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Why bass management makes my life tedious

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audio2920

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But that is what they are doing; boosting lower-level content that is still there. Not all Blu-rays exhibit this. My question is, what are the reasons why it would be reduced so much in level for the Blu-ray compared to the theatrical version?
Everything here is a great thought. But as @Soundmixer said there's too many other variables to say for sure.

My own suspicion would be that the Atmos HE mix was done on a different stage, perhaps even by a different mixer, and they felt they needed to reduce that content intentionally for some reason or other.

Just to speculate for what it's worth, most of the small rooms I've mixed in aren't massively acoustically treated down to those frequencies, so maybe the 20-30Hz content came out overwhelming for them and they reduced it. Who knows. Yet another possibility is they didn't feel like it brought much to the party but was using up modulation width (level) on the recording and reduced it to increase headroom / avoid pushing the limiter.

I would say though, my own experience of moving from the large stage mix down to the HE mix is that when you engage bass management, you quite often get more bass than you had before. Sometimes it's not a lot more. Sometimes it's actually good, and it's often the first time we get to hear what we had below about 40Hz on the LCR. But sometimes it's quite noticeable and needs addressing - which does tie in with my original rant (ahem, observation)

Oh, so I guess, maybe the Atmos HE mix was done to make it sound more like how it did in the cinema space without that low end content (perhaps by the original mixer who was used to that), and the person/team who did the DTS just enjoyed the extra LF extension. More speculation. :)
 
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audio2920

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HOWEVER: PCM LFE of the same mix, released on BluRay (or perhaps DVDA...or even perhaps DSD, though I forget whether I've seen it there) sometimes shows full range signal in the LFE channel (as seen in freq/amp plots of the LFE channel contents). E/g, a full range bass guitar and/or drum part has been mixed to LFE, but has not been rolled off.

I certainly wouldn't consider this "normal" either. A feature mix could fail QC for having significant unfiltered content in the LFE.

It could very easily be a plain and simple oversight with no thought behind it. Not trying to excuse it, but with the best will in the world, workflows can get really messy sometimes and things slip through the net in the rush to get things out the door. ( I shouldn't say that, obviously my workflows are bullet-proof :p )
 

Soundmixer

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Everything here is a great thought. But as @Soundmixer said there's too many other variables to say for sure.

My own suspicion would be that the Atmos HE mix was done on a different stage, perhaps even by a different mixer, and they felt they needed to reduce that content intentionally for some reason or other.

Just to speculate for what it's worth, most of the small rooms I've mixed in aren't massively acoustically treated down to those frequencies, so maybe the 20-30Hz content came out overwhelming for them and they reduced it. Who knows. Yet another possibility is they didn't feel like it brought much to the party but was using up modulation width (level) on the recording and reduced it to increase headroom / avoid pushing the limiter.

I would say though, my own experience of moving from the large stage mix down to the HE mix is that when you engage bass management, you quite often get more bass than you had before. Sometimes it's not a lot more. Sometimes it's actually good, and it's often the first time we get to hear what we had below about 40Hz on the LCR. But sometimes it's quite noticeable and needs addressing - which does tie in with my original rant (ahem, observation)

Oh, so I guess, maybe the Atmos HE mix was done to make it sound more like how it did in the cinema space without that low end content (perhaps by the original mixer who was used to that), and the person/team who did the DTS just enjoyed the extra LF extension. More speculation. :)

I could not have said this better!
 

Soundmixer

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How confident are you that everyone does it? Confident enough that I can raise my LPF for LFE to 250 Hz and not hear a difference?

On disc and streaming content, pretty confident. I have the LPF filter on both of my Home theater systems set at 250hz, that is how confident I am. However, there is always an outlier, and thank goodness I can compensate for that by turning the filter to between 80-120hz.
 

Soundmixer

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Checking for unwanted LFE content before actually watching a movie is not a practical solution ;)
Last time I checked a couple of Blu-ray discs it didn't appear to be an issue only affecting a small minority of movies. But admittedly that was some years back.

So my question would be, why waste your time on this issue in the present?

Just asking.....
 

Soundmixer

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I also believe the limiter is "channel linked" to try and maintain positional and spectral balance. (Not sure to what extent, maybe 100% link or maybe less but as you say, details on the limiter are quite thin on the ground so, I too could be wrong!). Since it's speaker based, not object based, in a mix where there's sound going all over the place, a room config with less speakers will likely cause the limiter to work harder than a higher speaker count would.

