Equalization

There are a number of different approaches to digital room correction in use these days. Some of the more sophisticated techniques used in modern receivers and SSPs attempt to reduce the extended decay times at the room’s modal frequencies by subtly modifying the original signal (modal equalization). Other more traditional methods involve flattening the magnitude of the frequency response using parametric EQ filters. The SMS-1 adopts the latter approach.

I have seen some recent research suggesting that, below 100 Hz, the ear is so insensitive to extended decay times, modal equalization is only worthwhile in really extreme cases. Flattening the magnitude response below 100 Hz may actually be all that is required. This is a contentious area with rival manufacturers making claims as to which approach is best. There may even be some merit in using a combination of the SMS-1 to tame the extreme low bass while letting your receiver or SSP's room correction deal with the remainder of the frequency range.

The important thing to realize about equalization is that you can usually only achieve a flat response within a relatively small listening area. Move even a small distance outside that area and your nicely equalized system may actually produce even more lumpy bass than you started with. Using two or more carefully placed subwoofers can sometimes help in these cases. However, you may still have to compromise and put up with a slightly bumpy response in order to achieve an acceptably wide listening area.

The SMS-1 incorporates an 8-band digital parametric equalizer. Each equalizer slider can apply a cut of up to -13dB, or alternatively, a boost of up to +6 dB. The sliders move in steps of 0.5dB at a time. When set to factory defaults, the sliders act at standard 1/3 octave frequencies (20, 25, 32, 40, 50, 63, 80 and 100 Hz).

It is most important to use the EQ boost capability cautiously. A +6 dB boost will require the subwoofer amplifier to quadruple its power output at that frequency. This will limit the overall headroom available and may be sufficient to cause amplifier clipping and/or driver overload when played at very high levels.

Realizing that there are some customers who will not want to get into the intricacies of the manual EQ set-up straight away, Velodyne equipped the SMS-1 with a couple of automated EQ set-up modes to minimize the effort required.

In order to use the simplest of these, the Self-EQ mode, you strictly only need to hook up the microphone to the SMS-1 and the LFE output to the subwoofer. However, I would always recommend connecting the video output to your TV as well. That way, you can monitor the sweep level on-screen to ensure it is not too low, or flat-lining along the top of the plot. You can then adjust the subwoofer gain to compensate accordingly.

To engage the Self-EQ mode, all you do is press the sequence 3-2-1 on the remote control while at the initial OSD display. The display will change to the sweep screen as follows.

The EQ sliders are reset to the standard 1/3 octave frequencies, and the SMS-1 then perform 25 sweeps, moving individual sliders up or down to try and achieve a flat response. Since the ear typically struggles to discern variations smaller than ± 3 dB at these low frequencies, the SMS-1 stops moving sliders when the response is within these limits. When it has finished, it saves the settings and returns to the initial OSD page. Note that only the subwoofer output is in play during Self-EQ. No EQ Output signal is sent to the receiver/SSP, so the main speakers produce nothing.

My system response following Self-EQ appeared as follows.

That represents a further incremental improvement, and if I didn’t want to venture any further into the set-up process, I could just decide to leave it at that.

Click Here to Go to Part V.

© Copyright 2005 Secrets of Home Theater & High Fidelity Go to Table of Contents for this Issue Go to Home Page

Terms and Conditions of Use