So, here's my attempt for a definitive guide to what's what in the exciting world of LFE. Any feedback welcome.
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.
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.
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.
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:
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.
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.
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.
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:
SW = LFE + Lower10dB(Redirected bass)
SW = Raise10dB(LFE) + Redirected bass
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.
SW = LFE + Redirected bass
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:
SW = LFE
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.
SW = Lower5dB(LFE + Lower10dB(Redirected bass)) = Lower5dB(LFE) + Lower15dB(Redirected bass)
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:
SW = Lower10dB(music LFE)
So the net boost of 10dB or 15dB is still needed in the receiver to achieve correct playback, consistent with other formats.
SW = Lower15dB(music LFE + Redirected bass)
Some players, usually SACD-only, do not do this 10dB adjustment, leading to the output
SW = music LFE
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.
SW = Lower5dB(music LFE + Redirected bass)
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:
- Player should drop LFE channel and downmix other channels to 2-channel
- Receiver should perform bass management
- Player should perform bass management
- Player's SW output should be LFE (if no bass management), or Lower5dB(LFE) + Lower15dB(Redirected bass).
- Player MUST take 10dB difference into account when redirecting bass. If wrong, the error is uncorrectable in the receiver.
- Player's SW output will be 10dB or 15dB low (possibly 0dB or 5dB for some awkward SACD players).
- Receiver should offer options to boost SW input by 0-15dB (a dedicated configurable setting for multichannel input).
- Player passes DD or DTS bitstream to receiver unmodified.
- Receiver decodes, boosts LFE by 10dB and performs bass management.
- Receiver may offer option to not boost LFE (to cater for some early DTS music tracks).
- Player decodes, and passes all channels to receiver as-is (but ideally SACD would be aligned with a 10dB LFE attenuation).
- Player's LFE output will be 10dB low, (except maybe SACD).
- Receiver must boost LFE by 10dB and perform bass management.
- For SACD use only, receiver may offer option not to boost LFE, for players which don't attenuate LFE output for those formats.
- The Misunderstood 0.1 LFE Channel in 5.1 Digital Surround Sound - good article from Secrets of Home Theater & High Fidelity
- Dolby 5.1-Channel Production Guidelines (discusses the LFE channel and calibration)
- Dolby Digital Professional Encoding Guidelines (figure 3.2 illustrates simple bass management, and the 15dB gain)
- ITU-R BS.775-2: Multichannel stereophonic sound system with and without accompanying picture
- ITU-R BS.1384-1: Parameters for international exchange of multi-channel sound recordings with or without accompanying picture
- CEA-861-D: A DTV Profile for Uncompressed High Speed Digital Interfaces
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
- Denon AVR-2307CI, AVR-2807, AVR-3806, AVR-4306, AVR-4806(AVC-A11XV), AVR-4806CI(AVC-A11XVA): 0,5,10,15dB steps
- Onkyo TX-SR604, TX-SR674, TX-SR702, TX-SR804E, TX-NR1000, TX-NR5000E: 0,5,10,15dB steps
- Pioneer VSX-84TXSi/VSX-82TXSi(VSX-AX4ASi/VSX-AX2ASi): 0 or +10dB selectable
- Pioneer VSX-74TXVi/VSX-72TXVi(VSX-AX4AVi/VSX-AX2AVi): 0 or +10dB selectable
- Sony STR-DA5200ES: 0 or +10dB selectable
- Marantz SR7001/SR8001: offers completely separate speaker level trims for 7.1 analogue input; SW -18dB to +12dB, others -12dB to +12dB
- Marantz SR7200
- Onkyo TX-NR1000, TX-NR5000E: can set +10dB, 0dB, -10dB, off; separate settings for PCM, DSD, DD, DTS and AAC
- Onkyo TX-SR804E: can set +10dB, 0dB, -10dB, off; unified setting for PCM, DD and DTS
- 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
- Panasonic SA-XR700: no option, but appears to get it right
- Yamaha RX-V1700/RX-V2700: No option. Latest tests suggest it is okay.
- 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
- 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.
- Anthem Statement D2 and Anthem AVM-50: firmware versions 1.10 and earlier lack the +10dB boost; version 1.11 fixes.
- Pioneer VSX-74TXVi (VSX-AX4AVi): no firmware fix, as far as I know
- Onkyo SR-804E: can't apply EX/PLIIx/etc to multichannel PCM input
- Sony STR-DA5200ES: can't apply EX/PLIIx/etc to multichannel PCM input
- Panasonic SA-XR700: can't apply EX/PLIIx/etc to multichannel PCM input
- Panasonic SA-XR57: can't bass manage multichannel PCM input - avoid
Players incorrectly summing bass and LFE
- Samsung DVD-HD850 - gets SACD wrong; sums as if it were DVD-Audio
- Denon DVD-2930 (except in SOURCE DIRECT mode)
- Samsung DVD-HD850, DVD-HD950
Other odd player behaviour
- Denon DVD-2930: Can bass manage and level adjust HDMI output
- 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