To your first comment, there is an object-based limiter (which maintains positional amplitude and spectral balance), the main channel limiter (to address bass build-up with BM), and an LFE limiter (to prevent overload of that channel while encoding the content). The specifics of how they all work together are not well understood, and I am sure Dolby wants it to stay that way (proprietary information?).


Your statement I bolded is a logical conclusion.
 
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Chromatischism

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Yet another possibility is they didn't feel like it brought much to the party but was using up modulation width (level) on the recording and reduced it to increase headroom / avoid pushing the limiter.
I had this thought, that it used up headroom or digital bits or something. Maybe they shouldn't worry too much about cutting it off for preference though. Clearly if people are going to great lengths to create custom software, they think it's worth the effort bringing it back!

In the United States we have an abundance of powerful, low-tuned subwoofers. In this day and age, I see no reason to cut the LFE channel so sharply unless the producers just didn't think it was appropriate for the movie (romance, etc).
 

Chromatischism

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On disc and streaming content, pretty confident. I have the LPF filter on both of my Home theater systems set at 250hz, that is how confident I am. However, there is always an outlier, and thank goodness I can compensate for that by turning the filter to between 80-120hz.
Would I audibly hear issues by leaving it at 80-120? I've never heard of this until now. Some feel 80 sounds better if they don't like their subs going too high.
 

Soundmixer

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Would I audibly hear issues by leaving it at 80-120? I've never heard of this until now. Some feel 80 sounds better if they don't like their subs going too high.

I would like to remind those people who say they don't want their subs going too high that we are talking about the LPF on the LFE, not the bass coming from the other channels directed to the subwoofer. The LFE already has an LPF inserted during encoding, so there is no need to repeat it on the AVR.
 

Soundmixer

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I had this thought, that it used up headroom or digital bits or something. Maybe they shouldn't worry too much about cutting it off for preference though. Clearly if people are going to great lengths to create custom software, they think it's worth the effort bringing it back!

The folks doing this are bassheads, and bassheads are a small minority within the AV community.

In the United States we have an abundance of powerful, low-tuned subwoofers. In this day and age, I see no reason to cut the LFE channel so sharply unless the producers just didn't think it was appropriate for the movie (romance, etc).

Those powerful low-tuned subs are not the big sellers in the marketplace, and no mixer mixes audio to appeal to a very specific group's sound system. Polk and Klipsch sell more subs than JTR, SVS, and Powersound put together times 9.
 

richard12511

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I realise I'm perhaps making a bigger deal out of this than it is most of the time. But, I honestly feel this BM issue could have a lot to do with people complaining about "muddy" mixes etc. Far more so than x-curves and general EQ considerations.

I mean, my own front room system isn't even FR corrected. It's just some speakers levelled and crudely time corrected, and I have no issue with listening to that.

Is x curve actually used frequently? I tried the x-curve preset in GLM for fun, but didn't like the sound at all.
 

Chromatischism

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I would like to remind those people who say they don't want their subs going too high that we are talking about the LPF on the LFE, not the bass coming from the other channels directed to the subwoofer. The LFE already has an LPF inserted during encoding, so there is no need to repeat it on the AVR.
Right, I was referring to the 80 Hz preference some have vs the default 120 Hz. Sorry to add another variable. By using such a LPF you are saying there is a phase problem introduced compared to setting it at 250. Is this problem audible?
 

Soundmixer

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Right, I was referring to the 80 Hz preference some have vs the default 120 Hz. Sorry to add another variable. By using such a LPF you are saying there is a phase problem introduced compared to setting it at 250. Is this problem audible?


Any time you introduce different filters, with different roll-off characteristics at different frequencies at different points in the software/hardware chain, there are going to be phase issues. If we were listening to this in an anechoic chamber, you could hear it, but our rooms are not anechoic. For sure, this has never been tested, but I do know that phase issues at low frequencies with mono signals are very difficult to detect in small rooms. My default is less is more when it comes to filtering and EQ. If the source already has filtering, don't use more filtering.
 
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markus

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So the short answer is, 115dB, because it's my understanding that the limiter (in Atmos) will kick in above that.

That's certainly true of the implementation in the mastering unit (RMU) we use in the studio. I guess it's possible that in a particular AVR the implementation of bass management could be done post Dolby's limiter, but I'm not sure a manufacturer would do that in practice, as they'd need some other way of managing the headroom.

But as @Soundmixer says, it's not even 115dB because we don't mix HE at 85dB ref. It comes out lower than that for all the current loudness normalisation specs.

