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Anti-mode 8033 Affordable Digital Signal Processing for Subwoofers
as reviewed by Roger S. Gordon
Digital Signal Processing (DSP) is the wave of the future for room treatment. For several years companies, such as Lyngdorf, Meridian, Copland, and TacT and many others, have been selling room correction devices utilizing DSP that attempt to remove standing nodes from the listening room. The removal of these standing nodes should result in better sound via a flatter frequency response. These DSP systems have not become overwhelmingly popular, however, for a number of reasons. These reasons include:
1. Cost - usually thousands of US dollars.
2. The fact that the systems will reduce peaks in the frequency response but usually no attempt is made to fill in the valleys and virtually every normal sized listening room has 20-40dB dips in the frequency response curve, usually at one or more of the room's primary dimensions.
3. The more filters the DSP device introduces into the system to provide greater smoothing of the frequency response curve, the more audible the device usually becomes.
4. The fact that most of these DSP devices require the analog signal to be converted to digital in order to be processed by the DSP device and then converted back to analog is usually audible, particularly if you are using an analog source.
Enter the DSPeaker 8033 Anti-Node (DSPeaker). The DSPeaker is a DSP room treatment device for subwoofers that eliminates three of the above four problems. First, the DSPeaker only costs US $350 MSRP. So it is affordable. Since you are only treating the subwoofer and not the main speakers the degradation in sound quality from the interjection of digital filters into the signal path and doing A to D and D to A conversions to the audio signal are not really noticeable. This is particularly true if you are using the subwoofer at frequencies less than 80Hz. The only one of these four problems the DSPeaker does not solve is filling in the valleys in the frequency response curve.
So what does the DSPeaker do? Please look at Chart 1.
This is a frequency response graph taken at my listening position without utilizing the single band equalizers built into the Dayton plate amps that I use to drive my subwoofer. The red line is the frequency response with the DSPeaker not engaged. The blue line is the frequency response curve with the DSPeaker providing digital signal processing. Needless to say, getting rid of that big 20dB bass hump from 25 - 55Hz did a lot to improve the sound. Specifically, removing a bass hump like that shown on Chart 1 does several things. First, it tightens up the bass. Because bass notes don't resonate as much anymore, their decay is much quicker. Now, you can really hear the tone and texture of each drum stroke as it is no longer muddied up by the resonating note. Secondly, with the excess bass energy no longer being present, the lower and middle midrange are no longer being obscured by the bass notes and their attendant 2nd order and 4th order harmonics. A much clearer, more detailed lower midrange is the result. The third benefit is that you get to turn up the volume of your subwoofer. With the DSPeaker reducing the amount of bass energy being put into the room, the manufacturer recommends increasing the subwoofer volume by 3-10dB.
How does the DSPeaker work its magic? Very easily. The DSPeaker itself is a small box with dimensions of: 126 mm x 80 mm x 28 mm (5" x 3.2" x 1.1"). It weighs only 230 g (8 oz). The rear panel has the line-in, the line-out, and the input for the supplied wall-wart (power) plug. The front panel has the input for the supplied microphone, an on/off power switch, two buttons, and four LEDs. The buttons are to start the calibration sequence, to select Bypass mode where the audio signal passes through the unit unprocessed, and to select two different levels of bass boost. The LEDs show whether the Bypass is engaged or if either of the two bass boosts has been turned on. To insert the unit into your system you need to place it between your preamp and the subwoofer. Plug in the wall-wart at the wall and plug the other end into the DSPeaker. Flick the DSPeaker power switch to ON and you are ready to calibrate the unit. Calibration requires you to plug in the microphone, place the microphone at the place where your ears would be at the listening spot, and then depress the two buttons for 3 seconds or until you hear sounds coming from your subwoofer. The unit will then run four 30 second sweep tones. After the tones stop, unplug the microphone and start listening to music. If you want to widen the listening area to be more than just the sweet spot the instruction manual tells you how to run an additional calibration test that will do this.
