Decoding LFE

[cujo75]

Sorry if this post is in the wrong place, but it does cover subwoofer input. Wondering if LFE needs to be decoded by a 5.1 receiver or if the info would be avaliable in a 2.1 style set up. I'm considering a 2 chanel for the fronts of course, and a x-over and pro sound amp for my subs. Also, any members in the Huntington/Wayne West Virginia area? There are no audio stores in the area and I would love to hear othe systems.

Thanks
Rick

 

[Volenti]

Typically when you down mix to stereo the LFE is mixed into the L+R channels, assuming you don't have a 5.1 receiver and are using the audio out on your DVD this will be performed in the DVD player.

I used a setup similar to this for some time and no problem getting lots of bass from movies.

Though I would think a full setup with a 5.1 receiver would veild better results if only from the greater array of setup options that the receiver would present.

 

[Hezz]

When listening to the 2 channel PCM channels the LFE is part of the front channel mix as was stated by the previous post.

However, the LFE channel is recovered only by a low pass filter since it is not a discrete channel. Unless you are downmixing from 5.1 to 2.1 in your HT processor then the LFE will be a discrete channel.

If downmixing from 5.1 to 2.1 then you don't have to have a low pass filter on your subwoofer since it will only contain LFE.

 

[dangus]

The processor should only divert the bass to the subwoofer output if the front speakers have been defined as "Small" in the "bass management" setup.

A good receiver will let you adjust this crossover frequency. So, if you had some truly fullrange speakers plus a Contrabass subwoofer, you might set the crossover down at 40 Hz or lower.

 

[cujo75]

thanks a bunch for your help. I want to do a surround pre-amp eventually but they are all so expensive. I have not seen one for less than $1000. Pretty expensive when considering a 5.1 receiver can be had for 400.

 

[Drew Eckhardt]

Typically when you down mix to stereo the LFE is mixed into the L+R channels, assuming you don't have a 5.1 receiver and are using the audio out on your DVD this will be performed in the DVD player.

Most decoders discard the LFE channel when down-mixing. Given consumer loud speaker capabilities and the channel's 10dB of headroom beyond the screen channels this is the preferred behavior.

This doesn't mean you lose bass - anything that's "important" and not an effect should be recorded in the main channels.

 

[Drew Eckhardt]

thanks a bunch for your help. I want to do a surround pre-amp eventually but they are all so expensive. I have not seen one for less than $1000. Pretty expensive when considering a 5.1 receiver can be had for 400.

So you use a receiver instead. Many have pre-outs, and a simple voltage divider (1K + 100 Ohms) can be used to get speaker level outputs down to line levels.

 

[leadbelly]

Another option is a DIY DAC made with the AD1954. This will give you a sub output.

 

[Pano]

So are we saying that the LFE track contains only LF info? Makes sense, but is there a standard? What's the BW of the LFE track? (He asks reachng for google)

 

[Pano]

OK. Good article here:
http://www.hometheaterhifi.com/volume_7_2/feature-article-misunderstood-lfe-channel-april-2000.html

I'm supposed to know this stuff, since I used to do Dolby alignments in cinemas. But never did one with an LFE channel. Seems the digital version is limted to 120Hz. In practice rolled off at 80Hz. Plays 10dB louder than the other channels.

So you should be able to tap the LFE output and feed your sub directly. Right?

 

[cujo75]

Drew, never thought about the line out converter for home use. Duh!!! I have done it many times in car audio applications. As for the DIY DAC, it is way beyond my skill level. Maybe someday I can find a good teacher locally, and try my hand at a few kits. thanks for all the help from everyone. Oh and one more thing. Does a line level converter, as mentioned by Drew Eckhardt degrade sound quality or frequency response in any way?

 

[Pano]

Does a line level converter, as mentioned by Drew Eckhardt degrade sound quality or frequency response in any way?

If it's coming off a power amp output, it may not be the "best" sound quality, but I doubt it matters for the LFE. If it is a line level LFE out (as on many receivers) then no worries!

 

[Drew Eckhardt]

So are we saying that the LFE track contains only LF info?
Makes sense, but is there a standard? What's the BW of the LFE track? (He asks reachng for google)

Yes.

Dolby Digital uses a 240Hz sampling rate. Per Nyquist this provides 120Hz of bandwidth. The default Dolby Digital encoder settings apply an 8th order 120Hz low-pass to the input. THX DVDs are mastered with an 80Hz low-pass.

Theatrical DTS is a 5-channel system, with two channels used for surround above 80Hz and LFE below. Home DTS is a different beast allowing for 80 or 150Hz of bandwidth