Use of LFE channel

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trwh
Posts: 3
Joined: Thu Dec 16, 2021 4:30 pm

Use of LFE channel

Post by trwh »

Hi Everyone,

Many thanks Carl for creating DCP-o-matic and releasing it for free - what a great bit of software! Thanks also to everyone here for the feedback and help - I've spent the last few days reading through many of the threads.

My question is about the use of the LFE channel in surround audio mixes. I've made a DCP with 5.1 audio via un upmix from stereo that I implemented manually in Adobe Audition which provides a silent LFE channel. I've included the upmix parameters below for interest, it's a basic one we implemented at the BBC for use with digital TV broadcasts.

The upmix sounds fine in the cinema but I've noticed that when playing music the bass sounds quite weak and quieter than I'd expect from an equivalent stereo PA setup in the same room. When upmixing for TV we always kept the LFE channel silent (as you can see below) with the logic that it was only meant for special effects and it was correct to place a full-range signal on the other channels which would be routed to the subwoofer if needed by the playback equipment (for example, if the main speakers aren't themselves full range). This made sense since many consumer surround processors let you define whether the front and surround speakers are "small" or "large" for exactly this purpose. However, I'm wondering if this logic still holds in the cinema? Is the LFE track just for bangs and rhumbles or is it also meant to contain low-passed sound from other channels? Is the silent LFE channel the reason for the slightly weak bass in my DCP?

To put it another way, can I assume that a cinema sound processor knows to route sound to the subwoofer where it's needed, or does it just pass straight through via the fader when the input channels map exactly to the sound system (e.g. 5.1 in -> 5.1 out)? The processor in the cinema I'm using is a Dolby CP650.

And finally, if I provide stereo sound in my DCP, will it be easy for the operator in the cinema to configure the CP650 for an upmix and might this sound better than my basic manual one? Is that called a Prologic decode?

Any thoughts are very welcome and thanks again!

Tim

Code: Select all

Lf = (L * a1) + (R * a2)
Rf = (R * a1) + (L * a2)
C = b * (L + R)
LFE = 0
Lsur = (L * c1) + (R * c2)
Rsur = (R * c1) + (L * c2)

where:
b = 0.25
c1 = 0.25
c2 = -0.2
a1 = 1.0 - (cmix * b) - (smix * c1)
a2 = -(cmix * b) - (smix * c2)

where (these are the downmix coefficients):
cmix = 0.707106781186548
smix = 0.707106781186548
jdent02
Posts: 22
Joined: Wed Nov 04, 2020 10:32 pm

Re: Use of LFE channel

Post by jdent02 »

Bass management isn't all that common in cinema sound systems, so the subwoofers will usually only play content in the LFE. Many soundtracks have more than explosions or big effects in the LFE for this reason. Whenever I watch the activity graphs on our CP750 I see LFE activity quite frequently during music passages. Granted it's nowhere near as high intensity as a big booming effect but it's definitely there.
Carsten
Posts: 2044
Joined: Tue Apr 15, 2014 9:11 pm
Location: Germany

Re: Use of LFE channel

Post by Carsten »

True - the idea in cinema is that the L/C/R front systems go deep enough for general low frequency content, and that the LFE only carries dedicated low frequency effect signal.

Some few systems employ so called bass management (usually, when the front systems have been chosen - or turned out - too weak for economical reasons), but in general, you can not count on it.

Cinema sound systems are very different from consumer systems. For that reason, cinema people stress that the LFE is NOT a subwoofer as used in home hifi/cinema systems.

Similarly, you can not count on cinema systems to create a 7.1 to 5.1 downmix if only a 7.1 DCP is made available. In that case, the back surround signals will not sound when being played back on 5.1 systems (which still the majority of cinemas have installed). 5.1, however, is usually spread onto 7.1 systems without a problem. However, usually not 'automatically', but only if the sound system has been configured properly by the cinema technician, and if no one has since tinkered with the system.

Our system has an optional dts Neo:6 upmixer which will send signals into the LFE channel created from low frequency content at the input - however, it is only available for non-sync/stereo signals, not for DCP audio.

The special cinema audio processors offer different setup options, but in general, they will not allow to apply 'strange processing' to DCP audio. Mostly, it is just EQ, level, and delay.

Regarding realtime matrix decoding for stereo DCPs - initially, most cinema audio processors were not able to perform matrix decoding. Therefore, using matrix encoded audio in DCPs was, and is still discouraged.

https://registry-page.isdcf.com/audioconfigs/

Now, as far as I know, most audio processors have now received firmware updates which allows basic matrix decoding from Lt/Rt tracks, however, only very few cinemas are set up to use it. The average cinema projectionist is not able to configure the audio processor. Basically, they can switch between presets, and set the playback volume. Unfortunately, many cinemas nowadays do not employ capable tech staff anymore.




- Carsten
jdent02
Posts: 22
Joined: Wed Nov 04, 2020 10:32 pm

Re: Use of LFE channel

Post by jdent02 »

A fully up to date CP750 can do Pro Logic II decoding on stereo tracks, which results in a pseudo 7.1 signal I believe. I've never used it but it's there :D
trwh
Posts: 3
Joined: Thu Dec 16, 2021 4:30 pm

Re: Use of LFE channel

Post by trwh »

Many thanks!

That's good to know re. bass management not being common in cinema systems. And that someone has noticed a little activity in the LFE channel during music - it shows that big creators are adding sound in there for more than effects in at least some cases.

Also very good to know about the ethos being that "pass through" is normal in the playback audio processing stage for DCPs, e.g. just EQ, level and delay.

I've recently moved an annual film event between cinemas in London (from Soho Screening Rooms to the Close-up Film Centre) and I suspect that the new venue just has a slightly more bass-light system. In future I'll experiment with adding a low-pass of the L/C/R channels into the LFE instead of leaving it silent. Wikipedia (https://en.wikipedia.org/wiki/Low-frequency_effects) tells me that 120Hz might be a good corner frequency to start with. The Wikipedia article states, "The LFE channel is conventionally played back 10 dB louder than the main channels, giving significantly more recording headroom" which surprises me a bit and suggests I should attenuate the output of the low pass by at least 10dB to keep the audio spectrum balanced. Finally, if I do this, I wonder if I should high-pass all the other channels... :D

As a separate chatty topic since I was a PA operator in a previous life, do you ever see cinema sound systems big enough that the main L/C/R speakers have active crossovers and multiple amplifier channels? Or are passive crossovers normal for cinema? To my ears mixes always sound slightly brighter than I'd expect in the cinema and I've got used to balancing expecting this over the years - I'm not sure if others have the same experience.

Thanks again,

Tim
Carsten
Posts: 2044
Joined: Tue Apr 15, 2014 9:11 pm
Location: Germany

Re: Use of LFE channel

Post by Carsten »

There are no solid statistics, but from what I see equipment wise, the larger the auditorium, the more likely active crossovers are used nowadays. Passive was common even for larger auditoriums until the late nineties or early 2000s. With the advent of digital soundtracks and more dynamic mixes, active systems came into use. Some older cinema audio processors, as the Dolby CP500, featured optional internal active crossovers.

All cinema audio processors currently in production offer digital crossovers as a standard feature.
trwh
Posts: 3
Joined: Thu Dec 16, 2021 4:30 pm

Re: Use of LFE channel

Post by trwh »

Fascinating, thank you!

I’ll have a read around but as if crossovers are included you might expect limiting / protection as well.

If anyone has pictures of any equipment racks and speakers please do share them.

Thanks again, Tim
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