Bass Management and the LFE Channel Page 2
The function of the limiter is to prevent the controller from outputting a voltage level that would overload the main speakers or subwoofers that are connected to it (see Fig. 5). Limiters are typically provided for the subwoofer output, but some controllers also include limiters for the main speakers in the event that there is no subwoofer and the LFE channel and bass from small main speakers is routed to other main speakers.
Limiters are generally set by listening to a bass test noise generated internally by the AV controller, increasing the volume of the test noise until distortion becomes audible, and entering into the controller the level where this distortion begins. From there, the controller engages the limiter to ensure that its output never exceeds the level where the speakers begin distorting.
Myths and Confusion
Now that we understand a little bit about bass management, it's time to dispel a few myths and examine common areas of confusion that relate to bass management, the LFE channel, and bass reproduction.
1. Low bass is NOT directional. If I had a penny for every time someone has told me they can hear bass directionality down to 20 Hz, I would be writing this article while flying to the French Riviera in my private jet. Yes, we can hear the overtones of bass instruments above 120 Hz, and those overtones should definitely be played by the main speakers correctly located in the room for proper imaging. However, we cannot-I repeat cannot-localize bass below about 80 Hz.
The most important thing we can do with non-directional bass frequencies is to produce them in locations in a room that don't favor strong coupling with standing-wave resonances. These resonances turn a bass punch into an event that takes two seconds to decay-that's bad, that's mushy, that's slow. Attempting to play stereo bass in a listening room without regard to 2-second-long bass resonances, which completely swamp any sense of separation, is pure madness! Of course, that's only my theory; others can draw their own conclusions. This is largely a free world despite what some may want us to think!
2. Full range speakers are NOT an excuse for eliminating bass management. In order to render bass management unnecessary, each full range speaker must have a 15-inch woofer (or dual 12-inch woofers) with 200 watts of power driving it. Anything smaller will simply overload on loud scenes. Also realize that each one of these behemoth speakers must be carefully placed so its interaction with the bass resonances of the room is optimized. Believe me, it's darned near impossible to place seven speakers in a room so that each speaker identically loads bass resonances. The result of improper loading is grossly uneven bass. Some speakers are cranking out thick and inarticulate bass, while others seem downright anemic.
The right way to handle this dual problem is to implement bass management and strategically place subwoofers in the room for optimized resonance loading. Research has shown that some locations in a room yield predictably good results after all is said, done, and installed. One of the best layouts is four subwoofers in a cross pattern (see Fig. 6).
3. The LFE channel is NOT a "subwoofer channel." The LFE signal should be thought of as a path for super-loud bass that would otherwise overload the main channels. Sound designers use this path when the main channels just can't put out enough bass to rock the house. Remember that in movie theaters, the volume control is fixed at a reference level. At that volume setting, the peak sound pressure level from the recorded medium should be 105dB in the listening area. In the mid-frequency range, 105dB is good and loud, but in the deep bass region, it just isn't enough to get the impact we all expect from big A-list titles with he-man characters wielding limitless firepower.
To get real chest-pounding bass, we need to get up to 115dB. The main channels are missing 10dB of headroom, and that's where the LFE channel comes to the rescue. With 10dB of extra headroom, it can really get a person's body bouncing around in the seat. LFE is only used during high-octane action with lots of bass; during the rest of a movie, the LFE channel has no content. The LFE channel may be fed directly to subwoofers in most systems, but there's no directive that it must be. An exceedingly large home theater (dimensions greater than 40 feet) with massive main speakers could theoretically run the LFE channel to those speakers and have no subwoofers at all!
4. There should be ONE, only ONE, and nothing but ONE audio connection between an AV controller and powered subwoofers. Some AV controllers offer an LFE-only output in addition to the subwoofer output, and some subwoofers offer multiple line-level inputs. Controllers with both LFE-only outputs and subwoofer outputs may tempt us to connect a separate subwoofer to each output and run one subwoofer for the LFE channel and another one for summed main-channel bass. This is an exceptionally bad idea in most cases, because we need both subwoofers playing the same thing, working together to cancel bass resonances. The most effective use of two subwoofers is to have them play the sum of the bass from the main channels and the occasional LFE hit. Bass character stays consistent throughout the movie, and the subwoofers are used to their full potential.
This brings us to subwoofers with multiple line-level inputs. The concept of a subwoofer with multiple inputs that have various filtering and summing functions makes no sense in light of our discussion of bass management. In a multichannel audio system with bass management, a subwoofer needs one input, a polarity switch, a power switch, and that's all folks! Those other things-diverse inputs with misleading names, a stereo summing input, a lowpass filter, a volume control, etc.-are only useful in a stereo system without bass management and just add to the cost and complexity of a subwoofer. It is the job of a custom installer or end users to search out the right input for the bass-managed feed from a controller, and that's not always an easy task.
