# LFE Channel, Rerouted Bass, and Room EQ



## hulkss (Feb 10, 2009)

I discovered a problem using Digital Room Correction and was wondering how you REW users deal with it.

I have a 7.1 audio system using a Home Theater PC (HTPC) that plays through a Digital to Analog Converter (DAC) and on to the power amps. I measured and EQ'd my system to a house curve but soon recognized a problem.

Reference acoustic level for the Low Frequency Effects (LFE) channel on DVD and Blu-ray discs is intended to be amplified +10dB as compared to the other channels (SMPTE RP 0200). See page two below from tech note TN99051701.

With my system EQ'd, I was not providing this gain offset between channels. It is possible, if you are not careful, to inadvertently remove the gain offset built into home theater equipment.

Since movie sound tracks may contain maximum level digital peaks (0dBFS) on the LFE channel, the only digital way to correct this is to turn down all the channels except LFE by 10dB. A poor solution in my opinion as I give up 10dB of signal gain and signal/noise ratio on seven channels.

The correct solution is to EQ the LFE Channel separately with 10dB of additional analog/acoustic gain. If this is done, any rerouted bass from other channels must be attenuated 10db to compensate.

I have options with my system to deal with this. How do you REW users handle this issue?


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hey Brad,

Your post might be better served in one of the system set-up areas of the forum, as it has nothing specifically to do with REW.

Regards,
Wayne


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## hulkss (Feb 10, 2009)

You can move it. It's here because I have observed systems set up with equal system gain on all channels (including the LFE) because a room EQ tool was applied and the user was not aware of this pitfall.


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## JohnM (Apr 11, 2006)

Moving to Audio Processing forum.


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## Wayne A. Pflughaupt (Apr 13, 2006)

hulkss said:


> I have observed systems set up with equal system gain on all channels (including the LFE) because a room EQ tool was applied and the user was not aware of this pitfall.


 Is this the Digital Room Correction system you’re referring to? If so, it appears to be a generic system that’s not specifically tailored for home theater systems, so that probably accounts for the discrepancy you’ve observed. Can’t say I’ve heard of anyone encountering a problem of this nature using the auto set-up/EQ process build into AV receivers these days. In any event, it should be obvious there’s a problem when the user fires up his system and notices there’s practically no low end. It’s easy enough to compensate by adjusting the subwoofer gain.

Regards,
Wayne


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## hulkss (Feb 10, 2009)

Wayne A. Pflughaupt said:


> Is this the Digital Room Correction system you’re referring to? If so, it appears to be a generic system that’s not specifically tailored for home theater systems, so that probably accounts for the discrepancy you’ve observed. Can’t say I’ve heard of anyone encountering a problem of this nature using the auto set-up/EQ process build into AV receivers these days. In any event, it should be obvious there’s a problem when the user fires up his system and notices there’s practically no low end. It’s easy enough to compensate by adjusting the subwoofer gain.
> 
> Regards,
> Wayne


Myself and others had this problem using Audiolense: Audiolense

I EQ'd all the 7.1 channels to the same system gain level. I also redirected bass to the LFE channel. This was all accomplished with user developed filters from Audiolense in a convolver plug-in in JRiver Media Center. I'm using an HTPC connected directly to a DAC and power amps.

Problem is, the LFE channel should be set +10dB relative to the other channels and redirected bass has to be attenuated by 10dB.
I was not using the base management built into JRiver MC.

I believe the same mistake can easily be made by REW users. A person could send measurement sweeps through a system and set the LFE to the same level as the other channels. This would wipe out the +10dB gain designed into a home theater receiver.


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## Wayne A. Pflughaupt (Apr 13, 2006)

hulkss said:


> Myself and others had this problem using Audiolense: Audiolense


This appears to be an issue that the end users of such systems should take up with their creators. 




hulkss said:


> I believe the same mistake can easily be made by REW users. A person could send measurement sweeps through a system and set the LFE to the same level as the other channels. This would wipe out the +10dB gain designed into a home theater receiver.


 Not a big chance that can happen, as typically REW signal generation is accomplished in the analog domain, where there is no LFE channel. I think a few folks have figured out a way to get a digital feed to their systems, but I expect it's PCM and not Dolby Digital. So again, no LFE. In any event, we have typically recommended that people re-calibrate their subs vs. mains levels via a test disc or their receiver’s on-board process after equalization. Probably good advice for users of HTPC systems as well...

Regards,
Wayne


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## hulkss (Feb 10, 2009)

Wayne A. Pflughaupt said:


> This appears to be an issue that the end users of such systems should take up with their creators.