So once the DSPeaker is calibrated and it is digitally processing the audio signal going to the subwoofer what is it really doing. The manufacturer's brochure states: "Anti-Mode™ technology eliminates the resonances of the speaker and the room by equalizing both amplitude and time domain responses using very accurate digital signal processing filter structures and anti-phasing technology." Per the literature the DSPeaker can construct up to 24 different filters (see below) to flatten out the frequency response curve from 16 - 144Hz. Five of these filters will be engaged only if you choose to widen the sweet spot. In layman's terms, the DSPeaker is an automatic 24 band equalizer—the DSPeaker creates a custom set of DSP filters and uses them to reduce the peaks in the frequency response curve.
Of course, DSPeaker cannot work miracles. I moved the microphone 18 inches to the left of my listening position, recalibrated the DSpeaker for that position, and generated Chart 2.
The blue curve is the DSP treated frequency response curve from Chart #1. The red line is the DSP treated frequency response curve after the DSPeaker was recalibrated for that new position 18 inches to the left of my normal listening position. That 25dB valley is at 56Hz and as you move your head back and forth between this position and the listening position the change in sound is dramatic. The wave length of 56Hz is 10 feet, the height of the listening room ceiling at its peak. You can't get away from the laws of physics. DSPeaker, like most other DSP room treatment devices does not fill in valleys. So the only choice I have to eliminate this major valley is to move my listening position.
While I had the unit calibrated for the position 18 inches to the left of my listening position I ran some frequency response curves with no bass boost, bass boost centered at 25Hz and bass boost centered at 35Hz. Chart 3 is the result.
The instruction manual says that for music you probably don't want any bass boost. However, if you want to make the music warmer you can try the bass boosts.
The single band equalizers that are built into my subwoofer amps are fairly effective. They can reduce the bass hump shown in Chart 1 from 20dB to only a 6dB hump. Being of a curious nature I wondered what would happen if I had the single band equalizers on when the DSPeaker was calibrating. I went back to my normal listening position, turned on the plate amp equalizers, recalibrated the DSPeaker at my normal listening position, and then measured the frequency response curve. The resultant frequency response curve looked exactly like the blue line on Chart 1. In other words, having the single band equalizers in or out of the system makes no difference. Good, that means I can use the single band equalizers to try to flatten out the 6dB hump centered at 64Hz. Another fun weekend project
My 21' x 17' listening room is heavily treated with sound treatments and has ten ASC Tube Traps tuned to different bass frequencies. Even with this amount of bass absorption, I still have the 20dB bass hump shown in Chart 1. Utilizing the single band equalizers can reduce the bass hump down to a 6dB hump. However, a 6dB hump is still audible as a warming of the sound and a slight muddying of the lower midrange. Digital Signal Processing is the only efficient way to deal with this problem. Insertion of the DSPeaker in place of the analog equalizers made a significant improvement to the sound of my system. For $350 this has to be the best bang-for-the-buck tweak I have made in my system for a long time. If you use a subwoofer for either your audio or your audio/visual system you have to give the DSPeaker a try. Highly Recommended. Roger S. Gordon
The charts were generated by downloading from www.realtraps.com sine-wave test tones from 10 to 299 Hz at 1 Hz intervals. By using a Radio Shack Sound Pressure Level meter while playing these tones it was possible to construct a frequency response curve for my listening room. The results were put into Excel 2007 and the charts were generated.
The filters consist of:
1. custom-design digital Anti-Modal oscillators
2. 6 custom-design digital Anti-Modal oscillators reserved for multipoint/ fine tuning in 1-point
3. 3 custom-design restricted-constraint digital IIR emphasizing bandpass filters
4. 2 alternative user-activated emphasizing lowpass digital IIR filters (15-25Hz (lift 25) / 25-35Hz (lift 35) / off)
5. 1 infrasonic digital IIR filter (active only when lifting is used)
6. 1 analog noise-shaping Bessel-filter (always active)