[MPlayer-users] Dolby Digital (DTS?) 10 dB Boost for LFE channel

[Wiebe Cazemier]

----- Original Message -----
> From: "Carl Eugen Hoyos" <cehoyos at ag.or.at>
> To: mplayer-users at mplayerhq.hu
> Sent: Thursday, 11 April, 2013 1:26:59 PM
> Subject: Re: [MPlayer-users] Dolby Digital (DTS?) 10 dB Boost for LFE channel
> 
> Wiebe Cazemier <wiebe <at> halfgaar.net> writes:
> 
> > The Dolby standard requires the LFE channel to have a 10
> > dB boost when played back. For example, Wikipedia says:
> 
> (It would be better to quote the specification, wikipedia
> is generally very useful but not as a reference for
> codec implementation.)

Just as quick reply:

http://www.atsc.org/cms/index.php/standards/standards/48-atsc-a52-standard

(don't need to read it now :) )

> 
> > > The LFE channel is conventionally played back 10 dB
> > > louder than the main channels, giving significantly
> > > more recording headroom.
> > 
> > Does mplayer (or a52dec?) apply this boost? Would it
> > even be possible; won't it clip the signal?
> 
> I cannot answer the question but please note that MPlayer
> does not contain a native Dolby decoder (and that the
> "a52" decoder is really obsolete), FFmpeg contains a
> decoder that is responsible for decoding (e)ac3 in
> MPlayer and other players.
> 
> Carl Eugen

Of course, I forgot about ffmpeg. I thought it weird the most recent a52dec was of 2002. Yet, Archlinux did install it for me.

Anyway, I did some more research, and if ffmpeg keeps to the rules, it doesn't apply the 10 dB boost, because it would require the rest of the channels to be reduced in gain by 10 dB. The .1 output can only be more than LFE (meaning LFE+bass from other channels) if the software applies bass management. This [1] is a good explanation. It makes this important statement:

====
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.

(...snip...)

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.
====

It also goes on to say that the digital output also passes the LFE unaltered. It's the receiver's job to apply the boost, which makes sense.

I think this does mean, though, that most people that use analog surround outputs, especially when they donwmix to 4.0 or stereo, have a unbalanced bass level. I suspect even a lot of "5.1" speaker sets don't do it right. I doubt a simple Logitech speaker set can handle the 10 times power increase required...

I think I know enough, and might continue the discussion on the ffmpeg mailing list.


[1] http://www.avsforum.com/t/748147/lfe-subwoofers-and-interconnects-explained