Yes, it is being looked into now.



Wayne A. Pflughaupt said:


> Not a big chance that can happen, as typically REW signal generation is accomplished in the analog domain, where there is no LFE channel. I think a few folks have figured out a way to get a digital feed to their systems, but I expect it's PCM and not Dolby Digital. So again, no LFE. In any event, we have typically recommended that people re-calibrate their subs vs. mains levels via a test disc or their receiver’s on-board process after equalization. Probably good advice for users of HTPC systems as well...
> 
> Regards,
> Wayne


Yes, the test disk is a solid recommendation. Missing correct levels on redirected bass is harder to detect.
I think most of the creators of Digital Room Correction software have their roots in stereo, not surround sound with LFE channels.

Audiolense is an amazing product. I have tri-amplified LRC loudspeakers (9 channels), 2 sides, 2 rears, and 3 separately controlled subs for a total of 16 channels. All digitally corrected in both time and frequency domains including crossovers with filters developed in Audiolense and convolved in JRiver MC. Pretty sweet, especially when I get the bass levels correct. :T


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## Nimish (Jul 2, 2011)

Query:

5.1 Room Equipments: 
QSC DSP 322us (Signal Processor)
Focal Speakers - L-C-R: CMS 65, Surr's: CMS 50, Subwoofer: CMS Sub
Calibration Mic: Dayton EMM-6
Software: TrueRTA
SPL Meter: RadioShack

Bass Management: Currently I have all the 5 speakers using Bass Management with a cross-over frequency of 65hz for the L-C-R and 75hz for the Surrounds.

I did a calibration today using Pink Noise & keeping a Small Room House Curve in TrueRTA.
While the tonal quality for all the speakers turned out perfect, the Low Frequency Content while playback was too much. I individually measured all the L-C-R's at 85dBSPL, Surrounds at 82dbSPL and Subwoofer at 88dbSPL (instead of 90-91dBSPL, due to the small room size.)

1. Is the Bass content adding up from the five speakers into the Subwoofer and thus making it too much?

2. Am I suppose to calibrate the Bass Content going to the Subwoofer from the L-C-R at 70dBSPL? (as read in some forums)

3. Any other way to Calibrate the Room?


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## hulkss (Feb 10, 2009)

Nimish said:


> Query:
> 3. Any other way to Calibrate the Room?


Look here: REW


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## SteveCallas (Apr 29, 2006)

If I am understanding your concern correctly, I think you may be misunderstanding things a bit. The LFE track is processed and sent to the subwoofer at +10db as compared to the other speaker channels. If you are using a tone generator to calibrate your speakers, and the subwoofer tone is not a discrete LFE track tone, but rather, just a low frequency tone, there is no problem calibrating it level to the other speakers. If you are calibrating with a discrete LFE tone for the sub, depending on the test disc, it is usually encoded 10db down on purpose, so when the LFE gets processed at +10db, you can continue to calibrate at the same overall level. 

So you're pretty much set either way - you don't want to calibrate so that the sub is 10db hot, otherwise your LFE signall will now be 20db hot.


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## hulkss (Feb 10, 2009)

Per DTS Technical Notice TN-E217:
"At a SMPTE meeting held in late 1998, all three digital sound companies agreed to comply with
the recommended practice for subwoofer level, SMPTE RP200. The subwoofer level has
changed to enable the use of one master recording when transferring to all three digital sound
processes and to provide playback consistency in theaters. As of January 1, 1999, the recorded
subwoofer level on 6-track masters has been lowered to the SMPTE recommended level of
10dB in-band gain (as compared to the screen channels)."

This means the LFE channel is lowered by 10 dB on all master recordings. The recording may still reach the level of 0 dBFS on the digital master in which case the SPL of a properly calibrated (to reference level) LFE channel will be 115 dB. All the other channels will play at 105 dB SPL at 0 dBFS.


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## SteveCallas (Apr 29, 2006)

I guess I'm not clear on what your concern is. You posted this:



> The correct solution is to EQ the LFE Channel separately with 10dB of additional analog/acoustic gain. If this is done, any rerouted bass from other channels must be attenuated 10db to compensate.


What are you using to generate a LFE tone or sweep to calibrate with? Most discs or even processors take into account the 10db difference and will utilize a LFE tone or sweep that is -10db on purpose for easy calibrating purposes. If you are attempting to manually compensate, you're creating a +10db difference on top of the already established 10db boost.


