Bass Management

An important aspect of a Dolby Digital decoder, speaker system and soundcard is bass management. In general, what we mean by bass management is not how high you turn up your bass but how the bass is sent to a system is distributed (or not distributed) between the satellite speakers and a subwoofer. This can be important if you are using small speakers that can’t generate much, if any, bass on their own. Systems that lack their own bass management systems (such as an internal crossover to a subwoofer) require the outputting device to handle the bass management. The actual complexity of bass management systems on outputting devices can vary from simple full frequency redirects where no filtering from the main channels occur to customizable crossover points with actual filtering of bass from the main channels.

You may often here the terms "large speaker mode" and "small speaker mode" used when it comes to bass management. Small speaker mode refers to when bass is filtered, or at least duplicated, from the satellites and sent to the subwoofer input while in large speaker mode only the LFE channel (see separate article) is sent to the subwoofer input.

If you use a 4-speaker system which lacks a subwoofer input and depends on the software DVD player (or in case of the Live and Audigy) soundcard to do the decoding you don’t need to worry about bass management. However if you use an external hardware decoder with a 4-speaker system you need to be aware that many external hardware decoders lack the option of selecting large speaker mode on all channels. That is it will filter away bass from some or all channels and send it to the subwoofer output but since you don’t have a subwoofer input you will never hear these bass frequencies. How significant the problem will be depend on where it starts to filter out the bass and if it at least offers the option to set the front channels to large speaker mode (most do).

We have found that as long as you can set the left and right front channel to large speaker mode the loss of bass is very limited. The reason is that in all the movies we have tested, we have not once got a bass heavy effect in the rear channel where there was not also bass heavy effects playing in the front. I don’t doubt that such effects do exist but they don’t seem to be all that common.

If you use or intend to buy a 4 or 5-speaker system with separate subwoofer input bass management becomes more critical. In some cases the speaker system can handle all the bass or bass management itself and you will then want a decoder or soundcard where the subwoofer output is only used to send the LFE, that is it can be set to large speaker mode. If that’s not possible you may get a small bass hump of about 3 dB. Not a major issue, but less than ideal.

If your speaker system lacks its own bass management you need a decoder or soundcard that does the job for you. How this works varies quite a bit between different decoders / soundcards. A few allow you to select the crossover frequency (that is the point where most of bass is filtered from the satellites to the subwoofer) while most have fixed crossover frequency or send a full frequency output to the subwoofer output without filtering bass from the main channel outputs or midrange and treble from the subwoofer output.   

The full frequency subwoofer output option is found on most sound cards and, in the absence of a variable crossover, ensures maximum compatibility with a variety of small speaker systems that lack their own bass management.  The trade off is that the risk of being able to localize sounds coming from your subwoofer is increased if the subwoofer does not have its own frequency filter.  Even if they don't have what's called a low pass filter those larger drivers are not that good at reproducing high frequencies anyway and the enclosure design will also often work to minimize the audibility of higher frequencies emanating from subwoofers.

Many home theater subwoofers can limit the frequencies send to the woofer. Some multimedia systems also offer this feature (e.g. ProMedia 5.1, DigiTheatre LC, MegaWorks510D, Polk Audio AMR-130). Oddly enough both the ProMedia 5.1 and DigiTheatre LC will redirect those filtered high frequencies on to the front satellites. In the case of the DigiTheatre LC its not very noticeable and in the case of the ProMedia 5.1 the system has its own internal bass management and is best left to do this on its own and set the sound card to not redirect the main channels to the subwoofers.

Here a quick look at what they do:

Analog Devices SoundMAX SPX

This motherboard / CNR card based solution does not have an explicit small speaker mode but in 6 channel output mode some, but not all, stereo source material has a subwoofer output generated. That excludes any DS3D material and also anything that has direct channel access (such as software DVD players. The end result is that at least with the current drivers this is not a good option for speakers / receivers that lack onboard bass management.

Canyon3D2/Canyon3D1)

Don’t know

Cmedia CMI8738

There is an option to generate a subwoofer output on the 6 channel versions of this chipset. Enabling this option does not filter any bass from the main channels.  However, high frequencies are filtered from the signal set to the subwoofer output channel.  The exiting signal peaks at around  78 Hz and by 200 Hz is down around 10 dB and another 10 dB further (-20 dB) by 335 Hz.  This will virtually eliminate the potential for localization effects from signal routed to a subwoofer but such a sharp crossover can result in a gap between the bass extension capability a small (2.5 inches or less) multimedia satellite and what will be produced via the signal sent to the subwoofer.  

Creative Sound Blaster Live 5.1

The Sound Blaster Live 5.1 cards have two non-proprietary options for 5.1 output.

One is bass redirection enabled. In this mode the full frequency response from all the five main channels is directed (mirrored) to the subwoofer output.

The second is using the same connections above but with bass redirection not enabled. This mode will only be used where you are:

1. connecting to a small speaker system that retains full control of bass management via the six analog inputs
2. if you are connection to a large speaker system that is capable of full frequency reproduction without the use of a subwoofer, or
3. some combination of (1) and (2).

It's worth noting that for the (2) and (3) above you won't find this kind of configuration in a multimedia speaker system. Rather, it's the setup you might use with some home theatre systems.

