For a few hundred dollars, your audio installations could improve exponentially. Stephen Dawson looks at a new subwoofer DSP from DSPeaker.

You’d think that with the extensive digital signal processing (DSP) and automatic calibration built into modern home theatre receivers, your subwoofer’s output would be optimally matched to your listening room. But in most cases that is not correct, because most receivers largely neglect important parts of subwoofer calibration.

The DSPeaker Anti-Mode 8033 Cinema addresses that in a big way, using a modestly priced, rather small box of electronics.

What You Get
The box uses a low voltage wall wart for power and also comes with a measurement microphone with 5m of cable. It has one RCA socket as in input, another as an output and a 3.5mm jack on the front for the microphone. There are two push button controls and four LEDs.

The unit provides three different functions. One is discovering and eliminating – or at least reducing – resonant room modes which boost some bass frequencies. The second is to optionally provide some deep bass boost. The third is to filter extremely deep bass below 10Hz in order to protect the subwoofer.

Now with just one input and one output, clearly the Anti-Mode is used primarily for one subwoofer. The company also has a two channel version, the Anti-Mode 8033S-II, available at higher cost (about $100 more), with a few more filter functions and covering a wider frequency range.

It’s easy to jump to the conclusion that this device is an automatic room EQ device, and it sort of has that effect in practice. But its main role is to identify those resonant modes in the room, as measured at the listening position, and control them. All rooms enhance certain frequencies according to their specific dimensions and the degree of these peaks varies according to where you are within the room. Unlike a full EQ system, the Anti-mode does not boost frequencies which the subwoofer is reproducing relatively weakly. It does reduce those which are too strong.

To do that it applies digital filters for which it can adjust the frequency, strength and ‘Q’ – essentially the width of the frequency band which is affected. It acts like a special kind parametric equaliser in which each band can only be turned down, not boosted.

Incredibly it can apply up to 28 filters in the frequency range from 16Hz to 160Hz. That works out to an average of more than eight filters available per octave. If all works as DSPeaker says, that ought to give it excellent fine control. The company says that each one is tuned to an accuracy of within 0.5Hz.

That’s the main function. The second function simply applies boost at low frequencies. There are two options for this: the band from 15Hz to 25Hz boosted, peaking at 7dB at 20Hz, and from 25 to 35, peaking at 7dB at 30Hz. Because really deep bass can damage your subwoofer, these both include an infrasonic filter that nominally cuts frequencies below 10Hz. That filter is applied alone is the third function.

Since the input and output are both analogue, the unit incorporates an analogue to digital converter and a digital to analogue converter. It specifies the system frequency response at 5Hz to 160Hz at the -3dB points and a dynamic range of greater than 90dB. The DSP is a 32-bit model for highly accurate calculations.

Doing it
That seems quite a lot of functions given that there are only two control buttons on the unit, but they are cleverly designed to be used singly, in combination, or held to give all the functions required. Clear labelling on the front panel means that you won’t have to pull out the manual time and time again.

That’s aided by the fact that setting up the system really could not be easier. You take the cable that normally connects your home theatre receiver’s subwoofer output to the sub and plug it into the Anti-Mode instead. You use another cable to plug the Anti-Mode’s output into the subwoofer. (Or you might find it more convenient to put the unit at the receiver end of the subwoofer cable).

Plug in its power pack. Plug in the microphone. Roll out that microphone’s cable and stick it to something so that it’s about where your head will normally be when using your system. Then press and hold the two buttons for three seconds. One of the indicator lights will start flashing and you will hear a tone sweep up from the deep bass to around 160Hz. Each sweep takes about 90 seconds and the sweep is completed six times.

And that’s it. Unplug the microphone, roll up the cable and put it away because you’re done.

Well, not really. You should then run the auto calibration on your home theatre receiver. The overall level of the subwoofer is likely different since some frequencies will have been reduced. The manual suggests that the average level is raised by 3dB to compensate, and my tests confirmed this, but this may still not be the correct overall level.

In addition the unit adds a very slight delay to the signal – just 2.7ms. If your receiver sets the subwoofer distance in its auto-calibration routine it will adjust for this. Otherwise just add a metre to the manual subwoofer distance setting.

Smooth
So did it work? Absolutely… and extremely well.

I should note that my subwoofer is considerably more capable than most. A 15” unit in a large sealed enclosure with 400W of power, it happily outputs 16Hz, which is the bottom frequency the Anti-Mode unit is capable of adjusting. The test listening position has a significant suck-out of frequencies centred at 35Hz. This should be a challenge for any bass adjustment system.

You can see for yourself the results. The measurements shown in Figure 1 are with the Anti-Mode’s ‘bypass’ mode engaged (i.e. the signal is untreated). Those in Figure 2 are with the DSP doing its work in ‘flat’ mode (i.e. neither of the two ‘lift’ modes engaged).

For those graphs the level is 3dB per division. And it is obvious how much smoother the response is. Aside from the suck-out, the response is 15Hz to 75Hz +/-2.5dB. It would probably have extended higher were it not for the receiver’s crossover. That was set at 200Hz (its maximum) when I made these measurements, so it was contributing to the roll-off at the higher frequencies. Untreated the same 15Hz to 75Hz required an envelope of +/-7dB.

The Anti-Mode reduced the suck-out depth is reduced considerably as well, from 14dB below the average level to about 10dB.

It wasn’t just the measurement that was improved but the general sound of the system, especially when using smaller speakers with an 80Hz crossover. The integration between subwoofer and speakers had occasionally suffered a little with some combinations of frequencies. With the Anti-Mode in place, doing its stuff, the whole system tightened up remarkably, with a smooth joining of speakers and subwoofer into a seamless whole across a wide range of music.

I checked out the boost functions and they did pretty much what DSPeaker says. Another way of putting that is that they took a nice accurate subwoofer signal and made it less accurate in ways that might be pleasing to some listeners, but would probably not be appreciated by enthusiasts.

If I had a suggestion to make, I’d consider dropping those and instead offer a couple of switchable high pass filters, say at 20Hz and at 40Hz. They’d be quite useful settings for late night operation. The deeper the bass, the more it penetrates structures and upsets neighbours. Also some music contains deep spurious noise, clearly not noticed by the engineers. Being able to control those deep frequencies while keeping the bulk of the subwoofer’s operating band working would be highly useful.

Conclusion
It’s hard to think of any other single $375 dollar enhancement to a subwoofer-equipped home entertainment system that could come close to producing as much audible and measurable improvement as the DSPeaker Anti-Mode 8033.

It’s hard to think of any system, in fact, which would not benefit from it.