LFE in a stereo downmix

[tarkus]

Hi all,

I have just received two subwoofers (Rythmik F12) which I have integrated with my mains (Revel F206) via a minidsp SHD Studio and a Toping E30. My source is a nvidia Shield TV Pro which is connected to the SHD studio (2 channel output).

I'm so far happy enough with the integration, however now have a problem as Plex and Kodi discards the LFE channel from movies when downmixing to stereo.

Does anyone if it is possible for Plex or Kodi to install the LFE channel to a 2 channel downmix? I do have a Vero 4K+ which uses OSMC (a kodi variant), which actually has an option to include the LFE channel in a down mix.

So I have these options for having the LFE channel in my setup
- find a way to play movies via Shield with LFE in stereo downmix
- use the Vero 4K+ for movies and TV shows
- I can do this, but would prefer to use the Shield if I can due to easy Youtube/Netflix/Amazon integration
- Add a processor/AVR to handle the Dolby/DTS downmix and include the LFE
- would not rather spend the money, but can anyone recommend the cheapest way to do this? a cheap AVR or is there a cheap processor that can do the job?

Many thanks

 

[Kal Rubinson]

I'm so far happy enough with the integration, however now have a problem as Plex and Kodi discards the LFE channel from movies when downmixing to stereo.

Try JRiver, instead.

 

[DVDdoug]

The LFE is not supposed to be included in the downmix. The other 5 channels are supposed to contain the normal bass and the LFE contains "extra" for "booms & explosions", etc.

 

[ElNino]

The LFE is not supposed to be included in the downmix. The other 5 channels are supposed to contain the normal bass and the LFE contains "extra" for "booms & explosions", etc.

I wish mixes consistently followed that principle....

 

[DJBonoBobo]

The LFE is not supposed to be included in the downmix. The other 5 channels are supposed to contain the normal bass and the LFE contains "extra" for "booms & explosions", etc.

But if you have mains with two potent subwoofers, it would be rather silly to miss out on these "extra"?

 

[tarkus]

Try JRiver, instead.

Thanks Ka. I've just tried it, but it seems like it's not ready for prime time Android for TV (on my Shield) yet. I can't add my network share from my NAS which is a critical feature.

The LFE is not supposed to be included in the downmix. The other 5 channels are supposed to contain the normal bass and the LFE contains "extra" for "booms & explosions", etc.

Yes I know this is the Dolby standard for standard 2.0 setups. However, niche setups of 2.1 or in my case 2.2 exist and there is no reason for LFE to be excluded in these cases.

Can anyone recommend a cheap processor that does DD/DTS decoding and downmixing?

 

[Kal Rubinson]

Thanks Ka. I've just tried it, but it seems like it's not ready for prime time Android for TV (on my Shield) yet. I can't add my network share from my NAS which is a critical feature.

I have never used it for TV but, over years in multiple systems, it has worked with all network shares from multiple NAS boxes. That's got to be your local problem.

 

[raif71]

https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg has lfe downmixing if you don't mind post-processing and command line processes

 

[NTK]

Block diagram from David Griesinger's 5.1 to 2 channels down mixer patent.

Patent Public Search | USPTO

pdfpiw.uspto.gov

 

[DVDdoug]

However, niche setups of 2.1 or in my case 2.2 exist and there is no reason for LFE to be excluded in these cases.

Send your complaints to Dolby.

Some AVRs might keep the LFE if you can configure it for no center or rear speakers. That might even be a common feature... Those Dolby "rules" are for a mix-down to regular 2-channel stereo.

Most "2.1" setups are actually 2 audio channels with the bass "crossed-over" and mixed for the woofer/subwoofer. I have "2.1" computer speakers but they plug-in to the soundcard's regular stereo output. (And the little woofer isn't really a subwoofer.)

 

[dasdoing]

you are going out 2 channel of your source, so you will need an app to make your own downmix there.
personaly I just add the extrachannels with -3dB to L and R. might have to atenuate overall level if it clips

 

[Yasuo]

You need something like MPC has:

 

[digitalfrost]

I can only agree that you need to do this either in the player or on operating system level. Plex and Kodi don't seem to be able to do this.

I am doing this with EqualizerAPO (under Windows) and I can tell you depending on source material having the LFE is either good or bad, it really depends.