Creative Sound Blaster Audigy 1 and 2

The Audigy series, just like the Live 5.1 series, offers bass redirect on and off. 

The big difference is that unlike the Live 5.1 it in bass redirect mode allows you to select the crossover point (10-200Hz) and it filters out bass from the main channels based on that crossover point and only redirects those frequencies to the subwoofer. 

For small speaker systems without onboard bass management the Audigy line, along with NVIDIA's nForce APU currently offers the greatest ability to tweak for optimum quality of the cards we have tested. 

By removing deeper frequencies from the satellites it reduces the chance of distortion, and by only sending those lower notes to the subwoofer, it reduces the risk of localization. Better yet, by allowing the user to select the crossover point the bass performance of any system can be optimized.

ForteMedia FM801 based cards

The ForteMedia FM801 based cards will send only the bass to the subwoofer output with a roll-off point high enough to ensure good compatibility with small speaker systems. That's a good thing as it reduces the risk of you being able to locate sound coming from the subwoofer, something you don't want to happen. 

On the down side the FM801 cards do not direct bass from the center channel to the subwoofer output so bass heavy effects that are in just the center channel (a common occurrence for games) will not sound the same as the identical effect in any other channel. When it comes to the bass output directed from the other four channels, it's always active and there is no way to adjust the subwoofer output level for games.

Hercules Game Theater XP, Fortissimo 7.1 and DigiFire 7.1

These cards have an optional small speaker mode that is available in 2.1, 4.1, 5.1, 6.1 and 7.1 configurations. The Game Theater XP with the 3.01 drivers onwards became one of the few cards (at the time joining the Audigy and FM801 based cards) that do more than a full frequency direct to the subwoofer.  The Fortissimo 7.1 and DigiFire 7.1 cards from Hercules now join products such as the Game Theater XP, Audigy and  nForce based motherboards in doing more than a full frequency redirect.

Specifically the Game Theater XP, Fortissimo 7.1 and DigiFire 7.1 all filter bass out from the satellite outputs and sends only that bass to the subwoofer in addition to any actual LFE channel material. This can result in cleaner sound from the satellites as they don't have to try reproduce those extra low frequencies and it can likewise reduce the chance of higher frequencies being localized from the subwoofer.

At one point Hercules was planning to release an update/utility to allow the user to configure the actual crossover point(s) but that project has been shelved.  

With the most recent drivers tested it's fixed at about 540 Hz. While that is a bit high the roll-off from the satellites is very gradual at that point and is only down 0.5 dB where the subwoofer is down 1 dB. The satellite output is down 1 dB at about 332 Hz, 3 dB at 189 Hz, 5 dB at 123 Hz and 10 dB at 52 Hz. The subwoofer output on the other hand is down 7 dB at 1 kHz, 10 dB at 1.3 kHz and 30 dB at 3 kHz.

NVIDIA nForce 1 and 2 APU

The NVIDIA nForce 1 and 2 APUs offer a variable crossover that can be set anywhere beween 50 and 500 Hz.  Like the Creative Audigy series this crossover not only send only the selected bass frequencies to the subwoofer output, it also filters out bass from the main channels based on that crossover point.

For small speaker systems without onboard bass management the nForce APU products, along with Audigy line, currently offers the greatest ability to tweak for optimum quality of the cards we have tested. 

By removing deeper frequencies from the satellites it reduces the chance of distortion, and by only sending those lower notes to the subwoofer, it reduces the risk of localization. Better yet, by allowing the user to select the crossover point the bass performance of any system can be optimized.

The variable crossover can work in both analog output mode and in Dolby Digital encoding mode.

Philips Acoustic Edge and Seismic Edge

With the latest drivers these cards have two bass management modes. 5.0 or 5.1. In 5.0 mode any material that would have been directed to the subwoofer output is instead mixed into the front channels. In 5.1 mode, when it comes to gaming the Acoustic and Seismic Edge subwoofer output will always work as a mix of all the other channels. For 3D audio (and DVD when DVD SSM is enabled) the subwoofer output will be just like the Live 5.1 in that it will be a full frequency signal. 

However the QMSS algorithms handle stereo sources and on top of generating a center channel (see Center Channel section for more) they also start to roll the subwoofer output off at about the 120 Hz point. As noted in the section for the Live, a full frequency subwoofer output that for 3D audio ensures maximum compatibility with a variety of small speaker systems with the same trade-off. Having QMSS process stereo material means that the risk of bass localization occurring is reduced for these materials and hence overall.

Turtle Beach Santa Cruz/VideoLogic SonicFury

These cards have a mode called virtual 5.1. Enabling this mode is the only way to get the cards to generate a subwoofer (or center channel) output. Oddly enough, with virtual 5.1 enabled the subwoofer channel is created from only the front channel material where as the center is created from both the front and rear channels. If your only concern is music then that won’t matter and if you have gaming in the mix then you can reverse the connections and get the sub output generated from front and rear channel material. Unfortunately this is not a good solution if you also want to use the cards output for software DVD 6 channel output as the center and LFE channels would be reversed. Just as with the Live5.1, Acoustic and Seismic Edge the subwoofer is a full frequency signal and there is no filtering of bass from the main channels.