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[hulkss]

When constructing filters for digital room correction you must be careful to preserve +10 dB of acoustic gain in the LFE channel as compared to the other channels. This is because LFE is recorded on DVD and Blu-ray intentionally mixed for subs that play at +10 dB (see references attached below).

Digital room correction software may inadvertently lead you to remove the +10 dB LFE channel playback.

For Audiolense users I have started a thread here: http://groups.google.com/group/audiolense/browse_thread/thread/5ce7cb90415c4e5e

[Matt]

I've been speaking with Bernt through email about this topic.

At some point I'll write a wiki about bass levels in Media Center.

The simple summary is that Media Center will normalize all signals (those with native LFE, JRSS signals, etc.) so that coming out of the 'Output Format' DSP it will be assumed that something later in the chain will make the subwoofer +10dB.

If you mix audio between channels, you need to take this into account.  The stock Room Correction does this properly.  Convolution filters also need to take this into account, and it sounds like they may not always do this (hence this thread).

Another solution would be for Convolution to assume that convolution filters do not understand the level difference between channels, and to manage these difference itself.  I'm not sure if these smarts would be welcome or if they'd add unneeded complexity.

[hulkss]

Maybe I'll get a response out of these guys: hometheatershack.com post

[adolfotregosa]

if the sub is self amplified it has a gain knob and since in audiolense we can remove gain using the 0.316 value , can't we just give more gain using the sub volume knob to compensate ? We do a measure with the volume on the sub lowered to the same level has the other speakers, and after all is set can't we give the sub more volume so the 10+ db is restored ?

Did i miss the point here or did i just said nonsense ?? are there subs without manual gain control ? ?

[hulkss]

if the sub is self amplified it has a gain knob and since in audiolense we can remove gain using the 0.316 value , can't we just give more gain using the sub volume knob to compensate ? We do a measure with the volume on the sub lowered to the same level has the other speakers, and after all is set can't we give the sub more volume so the 10+ db is restored ?
Did i miss the point here or did i just said nonsense ?? are there subs without manual gain control ? ?

The problem is....you can crank the sub volume knob all you want, Audiolense will generate a filter to undo it and make the LFE level the same as the other channels.

Yes, you can crank the LFE gain up after using Audiolense (10dB) and attenuate any redirected bass from other channels in the convolver configuration file.

[adolfotregosa]

The problem is....you can crank the sub volume knob all you want, Audiolense will generate a filter to undo it and make the LFE level the same as the other channels.

And ? Why do we need to crank the sub volume before the measure ? Until audiolense takes this into account when generating filters, why can't we manually put the sub volume at the same level has the other speakers and after generating the filter, manually restore the 10+ db ?? Is there ANYTHING wrong in doing this ?

[hulkss]

And ? Why do we need to crank the sub volume before the measure ? Until audiolense takes this into account when generating filters, why can't we manually put the sub volume at the same level has the other speakers and after generating the filter, manually restore the 10+ db ?? Is there ANYTHING wrong in doing this ?

You are correct, Audiolense will set all your channels (including the sub) to the same overall system gain regardless of the subwoofer volume setting. The sub needs to be +10dB relative to the other channels to play LFE content properly. If you turn up the sub +10dB after using Audiolense, any bass rerouted to the sub from other channels will be 10dB too loud. You will need to manually edit the filter configuration files to attenuate any rerouted bass channels.

Bernt from Audiolense has suggested an interim solution that can be done in MC Room Correction (see link in my first post).

[Trumpetguy]

I did exactly that. Measured as usual in Audiolense, levelling all channels. In MC, I have now put a -10dB attenuation on all channels except LFE. Everything else is left as before, with upmixing to 7.1 and convolution in the path. This way, only the LFE track should be receiving the +10dB gain, not the low pass parts of all the 7 other channels. The VU meter shows a much better picture now. Playing moder organ music (which sometimes is really similar to pink noise), all channels seem to level very well during playback. I have often wondered why the LFA channels seemd to be left much lower on the VU meter, but that is fixed now.

Movie tracks sound much more impressive...but I guess I will need to reduce the low frequency gain somewhat in my target curve...

