LFE, subwoofers and interconnects explained

KMO · Nov 9, 2006

It's clear that the introduction of HD DVD has prompted many to hook up their HD DVD/BluRay player to their amplifier via a multichannel connection. And this has introduced them to a world of calibration pain caused by the LFE (Low Frequency Effects) channel. Many have previously encountered this with SACD and DVD-Audio, but now a mass of newcomers need guidance. 
So, here's my attempt for a definitive guide to what's what in the exciting world of LFE. Any feedback welcome. Executive summary 
Many current receivers do not handle LFE over HDMI correctly. See the bottom for a list of good and bad receivers. Players are also considered. Read on for the explanation. Introduction 
Initially, I'm only going to consider LFE in a system without bass management. 
The LFE channel was originally found on multi-channel film recording formats, such as Dolby Stereo 70 mm Six Track. The purpose was to make more room for loud explosions, rumbles etc. The normal sound channels were calibrated so they could produce up to about 105dB SPL of output. Pretty loud, but not quite enough for the sort of low-frequency effects that you feel as much as hear. 
So, to increase the impact, they added an extra channel, dedicated to low frequency effects, to be sent to a dedicated subwoofer. And this channel was calibrated to play 10dB louder than the rest - it could output 115dB SPL of energy. And 10dB difference is quite a lot - it means a signal over 3 times the amplitude. Calibration 
This 10dB boost is achieved by calibration in the monitoring amplifiers at the studio; a full-scale LFE signal on the tape is set up to play 10dB louder than a full-scale signal on any other channel. The cost is increased noise - the channel has been turned up, so the general hiss and noise on the magnetic recording will also be 10dB louder. But this is not too much of a problem, as all sounds above 150Hz are filtered out anyway on playback. The channel is only handling low frequencies. 
To reproduce the effect heard in the studio, cinema playback systems are also calibrated the same way - a 10dB boost is added to the LFE channel amplifier. 
Calibration tests generally hide this 10dB difference; a basic pink noise test will contain a signal that should play at 75dB SPL for each channel. For the normal channels, that means a signal 30dB below full scale (105dB-30dB = 75dB), but for the LFE channel it is a signal 40dB below full scale (115dB-40dB = 75dB): 1/3 of the signal amplitude. If those test signals are used to calibrate the amplifier so they all output at 75dB SPL, then everything is set up okay. The LFE channel now has 10dB of extra headroom above the other channels and can produce the full 115dB SPL when required. Interconnects 
One important point is that the 10dB boost to the LFE is performed at the final amplification stage. The potential extra volume of the LFE channel could not pass through interconnects, analogue recordings or digital recordings without going over the specified limits by a factor of 3. This, indeed, is the whole raison d'être of the LFE channel. It's a channel where by convention everything is recorded 10dB low to make room for louder bangs. So at all points in a system - analogue interconnects, PCM signals, encoded Dolby Digital - the LFE channel is "10dB too low". A 75dB SPL signal on the LFE channel will be 3 times, or 10dB, smaller than a 75dB SPL signal on the other channels. It will only sound correct when fed to an amplifier that amplifies LFE by 10dB more than it amplifies the other channels. 
This 10dB difference is dictated by Dolby and DTS, and the international standard ITU-R BR.1384-1:
<div class="quote-container">Quote:
<div class="quote-block"> 
The LFE channel is recorded with a level offset of -10 dB. This offset is compensated for in the reproduction system, where the LFE loudspeaker has an acoustic output (within its low frequency passband) of +10 dB with respect to the other channels.</div>
</div>
 In the home 
The LFE channel progressed from early analogue magnetic formats, to the digital channel in cinematic versions of Dolby Digital, and in turn to the home versions of Dolby Digital, DTS and MPEG Multichannel found on DVDs. 
The rules remain the same: the encoded LFE signal needs a 10dB boost. In receivers with integrated decoders, this is handled automatically in the internals, and the user is generally unaware of it. It is only when we try to connect amplifiers to external decoders that things can go wrong. Many receivers do not offer the necessary boost for LFE coming in from an external decoder. LFE level adjustment 
