[Help] Encoding audio with LFE2?

[LucasGodzilla]

So this is totally something that has sort of fascinated me for a bit and I was wondering if anyone has ever managed to get working?

So apparently, certain audio codecs have the compatibility of including two separate LFE channels.

What's more, there's clearly documentation of it being coded into FFMPEG as part of the added support for 22.2 mixes—as insane as that sounds.

Now whilst I understand that there is the whole argument about whether or not this is something that'd even be worth the effort given low frequencies aren't particularly "directional" so-to-speak, I still at least like the prospect of its existence in a codec since it technically opens the door to properly supporting stuff like Dolby "Baby Boom" stuff which had dual LFEs.

Obviously, the opportunity of preserving something like that is practically nonexistent here, but I'm more curious about whether or not it can even work more than anything else at this point.

So I was spending a fair chunk of my afternoon testing different channel layout combos in FFMPEG to see if I can get it working, but for whatever reason, I can't seem to get it working whatsoever.

Maybe I'm doing something wrong with my code, but for some reason every time I try introducing the LFE2 in AAC, I get an error saying "Channel 3 cannot be referenced twice", so I assume the software's interpreting LFE2 as LFE still or something? I even tried trying to do a test creation of a 22.2 track—since it's the only standard channel layout that includes this mysterious channel—and not even that works.

I don't know. I don't suppose anyone else is willing to give this a shot on their end for experimentality's sake?

[stwd4nder2]

Interesting, I didn't even know this was a thing.

I think a bigger question is if AVRs will even support it. Pretty much every low to mid end processor will just have all the LFE channels Y'd out from a single output. Even higher end ones I'm really curious if/how they would handle it.

Also what is LFE2 even for? I assume stereo LFE but none of the documents seem to specify Left/Right. I know dual baby booms were relatively common, but I guess I always assumed it was pretty much the same channel duplicated. I know a few seconds of ESB in 70mm was scanned, not sure if the audio was scanned as well to see if there's any difference in the two channels.

A lot of interesting questions. Curious to see what else you dig up. Would be nice if there's an official release out there with an LFE2, that might help get us some answers.

[LucasGodzilla]

I think a bigger question is if AVRs will even support it. Pretty much every low to mid end processor will just have all the LFE channels Y'd out from a single output. Even higher end ones I'm really curious if/how they would handle it.

I'd imagine it would just be piped out to the single LFE speaker on most AVRs, but that's just me totally guessing. I certainly would like to imagine at least that the hardware would know what to do with that extra channel—at least on modern setups.

Also what is LFE2 even for? I assume stereo LFE but none of the documents seem to specify Left/Right.

As far as I'm aware of, going off of the MediaInfo page on the subject of channel layout, it is indeed just a stereoized LFE and not much else.

Furthermore, info about that absurdist 22.2 layout seems to also indicate that it's intended to create a stereo-ized LFE experience.

I know dual baby booms were relatively common, but I guess I always assumed it was pretty much the same channel duplicated.

Well it certainly at least makes the most sense to me that the dual baby-boom was actually stereo-ized since it'd be an odd waste of a track not to. Plus I've found this page which seems to offer a fairly detailed explanation of the format:

With a 70mm Dolby-Encoded “baby boom” print, tracks 2 and 4 carry this lower-pitched “boom” track information, which is handled by the magnetic sound “format” module located either in slot 18, 19, or 20 of the CP100 — just like any 70mm print. But tracks 2 and 4 were run through special “Link/Filter” cards (Catalog number 105D) in slots 21 and 22. The 105D Module contained a 200 Hz low-pass filter, providing a frequency range of 200 Hz down to 25 Hz, as well as individual equalization — presumably adjusted to make deeper sounds from the standing house speakers without blowing them to bits. We assume the Chinese was still using their five speaker array of Voice of the Theatre A2 speakers behind the screen. These speakers were rated to produce sounds as low as 30 Hz. Low indeed, but not low enough. Sensurround went down to 16 Hz with specially-designed speakers and amplifiers, but still, the 2 “baby boom” tracks provided an impressive rumble — in stereo!
[Image: dolby-stereo-4.1-70mm-track-layout.jpg]
[Image: dolby-stereo-4.1-70mm-diagram.jpg]

I know a few seconds of ESB in 70mm was scanned, not sure if the audio was scanned as well to see if there's any difference in the two channels.

As far as I'm aware of, that particular scan did not capture the audio—merely the picture given the video is silent. That said, the post did mention that mag cap was possible at the time?

Actually we may have a few more seconds from other parts of the film too, but only fragments. However, I wanted to post what we do have for a number of reasons. Firstly, it shows that we now have the equipment to scan a 70mm print (including the magnetic and optical audio tracks) so if you happen to have a 70mm print of Star Wars sitting in your garage that you don’t know what to do with, please let us know!