However, purely academically, I think the LFE uses up one of those 12 spatial objects, so you're left with 11, and the maths would be something like:

LFE = 115dB
11 Objects x 105dB = 20log11 + 105 = 126dB
126dB+115dB = 20log(10^(115/20)+10^(126/20)) = 128dB

I think..... :) So the limiter would pull 13dB off the LF output under these conditions.

Last thought - and it's outside my knowledge area and also irrelevant - I'm not sure the spatially encoded objects actually have a 0dBFS limit while "inside" the renderer and transport stream. If we only sent 12 objects to the RMU, then 0dBFS is the max for each one, as our MADI/Dante stream we feed it with is 24-bit fixed point. But if we were to send like 100 objects all really loud, would the spatial encoder maintain their level when summing these down to 12? I think it might, using 32-bit floating point which gives like 1000dB headroom above 0dBFS. The limiter would then just work really really hard on decode/render. I've no idea if this is what happens, nor do I intend to find out :D

Doesn't the limiter kick in if you have like 100 FS objects in the DAW? Does it change with monitoring configuration, e.g. 5.1.2 instead of 7.1.4?
I'd like to find out because this would allow us to determine the worst case scenario. Most AVR manufacturers seem to apply their own bass management downstream of Dolby processing and I've found that they simply use a static headroom. Sometimes the headroom even shrinks when turning up the master volume control. Not a desirable situation.

P.S. Yes, it's LFE plus 11 objects, not 12. Just a typo in my post. We arrived at the same 128dB.
 
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markus

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So my question would be, why waste your time on this issue in the present?

Just asking.....

Because I have no reason to believe that things have changed.
I've worked behind faders quite some time and errors happen all the time. I doubt these days there's less pressure getting a job done than back in the day.
 

markus

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To your first comment, there is an object-based limiter (which maintains positional amplitude and spectral balance), the main channel limiter (to address bass build-up with BM), and an LFE limiter (to prevent overload of that channel while encoding the content). The specifics of how they all work together are not well understood, and I am sure Dolby wants it to stay that way (proprietary information?).

I'm not asking about the inner workings of the black box of Dolby encoding and later rendering. Just asking what is coming out of it. That would allow us to determine the worst case scenario bass management in a SSP has to deal with. I doubt that the bass managed subwoofer channel gets more and more overloaded with increasing speaker channel count in a home theater environment.
 
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Soundmixer

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Doesn't the limiter kick in if you have like 100 FS objects in the DAW? Does it change with monitoring configuration, e.g. 5.1.2 instead of 7.1.4?
I'd like to find out because this would allow us to determine the worst case scenario. Most AVR manufacturers seem to apply their own bass management downstream of Dolby processing and I've found that they simply use a static headroom. Sometimes the headroom even shrinks when turning up the master volume control. Not a desirable situation.

P.S. Yes, it's LFE plus 11 objects, not 12. Just a typo in my post. We arrived at the same 128dB.



Dolby Atmos for streaming is 1 bed (the LFE) and 12 objects. For the disc, it is 1 bed plus 16 objects.
 

Soundmixer

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Because I have no reason to believe that things have changed.
I've worked behind faders quite some time and errors happen all the time. I doubt these days there's less pressure getting a job done than back in the day.


Things HAVE changed, you are just not aware it has. When Dolby TrueHD first showed up on Bluray, the encoder default for the LPF was off, and you had to turn it on. DTS HD Master Audio encoder default for the LPF was on, hence why you have never heard of Dts tracks having full range signals in the LFE. Today, Atmos and TrueHD encoder default is on, THAT is the change.
 

AdamG

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The folks doing this are bassheads, and bassheads are a small minority within the AV community.



Those powerful low-tuned subs are not the big sellers in the marketplace, and no mixer mixes audio to appeal to a very specific group's sound system. Polk and Klipsch sell more subs than JTR, SVS, and Powersound put together times 9.
No offense intended here. But your position is Static thinking. I personally have seen a huge move in the home theater area. Yes, agreed it represents a small portion of the market. But it is growing and people are buying larger and more capable subs, that just 10 years ago we’re not available or cost the same as a new car. No doubt that the current market is “Soundbar” centric, but the landscape is changing as more and more people decide to build out their own home theater. This move was amplified by the recent COVID crap and closure of most Movie theaters.

Full disclosure “I am a self diagnosed “Basshead””. Just give us the full range Sound and let’s the buyers/users determine if they can and will reproduce it fully. I am completely adverse to the recent trend of low frequency neutering of Sound tracks. JMHO.
 
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