Some receivers with integrated decoders do offer an adjustment for the LFE level - an option to NOT boost the LFE by 10dB. This is needed for some early music mixes using DTS; the studio mixers, being unaccustomed to 5.1 production, hadn't calibrated their LFE channel 10dB high. Thus the recorded LFE signal on those discs is at equal level with the other channels, and doesn't require boosting. 
The unboosted LFE option on the receiver is often called "LFE -10dB", with the standard boosted option being "LFE 0dB". Alternatively the options might be called "LFE 0dB" and "LFE +10dB". Whatever, the higher option is correct for films and almost all DD and DTS multichannel music. Since those early errors, both Dolby and DTS have specifed that LFE in music releases should be mixed for a 10dB LFE boost to match film soundtracks. However, SACD still differs... more on this below. Bass redirection 
When Dolby Digital and DTS reached the home, there was one important difference. Cinema speakers are MUCH bigger. A normal sound mix will generally contain deep bass in all the front channels, as well as low-frequency effects in the LFE channel. Home speakers usually don't handle deep bass that well, so it is usually advantageous to redirect bass from the main speakers to the subwoofer. Home DD and DTS receivers offer a bass management function to do this. 
With bass management enabled, there is now an important distinction between "LFE" and "Subwoofer" channels. LFE specifically refers to the LFE channel of the original soundtrack. But the subwoofer channel contains both the LFE channel and redirected bass from the other channels. Bass summing 
A very important thing to bear in mind when redirecting bass is the 10dB calibration difference between the LFE and other channels. A combined subwoofer channel must be calculated as: <blockquote>SW = LFE + Lower10dB(Redirected bass)</blockquote>
or equivalently
<blockquote>SW = Raise10dB(LFE) + Redirected bass</blockquote>
but not
<blockquote>SW = LFE + Redirected bass</blockquote>
This keeps the relative level of bass signals originating from the LFE and other channels consistent. If a piece of equipment gets this balance wrong, the error is uncorrectable further down the signal chain. LFE vs SW (subwoofer) labelling 
Equipment usually gets this labelling right. A receiver will have an SW pre-out, and an SW terminal on its multichannel analogue input. Multichannel outputs on players will include an SW terminal. This accurately reflects the fact that in general the player is expected to do bass management, so will be outputting LFE+redirected bass on that channel. And the SW input to the receiver will be passed straight to the SW pre-out - it expects bass management to have already been done. 
LFE interconnects can usually only be found in the digital domain. This reflects the fact that when using digital connections, the receiver is expected to perform bass management. The player passes the LFE channel unaltered, without redirecting any bass. Plain LFE information is carried in DD and DTS bitstreams over SPDIF/TOSLINK, and in PCM or DSD streams over Denon Link, i.Link or HDMI. 
Early DVD-Audio and SACD players did not offer any bass management for PCM or DSD, so in effect they only offered an analogue LFE out. This meant they could not be easily integrated into a conventional system with a receiver that only had an SW input, and no bass management for multichannel input. This lead to a market for separate bass management devices, such as the Outlaw ICBM. 
Current players now generally offer bass management for all formats on their multichannel outputs. SW analogue output from players 
Okay, what level should the subwoofer output be? If it were just the LFE track, then you'd expect it, as discussed above, to be 10dB lower than the other channels to fit the 10dB higher maximum LFE output. 
But if the player is performing bass management, then the subwoofer output also has to contain redirected bass; this extra signal could push the signal back above its nominal limit. To prevent overloading their output, or the receiver's input, players with bass management conventionally lower the SW output by a further 5dB. (This 5dB value is specified by Dolby, see references). Sometimes this 5dB drop switches in and out depending on whether bass management is being performed (ie if any speakers are set to SMALL). 
The final SW output will typically be: <blockquote>SW = LFE</blockquote>
or: <blockquote>SW = Lower5dB(LFE + Lower10dB(Redirected bass)) = Lower5dB(LFE) + Lower15dB(Redirected bass)</blockquote>
So, in those two cases the SW output from the player will be respectively 10dB or 15dB lower than the other channels. A receiver should offer the ability to specifically boost the SW input on its multichannel analogue interface to compensate. LFE digital output from players 