( I have run into situations in which the gain of the subwoofer is such that the bass level setting in the preamp-processor or AV receiver has inadequate range on its own to properly set the subwoofer level. Because of that, I personally prefer to have a level control on a subwoofer.—TJN)
5. There are NO clear rules governing the bandwidth of the LFE channel. The production statutes applied to LFE channels are so varied that determining the upper LFE cutoff frequency for a playback system is often an exercise in futility. Some LFE channels contain no content above 50Hz, while others (usually due to an error in mastering) are full range! The generally accepted safe approach on the playback end is to lowpass-filter the LFE channel at 80 Hz. However, some AV controllers do not apply a lowpass filter to the LFE channel at all, meaning that highly directional bass on some recordings could potentially be produced by the subwoofers. Beware of these controllers, as there is usually no way to add an external lowpass filter without serious repercussions to the main-channel bass.
6. The ratio of the LFE level relative to the level of the bass from main channels should NOT be adjusted in the AV controller. The ratio should be such that LFE signals are 10dB louder than signals of equivalent level in any other channel. A few early DTS music releases contained LFE channels that were 10dB louder than the industry standard. For this reason, some controllers include a DTS music mode that reduces the LFE channel by 10dB. There is no other logical reason to adjust the level of the LFE channel separately from the main channel bass. Doing so irreparably alters the mix intended by the sound engineer.
7. There is NO NEED for an LFE channel in the vast majority of music applications. There continue to be multichannel music recordings released with content in the LFE channel when the bass in the main channels isn't even close to overload. Inexplicably, some music-recording engineers think that they must put something into the LFE channel so that end users will hear sound coming from their subwoofers.
Frankly, that's terrible logic because the subwoofers in bass-managed systems (which represent the overwhelming majority) receive the LFE channel and the sum of the main-channel bass. Users and installers of multichannel systems don't really need to worry about a music recording with an LFE channel as long as they set up their systems correctly with bass management. At times, however, bass-managed playback systems dig up bass that recording engineers didn't hear because their monitoring systems weren't bass-managed and their monitor speakers weren't full-range. This unmonitored bass sounds ultra-funky, and there's absolutely nothing users can do about it without reconfiguring their systems every time they switch discs. It's time for us to lodge some complaints with the production community!
8. Analog signals from a DVD-Audio or SACD player that are input to an AV controller through its multichannel analog inputs may NOT be bass managed inside the AV controller. Unlike a digital input, where the incoming signal is run through bass management (which is always performed in the digital domain) before it is converted to analog, an analog input must feed its signals through the controller's analog-to-digital (A/D) converters before they can be sent to bass management. A/D conversion might be okay for some analog signals, but not those from high-resolution formats like Packed PCM (PPCM) or Direct Stream Digital (DSD).
Thus, most controllers do not convert their multichannel analog inputs to digital, or run them through bass management. This task is left to the disc player, most of which don't have enough DSP horsepower to provide filters as advanced as those in controllers. In fact, some players do not even provide the option of setting all the main speakers to Small-the front speakers are often restricted to Large. Furthermore, the digital-to-analog (D/A) converters on disc players' subwoofer outputs often do not have enough headroom to output bass at sufficiently high levels. The level of the subwoofer output is usually reduced by either 10dB (for a player without bass management) or 15dB (for a player with bass management), with the expectation that the signal will be boosted by the controller. This is all well and good except that many controllers are not set up to apply that boost to their analog subwoofer inputs!
The current solution to both of these problems is to set all the outputs of a disc player to Large and insert an external bass-management device between the player and the controller. (Bass-management devices are available from Outlaw Audio and Miller & Kreisel, among others.) It's not a very appetizing thought, but it's a heck of a lot better than lacking highpass filters for the front speakers or having bass that's 15dB too low!
Fortunately, there is another solution on the horizon. Universal digital interfaces, like FireWire, that can carry PPCM and DSD are appearing on the latest crop of disc players and AV controllers. If the external bass-management box is a turn-off, reach for these new players and controllers, which bass management for PPCM and DSD works just like it does for other digital-audio formats.
A number of thoughts, ideas, and schemes relating to bass reproduction are floating around these days. Some of them make quite good sense on the surface, and others actually work well in certain respects. Taken as a whole, however, nothing can top bass management as a means of reproducing the loudest, cleanest, tightest bass possible in a home theater or listening room. In a way, it's sad that the fully developed concept of bass management has yet to appear in products, but we should all be grateful for the things that have. For many of us, it's the only hope we have of adding the right amount of salt to our delicious listening experience.
Anthony Grimani is president of Performance Media Industries, a California-based acoustical engineering firm specializing in home theater design and calibration.
Chase Walton, Technical Editor, contributed to this article