[linux-audio-user] Creating surround sound

Fons Adriaensen fons at kokkinizita.net
Mon Dec 25 07:11:37 EST 2006

On Sat, Dec 23, 2006 at 05:58:34PM +0100, Pelle Nilsson wrote:

> Thanks! Apart from letting me know how to create the AC3 from the
> command line if I should need to do that, it also hinted at a standard
> recipe for dealing with the LFE channel: Mix all the other channels
> into one and apply a lowpass filter with a cutoff frequency of
> 150Hz. I will assume that this is a good method for now and look at
> the results.

This is actually not so good advice.

Moving the LF content from the 5.0 signals to the subwoofer may
be required in many systems if the 5.0 speakers can't reproduce
LF because they are too small. But it should be done at the
reproduction end, simply because it depends on the reproduction
system. Most 'receivers' can be set up to do this - it's called
'bass management'.

In a 5.1 system the subwoofer signal is not the same as the LFE 
channel. The signal that goes to the subwoofer should be the LFE
channel amplified by 10 dB, plus any bass that can't be reproduced
by the five main speakers. But the latter can only be decided at
the reproduction end. Users that have full range 5.0 speakers will
expect the LF in the main channels. The may still want to redirect
some of it to the subwoofer, but it should be their choice to do so
or not.

The LFE channel is meant for effects such as earthquakes and bombs.
In a correctly set up system it will also have 10 dB more gain than
the other channels. It should not be used for music. Even the center
channel has little use - if you calculate the psychoacoustically
correct signals for a 5.1 layout, it contributes little. It exists
mainly as a dialogue channel for movies. 


Lascia la spina, cogli la rosa.

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