When outputting a Dolby Digital or DTS bitstream from a player, the LFE channel remains encoded in the bitstream, untouched by the player. The receiver's decoder has the responsibility to boost the LFE by 10dB, and almost invariably they will get this right. DD and DTS remain the most foolproof way of transmitting LFE. 
When outputting a PCM multichannel bitstream over Denon Link, i.Link, HDMI or similar, the LFE channel will be transmitted "as-is", and will need boosting by 10dB in the receiver. The receiver should do this automatically. If it fails to do this before performing bass management, then you will be unable to calibrate the system correctly - boosting the subwoofer to compensate will have the side effect of over-emphasising bass from other channels. "DVD-Audio/HD DVD/Blu-ray's bass is too quiet" 
No. They're mixed exactly the same way as Dolby Digital or DTS on DVD-Video. The LFE track is recorded 10dB low. People are only noticing a problem because they've switched from a DD/DTS bitstream link which works to a multichannel interconnect lacking the necessary 10dB-15dB boost. If they had been listening to DD or DTS decoded in the player through the multichannel interconnect they'd have seen the same problem. And the problem is that their receiver isn't boosting its SW/LFE input sufficiently. 
It is not really an option for the player to boost its analogue SW output, as it would be in danger of overloading a receiver's input circuitry when a maximum volume LFE signal appeared - feeding a 6 volt signal into a nominally 2 volt input. You might get away with it if the amplifier was purely doing an analogue passthrough, but it would overload any receiver with multichannel ADCs. 
And similarly the player absolutely cannot boost its digital LFE output. There's no headroom to do this. "Super Audio CD's bass is too quiet" 
Same basic answer as the previous section, except for one wrinkle: SACD doesn't actually use a 10dB boost for its LFE channel (which poses the question - why have it at all?) 
To maintain compatibility, some multi-format SACD players apparently lower the LFE internally by 10dB, then carry on the rest of their processing as normal. This then leads to the output: <blockquote>SW = Lower10dB(music LFE)</blockquote>
or <blockquote>SW = Lower15dB(music LFE + Redirected bass)</blockquote>
So the net boost of 10dB or 15dB is still needed in the receiver to achieve correct playback, consistent with other formats. 
Some players, usually SACD-only, do not do this 10dB adjustment, leading to the output <blockquote>SW = music LFE</blockquote>
or <blockquote>SW = Lower5dB(music LFE + Redirected bass)</blockquote>
This could arguably result in better quality, by making better use of the range on that input, but means the receiver has to have its SW input switched to +0dB or +5dB respectively just for SACD with that player - inconvenient if it's multiformat. SACD over a digital interconnect 
This is unclear, and will depend on the digital interconnect type. Normally one would expect digital output to be "raw". But what happens when the raw LFE doesn't need a boost, because it's come from SACD? 
The full connection standards aren't publically available (eg CEA-861-D for HDMI). Is there a standard for LFE level over these connections? If there is, then the standard would be that LFE over digital interconnects should always need a +10dB boost at the receiver (as per ITU-R BS.775-2 and ITU-R BR.1384-1). Then any player outputting SACD data would have to attenuate its LFE by 10dB so the receiver could still apply a 10dB boost, as it does for DD+/DTS HD sourced data. 
If there's no standard, or players vary, then a receiver may need an option to disable the 10dB boost. To be on the safe side, I'd recommend that receivers offer this switch, maybe by tying it to the "0dB/-10dB" setting for its internal DD and DTS decode. Summary of requirements 
In summary, here is what should be happening with each interconnect type: Analogue 2-channel
<ul><li>Player should drop LFE channel and downmix other channels to 2-channel</li>
<li>Receiver should perform bass management</li>
</ul>Analogue multichannel
<ul><li>Player should perform bass management</li>
<li>Player's SW output should be LFE (if no bass management), or Lower5dB(LFE) + Lower15dB(Redirected bass).</li>
<li>Player MUST take 10dB difference into account when redirecting bass. If wrong, the error is uncorrectable in the receiver.</li>
<li>Player's SW output will be 10dB or 15dB low (possibly 0dB or 5dB for some awkward SACD players).</li>
<li>Receiver should offer options to boost SW input by 0-15dB (a dedicated configurable setting for multichannel input).</li>
</ul>Digital bitstream
<ul><li>Player passes DD or DTS bitstream to receiver unmodified.</li>
<li>Receiver decodes, boosts LFE by 10dB and performs bass management.</li>
<li>Receiver may offer option to not boost LFE (to cater for some early DTS music tracks).</li>