[Mikkel]

I was under the impression that the surround sound decoder automatically corrected the LFE-output by adding +10db?

[Trumpetguy]

I was under the impression that the surround sound decoder automatically corrected the LFE-output by adding +10db?

If decoded in a receiver, yes. But apparently, e.g. the LAV filters do not.

[hulkss]

I was under the impression that the surround sound decoder automatically corrected the LFE-output by adding +10db?

A surround sound decoder does not add 10dB to the LFE channel. It may do bass redirection from other channels and will reduce those signals by 10dB.

Here is the Dolby Professional 5.1 guide: Dolby Guide
Here is a Sony Tech Note: Tech Note

[hulkss]

I did exactly that. Measured as usual in Audiolense, levelling all channels. In MC, I have now put a -10dB attenuation on all channels except LFE. Everything else is left as before, with upmixing to 7.1 and convolution in the path. This way, only the LFE track should be receiving the +10dB gain, not the low pass parts of all the 7 other channels. The VU meter shows a much better picture now. Playing moder organ music (which sometimes is really similar to pink noise), all channels seem to level very well during playback. I have often wondered why the LFA channels seemd to be left much lower on the VU meter, but that is fixed now.

Movie tracks sound much more impressive...but I guess I will need to reduce the low frequency gain somewhat in my target curve...

Yes, you will have 10dB more LFE output from DVD and Blu-ray sources (and any bass redirected to LFE in JRiver DSP). Bass redirected in the convolver will not be any different.

[Mikkel]

A surround sound decoder does not add 10dB to the LFE channel. It may do bass redirection from other channels and will reduce those signals by 10dB.

Here is the Dolby Professional 5.1 guide: Dolby Guide

I see. Where does this leave us then? I'm using convolver [EDIT: I'm using Jrivers convolution-engine] with REW-generated freq-correction filters. What steps should I make to get proper LFE-sound level (as well as subwoofer-level for redirected sound?

[EDIT 2]: In the config-file I haven't added any rerouting of bass. I leave that to the room correction-engine in JRiver.

Thank you very much for your help (and patience with my lack of understanding!)


Best regads,
Mikkel

[hulkss]

I see. Where does this leave us then? I'm using convolver [EDIT: I'm using Jrivers convolution-engine] with REW-generated freq-correction filters. What steps should I make to get proper LFE-sound level (as well as subwoofer-level for redirected sound?
Thank you very much for your help (and patience with my lack of understanding!)
Best regads,
Mikkel

My comments here are based on the assumption the your convolver filters establish a system with equal gain on all channels including LFE. I'll assume REW does that.

Here is a solution from Audiolense that Trumpetguy used:

In DSP Studio in JRiver:
*Move the "Room Correction" module to the top.
*Enable
*Amplify sub with 10dB.

This will attenuate the all other input format channels except the LFE with 10dB, while the LFE will remain at 0dB.  This means those of you who have dedicated subwoofers with 10dB extra gain are wasting 10dB of gain. For those of you who feeds the LFE to mains speakers this is a perfect solution, where you have as much gain as you can have without risking audible clipping.

Here is another work around I suggested:

Put a -10 dB pad in the LFE channel interconnect before the power amps. EQ your room normally. Take out the pad for normal program playback.

Any rerouted bass now needs to be attenuated 10dB. JRiver does this automatically except with bass rerouted in user designed convolver filters.
Bass rerouting filters in convolver can be edited in the config file (.316 scale factor). http://convolver.sourceforge.net/config.html

[Trumpetguy]

Here is a solution from Audiolense that Trumpetguy used:

In DSP Studio in JRiver:
*Move the "Room Correction" module to the top.
*Enable
*Amplify sub with 10dB.

Not quite, I actually attenuated all channels except LFE by -10dB. With normalization and clip protection on, these two settings should be the same, but my solution will definitely ensure no digital clipping.

[Matt]

Room Correction always targets output that doesn't clip.  So If you turn the subwoofer up +10 dB, it just turns all the other channels down 10 dB. 

In other words, the highest level defines no change, and all the other level sliders are some level lower than that relative to the highest.

