LFE to LR downmix : What is it ?

[iparout]

Hi.

Can someone tell me in non-experts language what this command in Azid do ? And what happens if it is left unchecked ? It seems like many people recommend it for AC3 --> mp3 transcoding but I would like to know what it does so as to judge for myself if I need it or not.

Thanks in advance.

[Sycho]

The LFE channel is the channel that holds bass information, now, it is NOT a subwoofer channel, it just holds extra bass information which may or may not be used. You sould have LFE to LR @ -3dB

[DIggedy]

I don't understand what you're saying there, sycho...
The LFE (low frequency effects) channel IS intended as the subwoofer channel (the .1 in 5.1), but recievers have the option of mixing this extra bass into the front channels for those that don't own a subwoofer.
So iparout you should use LFE to LR to mix that bass into a 2 channel downmix (mp3 or whatever) to retain the soundtracks bass .

[dhv]

Simply - this option mixes .1 channel from 5.1 soundtrack into left and right channels. If you want to hear bass channel on stereo mix, must use this option (-3 db, because it mixes in left and right at the same time). I don't know enyone who wants to miss that channel, that's why this option always must be used for 2 channel downmix.

sycho wants to say that the subwoofer in the home cinema system may play the bass from other channels too, not just the LFE channel. This is when you have small speakers. This is the reverse situation, you have subwoofer, but main speakers are small.

[Sycho]

to the true dolby specs, when downmixing a 5.1 to stereo you are not suppose to included the LFE channel

[iparout]

Sorry to bring back an old thread but I would also like to ask this :

What happens if the movie's soundtrack is in AC3 2_0 channels format and I have LFE to LR = -3dB in Besweet ? Is the command automatically ignored ?

Thanks.

[DSPguru]

yes. ignored.

[iparout]

Ok I got it. But what if I disable the LFE to LR downmix for a 5.1 track ? The log file says : LFE to LR -INF. What does that mean ? Is the .1 channel completely lost ?

And something else : When I do mix the LFE to LR with -3dB, is the whole audio file somewhat affected (i.e. in volume) or not ?

[DSPguru]

gain of -inf db means gain = 0.

mixing the lfe means.. mixing :o.
given x=original signal, y=additional signal. mixing means x+y.
x+y sometimes have higher peak values then original x, hence, the maxgain value might become smaller (but not dramaticly).


* note that for mixing with (-3db), the mixed signal is x+sqrt(0.5)*y.