- Written by Dr. David A. Rich
- Published on 17 April 2013
Extending the ARC System to Include a Subwoofer
In Part 1 of this series, we discussed the Anthem ARC (Anthem Room Correction) firmware that is present in their SSPs as well as the AVRs which have a reduced filter bank size. In Part 2, we will discuss the addition of a subwoofer and how it is integrated into the ARC program using the full SSP version of ARC.
By far, the most significant sonic issue for acoustic music when a subwoofer is deployed occurs around the crossover from the subwoofer to the main speaker. Adding a subwoofer to a system using a standard bass management system without a room EQ in the loop (sub and main channels) degrades the flatness of the response in this critical area. At a minimum, the in-room transition band of the low-pass and high-pass filters should track the shape of a fourth-order Linkwitz–Riley (4th order LR - more on these two designers work is below) filter to about 20dB down. With a 4th LR at 80Hz crossover, both speakers are active between 60Hz and 110Hz (-10dB points), a frequency range that is populated with a variety of common instruments playing throughout the score.
A typical bass management system provides only a second-order filter for the main speaker, which will not sum to flat, even in an anechoic chamber. Developers of standard bass management systems (circa 1995) assumed the main channel speaker will rolloff naturally with a slope of 12dB response exactly below the crossover frequency, so they added only a 2nd order filter section to create the fourth-order filter.
This will never happen. Room EQs must provide additional correction for the main speaker channel to have the correct -6dB point and transition band associated with a 4th-order LR high-pass filter. Different main channel speakers require a custom filter synthesized by the adaptive room equalizer. Once the EQ performs this function, the system is flat in an anechoic chamber, but this is only a small part of what the room EQ must do.
Without additional electronic equalization, the room effects (uneven sound pressure preservation using Tom Nousaine's terminology; see my Technical Article Subwoofers: A Brief Look at the Effectiveness of Using a Subwoofer in a Music System) corrupt the shape of the low-pass and high-pass filter transition bands. The correct in-room high-pass filter transition band shape of the main speaker is as critical as the in-room low-pass rolloff of the subwoofer. Only an advanced electronic room EQ in a Pre/Pro or AVR can provide the required 4th order LR response at the listening seat for the main channel.
The crossover frequency must also be under the user's control. The user must select a crossover point high enough that the subwoofer enters before the main channel speaker's distortion starts to increase. At the same time, the crossover should be as low as possible to prevent localization of the subwoofer.
I have found lower crossover frequencies starting around 90Hz (but not much lower than 70Hz as explained in my article cited above) result in smoother response at the join between the main channel and subwoofer. This observation is room dependent. 100Hz – 130Hz tends to be an area with a lot of closely spaced peaks and dips on both the main and subwoofer channels. Given the different spots for the peaks and dips for the main and subwoofer channels (they are in different places in the room), matching becomes more difficult. If the EQ filter bank is IIR filters, we could run out of IIRs to fix all the response deviations of the subwoofer at the higher crossover frequencies in the critical area that both the main channels and subwoofer are active. I also note that speaker-to-room boundary interactions (Allison Effect) begin at about 100Hz, adding to the complexity of the main channel curve.
Some electronic room equalizers will not allow the crossover frequency to be adjusted. Since no room EQ system makes distortion measurements, the automatic selection is often wrong, with the setting far too low. Too much low frequency energy is applied to the main channel woofer, which will overdrive it.
Below I have zoomed in to the portion of the ARC Target Customization panel that involves subwoofer selection.
For this panel, only an L/R Front speaker (stereo configuration) was used, but you can see that you can select the crossover frequencies for each speaker pair and subwoofer separately. This provides a different high-pass frequency (HPF) for the fronts, center, sides, and rears. I call front, center, sides, and rears the main channels speakers to distinguish them from the subwoofer.
The bass management graphic user interface on the TV screen does not have to be set up. When ARC is enabled, the bass management filters are automatically selected per the panel above.
The subwoofer has a separate single low-pass filter (LPF) frequency. All the full range signals from the main channels are summed to a single mono signal, and the single low-pass filter follows before the signal is applied for the subwoofer.
If you set some speakers to a higher crossover value than the subwoofer LPF, you will get a hole in the response from the point the HPF crossover has rolled off the main speaker to the point the LPF allows the subwoofer to becomes active. This may be unavoidable if you are using small rear and surround channel speakers, with larger speakers for the left, right, and center.
If you set any HPF frequency lower than the subwoofer LPF frequency, you will get an overlap and a peak in that range between the LPF value and the lower HPF value. You should never be setting the panel to create this condition.
To prevent the problem of creating an overlap, or a hole, four LPFs would be required. One set for the front channels summed to mono, one for the center channel, one for the mono summation of surrounds, and one for the rears. Each of these four LPFs would be set to the value assigned to the corresponding HPFs selected in the control panel above. I have never tested an AVR that did not mix all main channels followed by one LPF. Adding the three additional low frequency LPFs requires the DSP to have six additional 2nd order IIR filters. This would require a more powerful DSP, or if the DSP cost is to be kept constant, a reduction in the size of the filter banks for the room EQ.
Yet another LPF to the subwoofer should be present for the LFE (low Frequency Effects) input (sometimes called subwoofer-out on a DVD or Blu-Ray player). The issue is the LFE channel has a spectrum up to 100Hz. In Anthem products, the LFE is summed with all the other channels before it enters the subwoofer LPF (note no selection box for the LFE channel in the panel above). If the LPF is selected lower than 100Hz (we want to match the main channels HPF to prevent overlaps discussed above), then information in the LFE above the selected crossover will be lost. Some AVRs have an extra LPF at 100Hz just for the LFE, which then bypasses the LPF for all the other main channels, and solved this problem. This is shown in the panel above.
If you want the full LFE frequency range reproduced, you will need to set the subwoofer to 100Hz to hear all the sound effects (music will never be in the LFE channel if the mix engineer is minimally competent). As explained above, this comes with a really significant cost. An overlap between the L/R/C and the woofer will resultant in peaking in the overlapped region. With ARC, the Cinema mode may become a useful workaround. This allows for using a subwoofer crossover of 100Hz for movies but matching it to the lower value selected for L and R for music.
Another way around the problem, exclusive to Anthem, is to use the only speaker setting you cannot control on the ARC PC display. In an on-TV screen setup menu generated by the Anthem AVR, there is an option called LFE Bypass Xover, which sends the full LFE channel to the subwoofer (this works under the assumption the LFE channel has been band-limited when the movie was mastered, as should be the case). Since the PC is gone for normal operation, you also need the menus to turn ARC on and off.