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## hulkss (Feb 10, 2009)

I'm using a HTPC, pro audio DAC, and power amps. My original message is related to the use of digital room correction software (I am using Audiolense). I had discovered that Audiolense EQ's the LFE channel at the same level as the other channels and reroutes bass that way also.

Yes, any test tone sent to the LFE channel should be at -10 dB as well as rerouted bass from other speakers.


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## SteveCallas (Apr 29, 2006)

The re-routed bass shouldn't be at -10db. It's not the subwoofer in general that gets boosted by 10db, it's only the LFE channel. Doing a sweep from say 20-20khz for a main speaker with the sub on, and let's say an 80hz crossover, using room correction software, you want to see a perfectly flat response. The bass sent to the sub needs to be at the same level as the rest of the frequencies sent to the speaker.

When doing a LFE channel sweep, which would be 3-120hz (or 3-80hz if you set 80hz as a hard stop in your processor), you should also expect to see the resulting average spl in the same range as each speaker, as most testing programs take into account the LFE should be 10db hot, so they lower it 10db. At no point should you have to calibrate the sub 10db hot. 

Hopefully I'm making sense.


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## hulkss (Feb 10, 2009)

LFE and rerouted bass are sent to the same subwoofers (in my system and most others). The subwoofer channel has 10 dB more acoustic gain (because the LFE channel is recorded -10 dB), when bass is rerouted to the subwoofers from other channels it is attenuated by 10 dB or it will be 10 dB too loud.

Like you say, this happens automatically in most systems, and LFE calibration signals are played at -10 dB.

In my HTPC I have to set everything manually and I was surprised to discover that Audiolense did not send -10 dB calibration signals to the subwoofers or attenuate the rerouted bass. Other measurement software may have the same issue.


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## SteveCallas (Apr 29, 2006)

> LFE and rerouted bass are sent to the same subwoofers (in my system and most others). The subwoofer channel has 10 dB more acoustic gain (because the LFE channel is recorded -10 dB), when bass is rerouted to the subwoofers from other channels it is attenuated by 10 dB or it will be 10 dB too loud.


Hopefully I don't seem argumentative, but this is incorrect. I use the same subs for redirected bass and LFE duties as well, but the LFE track is a discrete, independent channel. The subwoofer does not have 10db gain, only the LFE track does. If you didn't have a subwoofer, and routed the LFE to the mains, the LFE would still be +10db relative to the R&L tracks - that doesn't mean everything the main speakers played would be at +10db, only the LFE track would be.


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## hulkss (Feb 10, 2009)

My system is adjusted per this document: Setting the subwoofer levels

The subwoofer channel is setup with 10 dB more gain. I don't know what else to tell you.


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## Wayne A. Pflughaupt (Apr 13, 2006)

You do realize that document is for professional systems in theaters? There is no LFE routing option for “with sub” or “without-sub/direct-to-mains” in actual theater systems like we have with home systems.

Regards,
Wayne


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## SteveCallas (Apr 29, 2006)

hulkss, in a theater setting, all speakers are full range, and subwoofers handle ONLY the LFE track. For that reason, they may talk about the subwoofers being +10db, but what they really mean is that the LFE track is +10db.

In 99% of home theaters, subwoofers handle LFE and redirected bass, which means they may be playing portions of up to 6 or 8 channels at once depending if the source is 5.1 or 7.1. Only the LFE channel gets the boost, the other 5 or 7 channels do not. 

Is this making sense?


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## hulkss (Feb 10, 2009)

As I stated in post #1:

"I have a 7.1 audio system using a Home Theater PC (HTPC) that plays through a Digital to Analog Converter (DAC) and on to the power amps. I measured and EQ'd my system to a house curve but soon recognized a problem. Reference acoustic level for the Low Frequency Effects (LFE) channel on DVD and Blu-ray discs is intended to be amplified +10dB as compared to the other channels (SMPTE RP 0200)."

My custom system, like those described in the Sony document above, was calibrated using the same voltage reference on all channels. I neglected to set the acoustic level of the LFE channel 10 dB higher than the rest.

Now that this error has been corrected, if I reroute bass to those subwoofers, I must attenuate the rerouted signals by 10 dB. Keep in mind this system is completely configurable in software. The down side is I have to configure all of this stuff manually. A surround sound receiver does this automatically.

My main reason for posting initially is that it is easy to hook up a computer and play calibration signals through your system. If you are not careful, and you have separate level controls on each channel, a person could wipe out the LFE channel boost. The built in calibration signals in the receiver prevent this error.