As for the workaround, I think using Parametric Equalizer to -10dB everything except the subwoofer might be more clear than using Room Correction (which offers a lot of other settings like distance, etc.).

[hulkss]

...but I guess I will need to reduce the low frequency gain somewhat in my target curve...

What happened in my case:

I have been using a VST plugin from DTS called Neural Upmix to upmix stereo to 7.1. I noticed that it renders a LFE channel that seems low in output. I realize now that it is allowing for the expected +10 dB gain of that channel. So I have the same low level of LFE signal in my two channel playback as in my DVD/Blu-ray playback.

This led me to develop a target house curve in Audiolense that has about +16 dB in the LFE region. That did seem excessive but sounded OK. Now I understand why.

[Trumpetguy]

What happened in my case:

I have been using a VST plugin from DTS called Neural Upmix to upmix stereo to 7.1. I noticed that it renders a LFE channel that seems low in output. I realize now that it is allowing for the expected +10 dB gain of that channel. So I have the same low level of LFE signal in my two channel playback as in my DVD/Blu-ray playback.

This led me to develop a target house curve in Audiolense that has about +16 dB in the LFE region. That did seem excessive but sounded OK. Now I understand why.

Wasn't just me, then.  

What strikes me is that some people in here are probably more than average skilled in math. Still, I find myself doing + and - errors more often than derivatives, low-pass filtering etc.  

[Mikkel]

Sorry for my ignorance on this issue but would it be a hurddle if JRiver handled this automatically?
1. If LFE-channel is present: boost discrete LFE signal only by 10db
2. With bass-rerouting to subwoofer: no change

In that case if your setup use bass management to route bass to the subwoofer and a LFE-signal is present in the audio-stream only the LFE-signal would have a 10db gain.

If it was this simple I guess it would already have been done. Thx for listening, though 

[hulkss]

Sorry for my ignorance on this issue but would it be a hurddle if JRiver handled this automatically?
1. If LFE-channel is present: boost discrete LFE signal only by 10db
2. With bass-rerouting to subwoofer: no change
In that case if your setup use bass management to route bass to the subwoofer and a LFE-signal is present in the audio-stream only the LFE-signal would have a 10db gain.
If it was this simple I guess it would already have been done. Thx for listening, though  

The LFE can not be boosted digitally. It may already be at maximum level from the source material (0dBFS) and would clip. The only digital option is to cut the other channels. It would be best to EQ the playback system the way the Society of Motion Picture & Television Engineers (SMPTE) intended. They specified 0dBFS to be 115dB SPL on the LFE channel and 105 dB SPL on all the other channels (commonly called "Reference Level").

[Mikkel]

I see. But if reducing all other channels by -10 db (as Matt also suggests), wouldn't that mean that rerouted (to the sub) bass-reproduction would be 10 db too loud?

[Buckster]

I posted in another thread about this yesterday - but just as relevant here

I tried (as I need the 10dB boost to LFE as my amp does not apply it to the multichannel in)

so set all channels in JRiver to -10dB apart from the Subwoofer which I set to 0dB

ran an action Bluray for 10 minutes (randomly picked) and JRiver had already flagged up that it had had to apply a -3.4dB gain to avoid clipping ?!

so then reset all channels to -14dB apart from Subwoofer which I set to -4dB

same Bluray, JRiver then flagged up that it had had to apply -3.2dB gain to avoid clipping ??

[Gerbrand]

My question may have already been answered. In that case I apologize for missing the point...

Anyway, here it is:

I am currently taking the JRiver convolution engine + DRC generated filters route. My convolver path does not include any cross-mixing between channels, just a filter per channel.

I have ticked the "normalize filter" box in the convolution module.

My room correction module is above the convolver module.

I then use the test tone generated by the room correction module to level my subwoofer and other channels using the analogue level knob on the subwoofer.

Is there any chance that I will get a wrong LFE/rerouted bass level from this setup? My understanding is that the "normalize" box would  remove any level differences in the filters and that the the test tone from the room correction module is correct both for LFE and rerouted bass.



Thanks for your input,

Gerbrand