- April 2004 -
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Down the Bottom End
Bass Management attempts to insure that bass is sent to speakers that can handle it, but it is often misunderstood. This article introduces the subject, and points out some pitfalls for the unwary.
In the days of stereo, it was quite common - and practical - to have both speakers not only the same, but with the ability to reproduce the full range of audio frequencies. However, the advent of surround sound, primarily through home theater, has complicated this previously fairly simple area.
For many people, it is just not possible to set up five full-range speakers around the listening position - there just isn't the room in average apartment; very often one's partner will object to the intrusion; and finally, spending the extra money may not be justifiable.
However, active, DSP-controlled digital loudspeakers, such as Meridian's 5000 and 7000 Series, can be much more compact than equivalent conventional passive loudspeakers, because the drivers and their amplifiers are directly connected in the same enclosure, with the crossover operating in the digital domain and thus capable of specifications that are simply impossible with traditional analog designs. As a result, a Meridian DSP loudspeaker can deliver as much bass as a conventional speaker of seven times the volume - which goes a long way to making five, full-range loudspeakers in your living room a practical reality.
But in many cases, we will need to use smaller, conventional speakers, and these are often compromised at the bass end. We might have two main speakers at the front in the traditional stereo positions, and smaller speakers in the center-front and rear positions. Or we might have small speakers all round. In these cases, one or more "sub-woofers" generally provide the bass, and the signals used to drive it (or them) are generated by a system of "Bass Management".
The idea of bass management is simple: Bass frequencies, in whatever channels they occur, are fed only to the loudspeakers that can handle, and reproduce, them. Because our ability to localize low frequencies is limited, it really doesn't matter where the low frequencies come from: larger front speakers or from a sub-woofer in the corner, for example.
When we think of surround sound today, we generally think in terms of "5.1". The "point 1" refers to the additional channel, called the "Low Frequency Effects" or "LFE" channel, that is provided on DVD and similar multichannel discs.
The LFE was designed to handle exactly what its name suggests: low frequency effects such as those encountered on movie soundtracks - Tyrannosaurus Rex footfalls and asteroids crashing into the Earth spring to mind. These loud, super-bass signals were given their own channel for replay in analog movie theaters, to maximize headroom and reduce distortion.
Modern digital distribution and replay systems don't actually need this separation, but the majority of movies released on DVD simply have their soundtracks copied from the original, and as a result they inherit LFE information from these sources.
Thus, when your surround receiver or processor decodes a movie soundtrack, there are not only ordinary bass signals to cope with, but also these special super-bass signals in the LFE channel.
An important point to remember is that the LFE channel from the player is not normally fed simply to the sub-woofer. The LFE channel is not the "sub-woofer" channel. Instead, the LFE, and the bass from all the other channels, is fed to all the speakers that can handle it, including one or more subs if present.
How this is done is usually determined by the configuration in the processor or receiver. For example, you might tell the setup that your front speakers are "large", the surround speakers are "small" and that you have no sub-woofer - in which case all the bass will go to the front speakers and not to the rear. Or maybe you have five "small" speakers and a sub - so the bass, including signals from the LFE, will go to the latter and not to the small speakers that could not cope with it.
But what if you have a high-resolution audio component, such as a DVD-Audio or SACD player? Very likely, to get the highest resolution from your player, you will use its set of six analog outputs to connect to a multichannel analog input on your receiver or processor. And that multichannel input may bypass the bass management we just talked about - check to find out. (Meridian, and a small, but increasing number of other systems, have high-resolution, encrypted digital links from player to processor, but in many cases it will be an analog multichannel link like that described above.)
If the multichannel input bypasses bass management, then when you use that interface, for example to play a DVD-Audio disc, the LFE connection from the player will go straight to the subwoofer (if there is one) and the other channels will have no crossover in the path to send bass where it should go.
In addition, bass may be fed to "small" speakers as well as large ones. At best, they will not reproduce it very well and you will suffer loss of bass. At worst, bass signals can cause damage to small speakers not designed for them.
There may also be nothing on the LFE, so nothing will come out of the sub. This is not a mistake on the part of the studio engineer or producer mixing the recording for surround. If you read a review criticizing the bass of a high-resolution surround recording, lack of bass management on the reviewer's high resolution music system - not bad recording or mixing - is probably the reason.
It's also important to remember that the LFE is for special, super-bass effects. These do not occur in music. Even Telarc's recent all-digital high-resolution recording of Tchaikovsky's 1812 Overture, with real cannons, did not merit putting anything in the LFE. In any event, all the channels of a DVD-Audio disk can handle all frequencies, so there's really no reason to reserve a special channel for bass (instead, Telarc, among others, use it to carry optional height information - rather a good idea!). Many engineers and producers mixing music surround do not use the LFE at all, and this is a perfectly legitimate artistic decision, like deciding not to use the center-front channel in music mixes. They are both artistic options and not requirements.
What is the solution? Ultimately, it is very likely that all connections between player and processor/receiver will be digital and/or all inputs will enjoy the benefits of bass management. Until you have bought your Meridian system, however, you have to do it another way.
First, check to see if your player has bass management. (This is not the best situation, by the way: bass management should really be in the processor or receiver.) If it does, then use it, but then make sure that you use the multichannel analog link for everything you play back using the player - DVD-Video disks, for example, as well as DVD-A - you don't want to accidentally perform bass management twice (once in the player and once in the receiver if, for example, you play back a DVD-Video disk and listen via the normal digital interface).
Another way is to use an outboard bass management system. You can use this to provide bass management solely on your hi-res player's analog multichannel output in the same way as described above. Or alternately, connect it to the output of your processor to provide bass management for your whole system (in this case disable the bass management built into your processor or receiver), but note that you will need access to the preamp outputs and amplifier inputs to do this with a receiver.
As far as the future is concerned, we can hope that manufacturers will make bass management available on all multichannel inputs - something that is already tending to happen. At the same time, we would recommend music engineers and producers to continue to avoid placing any information in the LFE, even though there may be some loss of bass for some listeners whose systems are incorrectly set up.
In the meantime, hopefully these tips will help you get the best results from the bass end of your replay system when you're listening to a high resolution music source.
*Richard Elen is the head of Creative Services at Meridian Audio Ltd in the UK. He is a member of the Music Producers' Guild (UK) and a former editor of Studio Sound magazine. He writes frequently for consumer and professional audio journals on both sides of the Atlantic.