</ul>Digital multichannel
<ul><li>Player decodes, and passes all channels to receiver as-is (but ideally SACD would be aligned with a 10dB LFE attenuation).</li>
<li>Player's LFE output will be 10dB low, (except maybe SACD).</li>
<li>Receiver must boost LFE by 10dB and perform bass management.</li>
<li>For SACD use only, receiver may offer option not to boost LFE, for players which don't attenuate LFE output for those formats.</li>
</ul> References
<ul><li><a href="http://www.hometheaterhifi.com/volume_7_2/feature-article-misunderstood-lfe-channel-april-2000.html" target="_blank">The Misunderstood 0.1 LFE Channel in 5.1 Digital Surround Sound</a> - good article from Secrets of Home Theater & High Fidelity</li>
<li><a href="http://www.dolby.com/assets/pdf/tech_library/L.mn.0002.5.1guide.pdf" target="_blank">Dolby 5.1-Channel Production Guidelines</a> (discusses the LFE channel and calibration)</li>
<li><a href="http://www.dolby.com/assets/pdf/tech_library/46_DDEncodingGuidelines.pdf" target="_blank">Dolby Digital Professional Encoding Guidelines</a> (figure 3.2 illustrates simple bass management, and the 15dB gain)</li>
<li>ITU-R BS.775-2: Multichannel stereophonic sound system with and without accompanying picture</li>
<li>ITU-R BS.1384-1: Parameters for international exchange of multi-channel sound recordings with or without accompanying picture</li>
<li>CEA-861-D: A DTV Profile for Uncompressed High Speed Digital Interfaces</li>
</ul> Known equipment behaviour 
We can collect known equipment behaviour here. Please post your experiences, and I'll incorporate. Receivers offering dedicated 0-15dB analogue SW gain
<ul><li>Denon AVR-2307CI, AVR-2807, AVR-3806, AVR-4306, AVR-4806(AVC-A11XV), AVR-4806CI(AVC-A11XVA): 0,5,10,15dB steps</li>
<li>Onkyo TX-SR604, TX-SR674, TX-SR702, TX-SR804E, TX-NR1000, TX-NR5000E: 0,5,10,15dB steps</li>
</ul>Receivers offering dedicated analogue SW gain less than 15dB
<ul><li>Pioneer VSX-84TXSi/VSX-82TXSi(VSX-AX4ASi/VSX-AX2ASi): 0 or +10dB selectable</li>
<li>Pioneer VSX-74TXVi/VSX-72TXVi(VSX-AX4AVi/VSX-AX2AVi): 0 or +10dB selectable</li>
<li>Sony STR-DA5200ES: 0 or +10dB selectable</li>
<li>Marantz SR7001/SR8001: offers completely separate speaker level trims for 7.1 analogue input; SW -18dB to +12dB, others -12dB to +12dB</li>
</ul>Receivers with no separate analogue SW gain
<ul><li>Marantz SR7200</li>
</ul>Receivers boosting PCM LFE input by 10dB
<ul><li>Onkyo TX-NR1000, TX-NR5000E: can set +10dB, 0dB, -10dB, off; separate settings for PCM, DSD, DD, DTS and AAC</li>
<li>Onkyo TX-SR804E: can set +10dB, 0dB, -10dB, off; unified setting for PCM, DD and DTS</li>
<li>Denon AVR-2307CI, AVR-2807, AVR-3806, AVR-4306, AVR-4806CI(AVC-A11XVA): can set +10dB or 0dB; per-surround mode setting(?) - should allow separate settings for PCM, DSD, DD and DTS</li>
<li>Panasonic SA-XR700: no option, but appears to get it right</li>
<li>Yamaha RX-V1700/RX-V2700: No option. Latest tests suggest it is okay.</li>
<li>Denon AVR-4806(AVC-A11XV): LFE control only affects DD and DTS. One owner reports that PCM/DSD LFE gain is undocumentedly linked to analogue SW Level setting; another owner reports that this doesn't work for him</li>
<li>Pioneer VSX-84TXSi/VSX-82TXSi(VSX-AX4ASi/VSX-AX2ASi): latest USA firmware apparently fixes - with this, +10dB analogue SW boost also affects HDMI+ i.Link. New units may not have this firmware. Fix not yet available in Europe. Status of i.Link SACD after fix remains unknown.</li>
<li>Anthem Statement D2 and Anthem AVM-50: firmware versions 1.10 and earlier lack the +10dB boost; version 1.11 fixes.</li>
</ul>Receivers not boosting PCM LFE input by 10dB
<ul><li>Pioneer VSX-74TXVi (VSX-AX4AVi): no firmware fix, as far as I know</li>
</ul>Other odd receiver behaviour
<ul><li>Onkyo SR-804E: can't apply EX/PLIIx/etc to multichannel PCM input</li>
<li>Sony STR-DA5200ES: can't apply EX/PLIIx/etc to multichannel PCM input</li>
<li>Panasonic SA-XR700: can't apply EX/PLIIx/etc to multichannel PCM input</li>
<li>Panasonic SA-XR57: can't bass manage multichannel PCM input - avoid</li>
</ul>Players correctly summing bass and LFE Players incorrectly summing bass and LFE
<ul><li>Samsung DVD-HD850 - gets SACD wrong; sums as if it were DVD-Audio</li>
</ul>Players matching SACD analogue playback with other formats by 10dB SW drop
<ul><li>Denon DVD-2930 (except in SOURCE DIRECT mode)</li>
</ul>Players not matching SACD analogue playback with other formats
<ul><li>Samsung DVD-HD850, DVD-HD950</li>
</ul> Other odd player behaviour
<ul><li>Denon DVD-2930: Can bass manage and level adjust HDMI output</li>
<li>Oppo 970HD: Can bass manage and level adjust HDMI output; unfortunately this means LFE is 15dB low to make room, even with bass management off, but this is apparently fixed in latest beta firmware</li>
</ul>

