ===
> Calibration tests generally hide this 10dB difference; a basic pink
> noise test will contain a signal that should play at 75dB SPL for
> each channel. For the normal channels, that means a signal 30dB
> below full scale (105dB-30dB = 75dB), but for the LFE channel it is
> a signal 40dB below full scale (115dB-40dB = 75dB): 1/3 of the
> signal amplitude. If those test signals are used to calibrate the
> amplifier so they all output at 75dB SPL, then everything is set up
> okay. The LFE channel now has 10dB of extra headroom above the other
> channels and can produce the full 115dB SPL when required.
> ===
> But, I will do my best to find out, of course.
> I can tell you that the ALSA driver doesn't apply this boost to its
> .1 output (For my Audigy 2 ZS), because testfile [1] plays
> significantly louder when addressing the subwoofer.
> [1] http://www.halfgaar.net/media/chan-id.zip

To get back to this; finding a calibration file seems impossible. This mailing list discussion, archived at different sites, is basically the only thing I find when I Google for it.

I did test with the DVD 'Pulse' from Pink Floyd, which has a calibration routine on it. However, it says that the sub should play 4 dB (or was it 6?) louder than the rest. I don't understand why or what this is. It might be just a curious thing in their tests, because no standard specifies 4 or 6.

I also listened to a lot of movies. By the sound of it, the 10 dB boost is not applied and with it applied externally, it seems to give a pretty balanced result. Bass that is obviously meant to shake the room only does so with the boost, yet normal sound doesn't seem too rich in bass. So, it would seem ffmpeg does it right.

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