Using MSO to Help Find the Optimum Crossover Frequency
This technique no longer recommended and is unsupported. The information below is for reference only. The technique assumes you are one hundred percent sure of the following properties of your AVR or pre-pro.
- The low-pass filter used in the crossover must be a fourth-order Linkwitz-Riley type.
- The high-pass filter of the crossover must be a known type that is supported by MSO, and you must be absolutely certain of that type. This will usually be a second-order Butterworth type for THX-compliant AVRs and pre-pros, or a fourth-order Linkwitz-Riley type for some others. Some AVRs use a second-order Linkwitz-Riley high-pass filter in this application, which means that the corresponding low-pass filter is likely a second-order Linkwitz-Riley type as well. This would render the technique described below unusable because of the low-pass filter requirement above.
It is not reasonable to expect that typical MSO users will know this information or be able to do the measurements that allow them to find out.
Still other hardware implementations (such as Trinnov) allow configuring bass management so that the subs can be energized with no LPF at all using a main speaker input, while simultaneously energizing the corresponding main channel with no HPF. This kind of setup is arguably the best possible one for optimizing crossover frequencies, but some of the directions below do not apply to such hardware. Optimizing crossover frequencies in MSO is possible for many (but not all) hardware implementations, but is unsupported regardless of the hardware used.
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.
- Under the Subwoofer Channels node of the desired configuration, right-click the Shared Filters node and select Add Standard AVR Crossover LPF.
- Under the Mains Channels node of the desired configuration, right-click the Shared Filters node and select Add Standard AVR Crossover HPF menu option. Note: This is only correct if the AVR or pre-pro's HPF is a second-order Butterworth type. If it is not, use e.g. the Add Advenced Filter, HPF Linkwitz-Riley 24 dB/oct menu option. This latter option is a fourth-order Linkwitz-Riley HPF as used by some pre-pro vendors (e.g. Emotiva). If you use this latter option, you must select the filter and in the Properties window, set the Optimization allowed property of its cutoff frequency to False.
- In the Main Menu, choose Tools, then Project Options. Check the check box labeled Correct sub measurements to remove sub LPF response. Then enter the frequency in Hz that you chose for the crossover frequency for measurements. This correction boosts the amplitude of the measured subwoofer data above the cutoff frequency. To prevent excessive correction from being applied, the value entered into this field of the dialog box must be no less than 200 Hz.
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.
Evaluating Different Crossover Frequencies
MSO treats each filter as a completely independent entity. This has an unfortunate side effect when working with crossovers. If you were to set the Optimization allowed property to True for the cutoff frequency parameter of each of the low-pass and high-pass filters filters that make up the crossover, MSO would adjust each one independently to optimize the system. But this would generally result in different cutoff frequencies for the high-pass and low-pass filters. AVRs and pre-pros force the cutoff frequencies of the high-pass and low-pass sections of the crossover to identical values. In this case, MSO's optimized result would be incompatible with your hardware. The solution is to set the Optimization allowed property to False for these cutoff frequency parameters, then set the cutoff frequencies to the same value manually. To try out different crossover frequencies, use a separate configuration for each one.
Once you have cloned a configuration, you can change the cutoff frequency parameter of the crossover low-pass and high-pass filters of the clone to the desired value. Always make the cutoff frequencies of the low-pass and high-pass halves of the crossover equal to one another, and always ensure that the "Optimization allowed" property of each low-pass and high-pass cutoff frequency parameter is set to False. See the manual of your AVR or pre-pro for allowable crossover frequencies.