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## SteveCallas (Apr 29, 2006)

Enjoy your system, I give up :dontknow:


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## hulkss (Feb 10, 2009)

A good explanation is here: LFE, subwoofers and interconnects explained


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## Mark Techer (Jan 17, 2008)

hulkss said:


> As I stated in post #1:
> 
> My main reason for posting initially is that it is easy to hook up a computer and play calibration signals through your system. If you are not careful, and you have separate level controls on each channel, a person could wipe out the LFE channel boost. The built in calibration signals in the receiver prevent this error.


When LFE is encoded, it is done so at -10dB to which the format decoder then applies the needed 10dB boost to bring it back to reference.


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## hulkss (Feb 10, 2009)

Mark Techer said:


> When LFE is encoded, it is done so at -10dB to which the format decoder then applies the needed 10dB boost to bring it back to reference.


That's true. Actually the LFE channel may contain content all the way to 0 dBFS (maximum level) so boost can not be applied digitally without clipping. It is done in the analog gain. The result is that a 0 dBFS LFE signal plays at 115 dB and the other channels at 105 dB. Digitally rerouted bass to the channel used by LFE is attenuated 10 dB to remain at the proper level during playback.

I my case I used a computer program (Audiolense) to calibrate my audio system. It sent equal level measurement signals (sine sweeps) through all channels. It then corrected all the acoustic levels to be the same magnitude. This eliminated the 10 dB boost to the subwoofer channel.

To do this correctly the LFE sweep signal should have been at -10 dB.


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## gdstupak (Jul 13, 2010)

MT:"_When LFE is encoded, it is done so at -10dB to which the format decoder then applies the needed 10dB boost to bring it back to reference._



hulkss said:


> I my case I used a computer program (Audiolense) to calibrate my audio system. It sent equal level measurement signals (sine sweeps) through all channels. It then corrected all the acoustic levels to be the same magnitude. This eliminated the 10 dB boost to the subwoofer channel.
> To do this correctly the LFE sweep signal should have been at -10 dB.


Brad,

With a normal system, Audiolense's process is proper. Normally you want a flat response whether you are measuring the redirected bass from the other channels or measuring dedicated LFE signals (because the LFE signal should have already been boosted by the format decoder, you (or Audiolense) should not need to manually boost the LFE signal).

Are you saying that your format decoder is not adding the 10dB boost to bring the LFE back to reference (and how do you know your decoder is not readjusting as it should)?

What format decoder are you using that does not do this automatically?


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## gdstupak (Jul 13, 2010)

I first read this thread at work which wouldn't let me see most of Brad's outside information. Now that I'm home and reading more of it, I am getting a bit confused and I can see what Brad is talking about.



Mark Techer said:


> When LFE is encoded, it is done so at -10dB to which the format decoder then applies the needed 10dB boost to bring it back to reference.


Mark, are you saying that either our AVR or our Blu ray player will add that 10dB boost to the LFE signal before it leaves the AVR's LFE output (or if not using the LFE output, it would still be added to the main channel's signal).

Brad's info does make it sound as if the 10dB boost needs to be added after the AVR (or decoder). 
Or is this part of the difference between the pro theater equipment and our home equipment (where the theater equipment decoder doesn't add the boost, but our home AVR/Blu ray player decoder will add the boost?


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## hulkss (Feb 10, 2009)

gdstupak said:


> Brad,
> 
> With a normal system, Audiolense's process is proper. Normally you want a flat response whether you are measuring the redirected bass from the other channels or measuring dedicated LFE signals (because the LFE signal should have already been boosted by the format decoder, you (or Audiolense) should not need to manually boost the LFE signal).
> 
> ...


I am using Jriver Media Center and Audiolense XO only, then straight out with ASIO over USB to a 16 channel DAC and power amplifiers. There is no other format decoder or "home theater" hardware.

Here is a quote from Matt, the JRiver audio developer "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."

Here is a quote from Bernt, the developer of Audiolense: "Audiolense doesn't address how the playback format is played back. It is assumed that the 10 dB reduction (referring to rerouted bass) is handled by the player or some other application."

Hence the problem: Audiolense assumes that someone else is dealing with LFE levels. JRiver MC will only set you up properly if you use their calibration signals and bass rerouting. I am using calibration signals and bass rerouting from Audiolense.

Bernt at Audiolense is currently working on calibration signals and bass rerouting to correct this issue. I started this thread in his forum: Bass Management

As I've cautioned in my first post of this thread when using calibration signals: "It is possible, if you are not careful, to inadvertently remove the gain offset built into home theater equipment."


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