Schwingding · Nov 9, 2006

That was GREAT. Wish it were available when I was learning all of that stuff. The only criticism I might have of it is that were I not previously aware of this issue, I might still be confused. There is a lot of great, very easy to understand info here, but there is a LOT of info. 
Nice work.

trekguy · Nov 9, 2006

Onkyo TX-SR702 options. 
Multichannel Analog-- 0dB (default), +5, +10 or +15dB. Manual says if too loud (sic) set to 0, +5, +10 or +15. 
DTS-- -infinitedB, -10db0dB (default), 0dB (default). Manual says if too loud set to -infinitydB or -10dB. 
Dolby Digital-- -infinitedB, -10db0dB (default), 0dB (default). Manual says if too loud set at -infinitydB or -10dB. 
I don't know how to get the other information, or when THX Select settings are used what changes occur.

Ktulu_1 · Nov 9, 2006

If I'm reading this right, this explains why a HD-DVD player connected to a Pioneer VSX-84TXSi via HDMI runs the LFE channel low. It's receiving raw PCM and doesn't apply the 10db boast to the LFE channel like it would on a DD or DTS bitstream.

KMO · Nov 9, 2006

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by trekguy <a href="/forum/post/0"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style=""></a> 
Onkyo TX-SR702 LFE options. 
Multichannel Analog-- 0dB (default), +5, +10 or +15dB. Manual says if too loud set to 0, +5, +10 or +15.</div>
</div>
 
Thanks, that's good info. And a good receiver by the sound of it. But in this one case, don't you mean too quiet? And to keep the LFE/SW terminology accurate, presumably that's SW rather than LFE for that one.

KMO · Nov 9, 2006

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by Ktulu_1 <a href="/forum/post/0"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style=""></a> 
If I'm reading this right, this explains why a HD-DVD player connected to a Pioneer VSX-84TXSi via HDMI runs the LFE channel low. It's receiving raw PCM and doesn't apply the 10db boast to the LFE channel like it would on a DD or DTS bitstream.</div>
</div>
 
That's what I've gathered from other posts here. Don't have one myself, but it's on my shortlist (albeit a bit further down now if that 10dB boost is missing...) Although I understand there may be worse problems in that the channel assignments for multichannel-PCM bass redirection may be scrambled in some way. Not sure of the details.

trekguy · Nov 9, 2006

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by KMO <a href="/forum/post/0"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style=""></a> 
Thanks, that's good info. And a good receiver by the sound of it. But in this one case, don't you mean too quiet? And to keep the LFE/SW terminology accurate, presumably that's SW rather than LFE for that one.</div>
</div>
 
Sharp eyes! No, it says "too loud" although that does seem incorrect. You are correct LFE was my error, it should be SW. 
OP now corrected.