Would be nice if there's an official release out there with an LFE2, that might help get us some answers.

Finding any official use of this extra LFE channel is practically nonexistent out there. It's really weird too since you CAN find documentation on it for specific codecs as mentioned before, one of the most notable being DTS-HD even, but it's really sort of buried since I don't think the Mastering Suite offers that capability (don't know as I never messed with that software before)?

LFE2 | Second low frequency effects subwoofer | 0x00010000 | 0x00010000

But you'd think it'd be something you'd see being prodded more, especially knowing the highlights of some modern BD releases are the restorations of those 6-track mixes like with Streets Of Fire for instance.

[Yarp]

i don't have a crazy amount experience in the world of LFE but it's probably more than most on this forum.

the majority of people need more than one subwoofer to fill their room properly for a basic 5.1 setup. this is mainly due to the fact that 1 subwoofer in 1 location (especially in a bigger room) isn't enough to properly cover the full spectrum from 120hz-20hz). you are more than likely to have nulls where you have almost no bass whatsoever (in my setup, it was around 35-40hz, but i'm flat to about 16hz otherwise). a 2nd subwoofer is useful to most because when placed in a different location, it can cover that 30-40hz that the main subwoofer can't...filling in that null. this is why bass management (via something like a minidsp 4x2 HD) is important for anyone with multiple subwoofers.

given all this, i can't even comprehend what a 2nd LFE track would be useful for and i'm guessing any AVR/receiver (even modern ones) would have no idea what to do with it anyways and probably just mix it into one LFE track. if dual sub tracks were a thing, they would have found their way into theaters long before chairs that shake you or speakers in the ceiling. but maybe there's someone that knows this stuff better that can chime in.

[zoidberg]

This piqued my curiosity so I had a little dig around. 22.2 is the audio configuration for Super Hi-Vision (i.e. 8k) as proposed by NHK, which features dual LFE channels, but it also goes back to Tomlison Holman's 10.2 format which specifically features left and right LFE channels. In this configuration the left LFE channel supports all left speakers while the right LFE supports right speakers (any centre information is split between L/R).

I can almost see the sense in having an additional discrete subwoofer channel when you start getting into such large multichannel configurations, as it shares the burden over two channels and probably allows for better defined bass reproduction overall, especially when all bass information is directed to the sub as is often the case with home cinemas. It won't make the bass 'louder' as the calibration would most likely take into account the summed output of both subs (as is the case with surround speakers). 

I can absolutely see the merit in having two subwoofers in a home for exactly the reason Yarp points out, to cancel out standing waves formed in smaller rooms by the low frequencies/long wavelengths at which subwoofers operate. I can't really see the need to artificially create a second LFE channel for most 5/6/7.1 content that's out there now, although a lot of blu rays feature D-Box support (but that's a whole other thing).

Regarding 70mm 'baby boom', at the time it was implemented 70mm screens used the Todd-AO format with 5 front channels (Left, Centre, Right and what were know as Left-Centre and Right-Centre) and a monaural surround channel. Dolby changed the speaker configuration to the same as matrixed Lt-Rt i.e. Left, Centre, Right, Surround (with the added benefit of being fully discrete allowing wider 'spread'), with the now-redundant Left and Right Centre channels used solely to playback low bass frequencies in order to reinforce the output of the main channels. Despite being spread over 2 tracks the 'baby boom' signal would have been mono. It's important to note that there is no dedicated subwoofer in this format, just repurposed full-range speakers. This way no new speaker arrangement was needed and older prints with the Todd-AO format could still be run.

[stwd4nder2]

Here's an ffmpeg issue about 22.2 AAC: https://trac.ffmpeg.org/ticket/8714

What's interesting is that it include a google drive link for an example file encoded in 22.2. I gave it a download and I wasn't able to play it back or open it with any of the usual tools.

[LucasGodzilla]

Regarding 70mm 'baby boom', at the time it was implemented 70mm screens used the Todd-AO format with 5 front channels (Left, Centre, Right and what were know as Left-Centre and Right-Centre) and a monaural surround channel. Dolby changed the speaker configuration to the same as matrixed Lt-Rt i.e. Left, Centre, Right, Surround (with the added benefit of being fully discrete allowing wider 'spread'), with the now-redundant Left and Right Centre channels used solely to playback low bass frequencies in order to reinforce the output of the main channels. Despite being spread over 2 tracks the 'baby boom' signal would have been mono. It's important to note that there is no dedicated subwoofer in this format, just repurposed full-range speakers. This way no new speaker arrangement was needed and older prints with the Todd-AO format could still be run.

Yeah I recall reading about how Baby Boom was technically a result of the Todd-AO format being recycled. Funnier thing is technically that could mean that you'd get arguably "authentic" results if you just re-routed the channels into a FL+FR+FC+FLC+FRC+BC layout—which I believe can be done fairly easily in FFMPEG for most codecs like PCM. Though that said, because the Baby Boom channels were in those subwoofer Hz territories, it does just make more sense to re-route them to LFEs.