Here's the command I use:

Copy: L=1*L+0.707*C+0.707*RL+1.58*SUB R=1*R+0.707*C+0.707*RR+1.58*SUB

+4dB = 1.58

(+10dB standard LFE boost -6dB because of additive bass output from the two stereo speakers)

Depending on the rest, you might need to apply a negative preamp to prevent digital clipping.

 

[dasdoing]

+10dB standard LFE boost

don't do this. that's why you have mixed results

 

[dasdoing]

Today, there is no longer any optical tracks (or any Dolby theatrical 5.1) but nevertheless, the -10dB printing of the LFE channel is still always observed. However, unless you actually have access to an original theatrical DCP (and the means to play it of course), the chances are that you are NOT listening to the theatrical mix anyway. What you're probably listening to on your DVD or BluRay is a mix specifically "re-versioned" for home consumer use! IE. Designed for consumer bass managed systems, which don't use -3dB in the surrounds or +10dB in the LFE.

https://www.hometheatershack.com/threads/all-about-10db-sub-boost.117769/#post-1355649

 

[KMO]

That post you're quoting is nonsense, at least with respect to the specific LFE bit you're relying on.

Home systems always have +10dB for the LFE out of the box, and should do for standard Dolby/DTS mixes. The reference 0dBFS for the LFE is 115dB(C), compared to 105dB(C) for the mains, same as the cinema. (You don't usually play the entire system at reference in a small non-cinema like room, but that doesn't affect the point about the 10dB LFE<>main offset).

However, he is right about the surround offset difference - home systems are calibrated for equal-level mains, unlike the cinema, and that should be allowed for when preparing the consumer mix.

digitalfrost's mix calculation looks reasonable to me. Including the detail of assuming 3dB non-coherent summing for the full-bandwidth C and surround signals, but 6dB coherent summing for the low frequency. The number should possibly be somewhere between 3dB and 6dB in practice. There was a thread here somewhere discussing that value from the other direction for main->subwoofer bass redirection. (The error accumulates the more speakers are involved in the redirection.)

 

[tarkus]

Copy: L=1*L+0.707*C+0.707*RL+1.58*SUB R=1*R+0.707*C+0.707*RR+1.58*SUB

+4dB = 1.58

(+10dB standard LFE boost -6dB because of additive bass output from the two stereo speakers)

Just wondering why the -6 dB is necessary? I'm looking for the same bass output as a 5.1 system which would include the bass from both the main Left and Right as well as the LFE?

 

[Feyire]

Yes I know this is the Dolby standard for standard 2.0 setups. However, niche setups of 2.1 or in my case 2.2 exist and there is no reason for LFE to be excluded in these cases.

I'm also one of the people with a niche 2.2 setup, so 2 large floor standing speakers with 2 subwoofers. I certainly didn't purchase the subwoofers, only to miss out on the LFE channel goodness simply because I'm only using a stereo setup.

Unfortunately, it is difficult to find a perfect solution, but looking for a fully compliant HDMI 2.1 receiver (ARC and eARC support and thus support for all the latest audio formats) is certainly one way to do it, perhaps even the easiest if you are also happy to use the receiver's amplification.

However, you'll need to check the receivers manual to see what kind of functionality they support in regards to LFE and downmixing. For example, lots of receivers will have the option to set your speakers as "Small" or "Large" or similar terms. Here's a useful breakdown from an article:

“Here is the best way to understand Small/Large , LFE/LFE+Main and the LPF for LFE.

• Small applies a 12 dB/octave high pass to the speaker at the selected crossover frequency.
• Small sends a duplicate signal to the subwoofer and applies a 24 dB/octave low pass to the subwoofer at the selected crossover frequency.
• LFE means the subwoofer gets the LFE channel and redirected bass from any channels set to Small.
• Large sends that channel a full-range signal. This is also known as ‘Full Band’.
• LFE+Main sends a duplicate signal to the subwoofer for any channel set to Large and applies a 24 dB/octave low pass to the subwoofer at the selected ‘crossover’ frequency **.
• When set to LFE+Main, the subwoofer still gets the LFE channel and redirected bass from any other channels still set to Small.