CAVX · Nov 11, 2006

Great thread KMO, well written <img alt="" class="inlineimg" src="http://www.avsforum.com/avs-vb/images/smilies/smile.gif" style="border:0px solid;" title="Smilie"> Any modds out there, please make this thread into a sticky... 
Mark

bwclark · Nov 16, 2006

KMO, 
According to this thread, the Sony STR-DA5200ES has a +10db for the SW: <a href="http://www.avsforum.com/avs-vb/showthread.php?p=8913999#post8913999" target="_blank">http://www.avsforum.com/avs-vb/showt...99#post8913999</a>

KMO · Nov 16, 2006

Thanks! Actually, I already had the manual for that, so I really should have added it myself...

soundlovr · Nov 17, 2006

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by KMO <a href="/forum/post/0"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style=""></a> 
And 10dB difference is quite a lot - it means a signal over 3 times the amplitude.</div>
</div>
 
Amplitude is a tough term to use there... I'm not sure it fits in relation to a logarithmic scale. But if you're talking about the perceived loudness, then the human ear perceives a doubling of loudness for approximately every 10dB increase, not 3 times, nor over 3 times.

jamin · Nov 17, 2006

soundlovr - good point regarding our basic SPL sensitvity. You may not realize that KMO has been translating to eqivalent voltage waveforms or digital numerics --- as in sqrt(10) = 3.16. But hey it is still a 10x power change!

krabapple · Nov 17, 2006

what's a good test to determine if one's avr is applying the 10dB boost to LFE transmitted via a digital input?

KMO · Nov 17, 2006

Get a player with a built in Dolby Digital decoder, and some Dolby Digital test tones (eg Avia). Note that on Avia, the subwoofer calibration tone does not use LFE. Only the LFE frequency sweep tone does - use that. 
Ensure the receivers "LFE level" setting for its own DD decoder is set appropriately - if it offers "-10dB" and "0dB" you want "0dB". If it offers "0dB" and "+10dB" you want "+10dB". Otherwise, check the manual, but it's probably the highest option that you want. 
Switch between getting the player to decode DD and pass PCM to the receiver, and passing the DD bitstream up to the receiver for it to decode. 
You'll probably see an absolute level difference on all channels in the two cases, due to dialogue normalisation, or other subtleties - maybe 4-5dB. If you see a bigger discrepancy (>=10dB), the player's doing something odd, or you've got some level trims set up that affect the HDMI output (set them all to 0dB). Figure out what that difference is. 
The LFE tone should change by the same amount. If the receiver isn't boosting LFE from PCM, but is when doing its own DD decode, then the LFE in the PCM case will be 10dB lower relative to the other channels. 
It may be that the receiver has some setting to adjust LFE level over HDMI PCM/DSD. It may be unified with the level for its own DD decoder (as in the Denon AVR-3806), or it may have a separate setting (as in the Onkyo NR-5000E). Conceivably it may even be unified with the multichannel analogue input SW level. 
Try whatever settings you can in the receiver to adjust it. But note that boosting the subwoofer speaker level is not a solution if you've got any bass processing happening, as that will also boost the redirected bass by 10dB. The receiver has to understand and get the LFE level right before it redirects bass (see the maths in the main article). 
An LFE control setting in the receiver is desirable because it may be the only place to deal with SACD or old DTS music discs that don't need an LFE boost, if the player doesn't offer its own controls, or automatically lower LFE output for SACD to match. 
If the player has level trims that affect HDMI output, then you can compensate in the player (as long as the player isn't bass redirecting). Set all level trims other than LFE to -10dB, and LFE to 0dB.

