Multi-Sub Optimizer Reference (page 2)

Optimization Strategies

MSO allows for a number of optimization approaches. You can choose to optimize the integration of subwoofers with one another and with the main speakers, or you can include only the subwoofers in the optimization. Usage of MSO requires that you have some sort of DSP processing with IIR filters available so that EQ, gain and delay can be applied to each subwoofer individually. The alternatives can be generally categorized as follows.

Optimizing Subs Only

When optimizing only the subs, you should set the crossover frequency of the AVR or pre-pro (or its LPF of LFE) to the highest allowable frequency. Whether you choose the crossover frequency or LPF of LFE depends on how you apply an input signal to the AVR. If you use one of the main speaker channels via either analog or HDMI excitation, set the crossover frequency to the highest allowable. If you apply an HDMI signal via HDMI channel 4, set the LPF of LFE to the highest allowable frequency.

Fixed Crossover Frequency

For this scenario, set your AVR's crossover frequency to the desired value, then perform the measurements with the crossover in place. When adding filters to subwoofer channels in MSO, do not add low-pass or high-pass filters, as their effect will already be present in the measured data. Any filters added within MSO represent changes from the as-measured condition. When using a fixed crossover frequency, choose Tools, Project Options from the MSO main menu and ensure that Correct sub measurements to remove sub LPF response is unchecked.

Using MSO to Help Find the Optimum Crossover Frequency

Note: This technique is not recommended unless you are one hundred percent sure of the following properties of your AVR or pre-pro:

If you use this strategy, you'll need to set your main speakers to Large before making measurements, while still allowing the subwoofer to play. Ideally, the sub should have no low-pass filter in effect at all, but this is generally not possible. The solution is to enable the mode sometimes called "LFE + Main", "double bass" or "plus mode" depending on the AVR manufacturer. Then set the crossover frequency as high as possible, preferably 250 Hz. When this is done, the mains will be full-range, and the subs will have a low-pass filter of e.g. 250 Hz applied. This low-pass filter will not be present when the final crossover frequency is chosen, so MSO has a facility for correcting the measured subwoofer data to remove its effect. This feature only works when the AVR or pre-pro's low-pass filter (LPF) is a fourth-order Linkwitz-Riley type.

After importing the data into MSO, the following steps must be performed (see "adding filters") in MSO's Config View.

The last step ensures that the subwoofer low-pass filter effect modeled by MSO will be only the crossover low-pass filter without the added effect of the undesired low-pass filter present in the measurements.

After adding the crossover filters per the above, the cutoff frequency parameter of each filter will be set to the default value of 80 Hz, and the Optimization allowed property of the cutoff frequency will be set to False. Change the cutoff frequency of both the low-pass and high-pass filters to the desired value if that value is different from the default 80 Hz, but keep Optimization allowed set to False for each one. See Changing Filter Parameter Values and Constraints for how to edit filter parameters. Unfortunately, if you set Optimization allowed to True for the crossover low-pass and high-pass filters, MSO will adjust each cutoff frequency individually, resulting in cutoff frequencies for the crossover low-pass and high-pass filters that are different from one another. Such a configuration is incompatible with the capabilities of AVRs.

If you want to evaluate different crossover frequencies, you must use MSO's Configurations feature. From the main menu, choose Config, Clone, and in the resulting dialog, make sure Clone associated graphs is checked. Then, in the newly created configuration, change the cutoff frequency parameter of the low-pass and high-pass filters to the desired value. For a detailed explanation of the Configurations feature, see the Configurations topic.

Using an AVR or Pre-Pro Without a Conventional Crossover

This is the strategy recommended by Earl Geddes for use with his speakers. In this approach, the AVR or pre-pro is set to the "LFE + Main", "double bass", "plus" or similarly-named mode, depending on its manufacturer. The main speakers are run full range, and the AVR crossover frequency is set to the highest allowable value, often 250 Hz. A separate low-pass filter for each sub is configured in the DSP device connected to the AVR's sub out, but only after optimization. Prior to measurements, there is no LPF configured in the DSP. Instead, the DSP is configured to have all its filters bypassed. These low-pass filters could in general have a different cutoff frequency for each sub, and the cutoff frequency is chosen so the frequency ranges of subs and mains overlap. Filters of the desired type are added in MSO, which will then determine the optimum cutoff frequency for each one. It should be noted that the Geddes loudspeakers are of closed-box type, with high power handling and an anechoic low-frequency cutoff of 80 - 90 Hz. If your mains are of the vented-box type, you'll still need to high-pass filter them, which for this configuration would require your DSP device to filter the main speakers as well, requiring more DSP channels.

In the past, Earl Geddes has recommended that dissimilar subwoofers be used, with one high-output "super sub", and two or three additional subs having lesser output. MSO assumes linear operation, so running an optimization without taking maximum output into consideration could lead to a solution that is sub-optimal with respect to maximum output capability. Since this potential problem is not limited to the Geddes-style configuration, it is discussed separately on the Working With Dissimilar Subwoofers page.

Using an HTPC With Filters Implemented in Playback Software

Modern pro audio multi-channel audio interfaces, along with playback software having DSP capability such as JRiver Media Center, provide interesting opportunities for multiple-subwoofer systems without the constraints of AVRs. Because they are so flexible, it's hard to make specific recommendations with regard to their operation with MSO. However, there are a couple of suggestions that are generally applicable.

Some playback software has the ability to incorporate FIR filters, but MSO only supports IIR filters of the type normally used to emulate their corresponding analog filters (Butterworth, Linkwitz-Riley, Bessel and so on).