** Note if the mains are set to Large and the subwoofer mode is set to LFE, there is no crossover selection available for the mains. If the mains are set to Large and LFE+Main is selected, then the ‘crossover’ selection becomes available for the mains. This really isn’t a true ‘crossover’ at all, because the mains are still being sent a full-range signal. What the ‘crossover’ becomes when LFE+Main is selected is the low pass filter setting for the duplicate signal being sent to the subwoofer. In this sense, the user can select the amount of ‘overlap’ between the mains and the subwoofer.

This distinction is not well understood by most enthusiasts, nor is it well communicated by the AVR GUI menu. It suggests or implies a crossover is still being applied to the mains when they are set to Large, when in reality that is not the case.

The LPF for LFE is the low pass filter setting for the LFE channel. Normally this is set to 120 Hz, since that is typically the upper limit used by DVD mixing engineers

https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg has lfe downmixing if you don't mind post-processing and command line processes

In my case, because of my current hardware and setup, I make use of ffmpeg to manually downmix a select list of my movies (e.g. with 7.1 channels) to 2.0 as described in the above linked post.

Here is the ffmpeg command and parameters I use, which copies the video source along with any subtitles, and downmixes multi-channel audio to regular stereo (2.0) with LFE mixed in. As such, I send a full range stereo signal with LFE mixed in to my full range speakers and subwoofers, allowing the crossovers built into the speakers and subwoofers to deal with it.

ffmpeg.exe -i "MyMovie.mkv" -map 0:v:0 -map 0:a:0 -map 0:s? -vcodec copy -acodec ac3 -ab 640k -af "pan=stereo|FL = FL + 0.707*FC + 0.5*SL + 0.5*BL + 0.5*LFE|FR = FR + 0.707*FC + 0.5*SR + 0.5*BR + 0.5*LFE" -scodec copy "MyDownMixedMovie.mkv"

This is of course a non-ideal solution, as it only works on the movies I specifically decide to process, so live audio coming from my TV (e.g. via satellite, or cable, or streaming services or wherever) do not get this treatment and LFE is still lost in those cases. In your case, if the Shield is your only/primary source and if it could downmix the LFE into stereo, then that would be best solution. Might be worth asking about this on the Nvidia forums.

 

[KMO]

Just wondering why the -6 dB is necessary? I'm looking for the same bass output as a 5.1 system which would include the bass from both the main Left and Right as well as the LFE?

Cos he's sending one channel (LFE) to two channels (L and R), and doesn't want to double the output as a result.

For full-bandwidth signals, -3dB is usually used to compensate for that (which you see as the 0.707 for the centre signal there). Two non-coherent sources sum as +3dB. (Although, come to look at it, not sure why he's done -3dB for the surrounds.)

But phase-coherent signals actually double and sum as 6dB. At the lowest frequencies, two bass sources will be roughly phase coherent, so will sum closer to 6dB.

This is one of those wrinkles you get when trying to spread stuff around different numbers of transducers. And neither 3dB nor 6dB is a totally perfect answer. You probably should use a frequency-dependent slope, but most equipment uses a constant somewhere in that range.

When redirecting bass to a subwoofer, most stuff just sums the bass from all channels. So if you had mono bass in L+R, say "X", then that would be extracted and 2X total would be sent to the sub. In other words a single +6dB signal, due to the 2 inputs. Reversing that to send it back to L+R needs -6dB to each.

But it has been noted that the sum in that case is a bit excessive, as the L+R aren't fully coherent, so the redirected bass ends up a tad high. Conversely, you may want to use a slightly smaller than -6dB adjustment when sending LFE to L+R.

Here was a thread about the summing effect, with some tests:

Why bass management makes my life tedious

I'd like to talk a bit about one problem (amongst many) I find when mixing films in surround sound: Bass summing. It's not a new problem, but as speaker counts have gone up, it's definitely been exacerbated. While the issue is kinda multidimensional, discrepancies in bass level at the point of...

www.audiosciencereview.com

 

[digitalfrost]

For full-bandwidth signals, -3dB is usually used to compensate for that (which you see as the 0.707 for the centre signal there). Two non-coherent sources sum as +3dB. (Although, come to look at it, not sure why he's done -3dB for the surrounds.)

Both Dolby and ITU-R BS.775-3 specify -3dB for the center and side channels when downmixing.