KMO · Nov 17, 2006

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by jamin <a href="/forum/post/0"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style=""></a> 
soundlovr - good point regarding our basic SPL sensitvity. You may not realize that KMO has been translating to eqivalent voltage waveforms or digital numerics --- as in sqrt(10) = 3.16. But hey it is still a 10x power change!</div>
</div>
 
Yes, that's my point - I'm highlighting the issue of squeezing this loud LFE signal through a nominally 2V analogue connector or a PCM signal with a hard fixed scale. Try to imagine what happens if you try to increase the LFE level by 10dB to match the other channels. You're fine with a nice quiet 75dB or 85dB test tone, but what happens when you get a full-scale 115dB LFE rumble? 
Here's a quick table highlighting the point. The maths may not be precise, but it's illustrative: 
For a normal channel: 
75dB SPL = 0.07V, or 1/27 scale digital signal 
85dB SPL = 0.2V, or 1/9 scale digital signal 
95dB SPL = 0.7V, or 1/3 scale digital signal 
105dB SPL = 2V (maximum), or full scale digital signal 
For LFE channel: 
75dB SPL = 0.02V, or 1/81 scale digital signal 
85dB SPL = 0.07V, or 1/27 scale digital signal 
95dB SPL = 0.2V, or 1/9 scale digital signal 
105dB SPL = 0.7V, or 1/3 scale digital signal 
115dB SPL = 2V (maximum), or full scale digital signal 
So for any given intended volume, the LFE waveform will be 3 times smaller than the other channels. You could try to adjust trims on the output device to make it match the other channels, and that would be fine while working with nice gentle 75dB or 85dB test tones, or when working with content that never uses the extra range of the channel. 
But when the LFE wants to output 115dB, by turning it up you've just generated 6V and overloaded the receiver's input, or massively clipped the digital waveform with a 3-times overload. 
That's why the LFE is recorded and transmitted down 10dB. The amplifier at the end is calibrated to turn it up an extra 10dB, so it can push out 10dB more than the other channels. 
But that extra 10dB can't be applied until you've reached a point where there's room for the extra signal. And there isn't room over any digital PCM link (unless you're actually turning down all the other channels by 10dB instead), and it's something to be wary of over an analogue link, unless you know the receiving device can handle 6V input okay. (Any device that digitises the input is likely to suffer; an analogue device like a power amplifier or powered subwoofer may well cope okay).

krabapple · Nov 19, 2006

For the Pioneer 74TXvi, which I only ever use with digital connections (ilink or HDMI), I can report that it has a dedicated variable SW channel level adjustment from -10 dB to +10 dB which affects digital input and, I presume, multichannel analog input too. Given the performance of the 80TX series, I would be surprised if the 74txvi correctly applied automatic +10 dB boost to LFE data over the digital input, though I still haven't formally checked it. 
Given the sophistication of AVRs these days, it's sad that they still don't tend to correctly process bass for all formats.

KMO · Nov 19, 2006

I think you're referring to the global subwoofer output trim? What I'm looking for is a separate setting that adjusts the multichannel analogue subwoofer input specifically, to make sure you can give it the special 10-15dB boost it needs, without messing up the other sources. That could either be a special setting, or offering a complete set of different speaker trims for the multichannel input. 
Yes, I'm amazed they can't get this sort of basic stuff right, given how complex so much of the stuff they do is.

krabapple · Nov 20, 2006

Right, I wasn't so much answering your query as just adding to the 'devices' info database. There is no dedicated 'LFE' or MC input level setting..it's all down to the global channel level settings. [Edit: this refers only to digital input, not analog]

KMO · Nov 20, 2006

Hi krabapple; that sounded a bit odd to me, so I double checked. It actually does have a +10dB boost option for the multichannel analogue SW, like the 84, at least according to the manual for the European version <a href="http://www.service.pioneer-eur.com/peeservice/instrmanuals.nsf/0/18F62CD32737E1BAC12570A7003221BE/$File/ARB7335_VSX-AX4AVI-S.pdf" target="_blank">here</a> (page 51). 
But bizarrely, like the 84, it's a totally hidden feature, so you wouldn't find it unless you happened to read that page of the manual.

jacksonian · Nov 20, 2006

KMO, 
I'm growing increasingly tired of trying to figure this out with my 84TX. I compared the DD from an SD DVD last night to the PCM 5.1 Blu Ray version last night of Black Hawk Down and the PCM was significantly weaker in the LFE. 
I'm about ready to jump ship unless you think there's a way to make this work. I don't really listen to music on this system that often, mostly movies. Would you recommend the Denon 3806 as one that handles this issue correctly? And if so, do you want a great deal on a barely used 84TX? <img alt="" class="inlineimg" src="http://www.avsforum.com/avs-vb/images/smilies/biggrin.gif" style="border:0px solid;" title="Big Grin">

LFE, subwoofers and interconnects explained

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