Interesting reading about how the Baby Boom tracks would've been mono though. It's a bit strange though given the quote I pulled directly stated it was in stereo, though who knows what source that may have been. It'd certainly explain why 6-Track remasters tend to just stick to the one LFE channel I guess though—and it serves as a comfort knowing that nothing would technically be lost.

All that said, it would still be fascinating to see if one of us could actually manage to get FFMPEG to pan something to LFE2 to see if it's actually a real thing that's technically supported by stuff like DTS-HD. Would be amusing to see—if it is possible—dual LFE being utilized in the same sense of how dual mono is being done sometimes lol

Here's an ffmpeg issue about 22.2 AAC: https://trac.ffmpeg.org/ticket/8714

What's interesting is that it include a google drive link for an example file encoded in 22.2. I gave it a download and I wasn't able to play it back or open it with any of the usual tools.

Rather interesting to see that ticket got resolved years ago.

I feel like that file is corrupted or something, because whilst it does—amazingly—list out every channel in MediaInfo, if I try directly copying it over to something like MKA, I get an "invalid data" error after like 1:40, and had to trim the conversion to go up to only 1:00 for it to complete successfully. Even then though, the MKA doesn't seem to detect the channel layout so-to-speak (only showcasing the number of channels with no details).

Haven't been able to re-encode the audio with much success either. I would normally wonder if these more "out-there" layouts are only encodable by libfdk_aac (which I admit I don't have set up on my end whatsoever), except I can clearly see in the ticket that it was stated that FFMPEG's AAC should natively support this?

I also have issues trying to get the LFE2 on other formats outside of AAC, which is weird given they are stated here and you can find mentionings of it in several of the codecs listed here with a little of digging.

[Yarp]

I can almost see the sense in having an additional discrete subwoofer channel when you start getting into such large multichannel configurations, as it shares the burden over two channels and probably allows for better defined bass reproduction overall

in an ideal/modern home setup, you've got your 1 lfe track that gets piped to multiple subwoofers which are all independently tuned to the room they are in so that bass response is as flat as possible from 0hz-120hz. how does adding another lfe track allow for better bass reproduction in this situation where i already have flat bass response?

All that said, it would still be fascinating to see if one of us could actually manage to get FFMPEG to pan something to LFE2 to see if it's actually a real thing that's technically supported by stuff like DTS-HD.

Subwoofers, unlike speakers, are omnidirectional. It's why you need your speakers in very specific locations, yet your sub can be placed almost anywhere and in any position and still get results. A 2nd LFE track won't be able to fix problems you have with your current subwoofer setup and I would guess it would have little to no benefit for home use. If you really want to transform your subwoofer experience, you need 2 subwoofers and a something like a minidsp so you can tune them both independently.

Would be amusing to see—if it is possible—dual LFE being utilized in the same sense of how dual mono is being done sometimes lol

dual mono is just 2 identical audio tracks. 2.0 mono doesn't really add anything over 1.0 mono. digital mono tracks will be 100% identical anyways and analog tracks will have minor fluctuations due to noise, but they are still the same tracks. transferring this idea to the LFE channel...i don't see how 2 identical LFE tracks would do anything or sound any better over 1 properly mixed LFE track.

also note that LFE2 is listed in the TrueHD spec (page 7), and they reference this ITU standard for how/why the channel list was created.

if you really want some input, you should post this question over on /r/hometheater and see what they think. there's plenty of very knowledgeable users over there, especially when it comes to subwoofers and placement.

[zoidberg]

The way I see it, and it is purely theoretical, is that a subwoofer's driver can only move so fast and if you're trying to provide LFE for 22 channels there's a potential for things to get muddy, having an additional discrete channel would split the burden, it's more about definition rather than frequency response. I guess the analogy is having a three-way speaker vs a two-way or even a single cone. In any case the engineers designing these 10.2/22.2 systems seem to think it's necessary. I don't think a standard 5/6/7.1 setup would ever warrant dual discrete LFE channels, but pretty much any home theatre would benefit from either dual subs or very careful placement of a single sub to even out the response. It's one of the reasons I think about getting an RTA from time to time.

The quote about 'baby booms' being stereo is technically correct in the sense that there are more than one track but I think maybe whoever wrote the quote perhaps misunderstood the concept, as bass is non-directional. Twin tracks meant they could utilise both 'dormant' speakers (left-centre and right-centre) and get a healthy output gain from the sum of the two channels. I have a feeling that sensurround had different low-frequency tracks for audible bass and the floor-shaking tactile bass (in the same way some blu-rays have d-box code) but I don't know enough about that format to be sure.