Multi-Sub Optimizer Reference Manual (page 18)

Special Topics: Tips and Tricks

In this section, you'll find discussion of miscellaneous program features that can help you be more productive with MSO.

Copying and Pasting Filters

You can copy filters in the following ways:

In combination with the filter paste feature, this is a quick way to add many filters to one or more channels. If you choose to copy selected filters of a channel, a dialog box pops up listing the filters in the channel, with a check box next to each one. You can then paste them into the same channel or a different channel of any configuration with Ctrl+V. When pasting filters to a destination channel, sometimes not all filters will be pasted. This can occur if e.g. you try to paste delay blocks, gain blocks or LPFs to a main channel, or if the paste operation would result in too many delay or gain blocks in a sub channel. If not all copied filters are pasted to the destination channel, a tabbed text view is created on the right, listing the filters that weren't pasted and why. This is to show what happened without a dialog box getting in the way. The information in this text view is temporary and not saved with the project. You can also copy filters to another project if desired. To do this, just copy the filters you want, close the project, then open a different one. The filters are still in the clipboard, and so they can be pasted into the newly-loaded project.

The copy and paste commands appear on the edit submenu of the main menu, on context menus in the Config View, and on toolbar buttons. You can see whether the copy or paste commands are available by looking at the appearance of the copy and paste toolbar buttons. If the copy or paste command is enabled (cursor is on an appropriate node), the buttons will show in color. If not, they will display in gray scale. The logic of whether filter paste is enabled is more complex than it first appears. If you have copied e.g. only a delay and a gain block to the clipboard, and you move the cursor to a main channel filter node with the intent of pasting the filters to the main channel, the paste command will be shown as disabled, as neither delay nor gain blocks are compatible to paste into a main channel. To paste the filters using the Ctrl+V keyboard shortcut, be sure the focus is on the Filters node of the desired destination channel in the Config View and not one of the tabbed views on the right.

Specifying Filter Parameters and Limits

Make sure you understand the two ways of specifying filter parameter limits, and how to use the filter Properties Window and Application Options dialog to perform those tasks. See Parameter Locking for how to prevent the optimizer from changing the parameters you've specified.

Avoid Excessive Sub Delays

The introduction of the miniDSP 2x4 HD version has given users the ability to set much larger sub delays than were possible with the non-HD 2x4 versions. Depending on the plug-in used, the non-HD 2x4 hardware is capable of a maximum delay of either 7.2 msec or 7.5 msec. In an anechoic environment at a typical room temperature, these numbers correspond to distances of 8.11 feet and 8.45 feet respectively. These distance numbers can be interpreted as the maximum difference in distance between the closest and most distant sub at a given listening position (usually taken as the MLP) that the hardware can compensate for in an anechoic environment. This adjustment range may not be enough for all applications. On the other hand, the miniDSP 2x4 HD version has a maximum delay of 80 msec, which corresponds to a distance of 90.08 feet. This is larger than would ever be needed in even the most extreme home theater setups. For this reason, the maximum allowable delay of the MSO delay blocks should be much less than this 80 msec value.

A good starting point for figuring out what the maximum allowable delay should be is to use the procedure in AustinJerry's guide called "Using MiniDSP 2x4 to Time-Align Multiple Subs on One Channel before Room Correction". Suppose you were to calculate the maximum delay needed using his approach, and the number turned out to be 10 msec. MSO adjusts delay based on frequency-domain considerations which are affected by room modes (not taken into account when using the anechoic assumption in the above mentioned document). Therefore, it's possible that the actual delay needed by MSO could be somewhat larger than that predicted using the anechoic assumption. In the case of the hypothetical 10 msec number calculated above, even if you double it, which might be more than needed, the resulting 20 msec maximum delay is still far, far less than the maximum 80 msec value of the miniDSP 2x4 HD. It should be emphasized that the use of MSO is intended to be a supplement to, not a substitute for, common sense. Using excessive maximum delay values can degrade the time-domain performance of the system, and also possibly cause MSO to get stuck on poor solutions, especially near the high end of the subs' frequency range.

Example 1: Setting Delays When Using Near-Field and Far-Field Subs Together

When adding delay blocks to subwoofer channels in MSO, the software will by default make the delay value limits for each of the delay blocks equal, as specified in Tools, Application Options, Filters, Delay. For systems using a combination of near-field and far-field subs, having equal delay limits for all subs may not be an appropriate choice. With such a setup, you'll want to experiment with setting the delay of the near-field subs to a larger value than the far-field ones, to nominally match the times of arrival of the early sounds from each of the subs.

In more conventional setups for which all the subs are in the far field, delays are used in MSO in order to manipulate room modes to get consistent seat-to-seat frequency response. But for near-field subs, the room modes have a much reduced effect on frequency response compared to the far-field case. The "time alignment" technique makes the implicit assumption of anechoic behavior, which breaks down for far-field subs. Near-field subs more closely approximate the anechoic case, making the assumptions underlying the "time alignment" procedure more accurate. Thus it's worth experimenting with the "time alignment" technique when using near-field subs.

The traditional solution to the "time alignment" problem has been to calculate the delay for each sub based on the speed of sound and the relative distance of each sub to the main listening position (MLP). The most distant sub to the MLP is assigned a delay of zero, and the other delays are calculated based on the difference in distance to the MLP between that of the most distant sub and each of the others. This calculation is best described in AustinJerry's guide titled "Using MiniDSP 2x4 to Time-Align Multiple Subs on One Channel before Room Correction". The title of the relevant subtopic is "Set Output Delays". This topic gives explicit instructions for how to compute each delay. After setting the delays according to the calculation, you may want to only allow the optimizer to make small delay adjustments, say a few msec, from the calculated delay values. Or, you might want to completely disallow the optimizer from changing the delays from these calculated values. In the latter case, you would use the Parameter Locking feature.

Example 2: Setting the Center Frequency Range for PEQ Filters

Once you have set the minimum and maximum optimization frequencies fmin and fmax respectively using Tools, Optimization Options, Criteria, you'll need to set the allowable center frequency range of the PEQ filters so the optimizer can do its job over the full frequency range you've specified.

For subwoofers, set the minimum PEQ center frequency to the minimum optimization frequency fmin as specified above. Set the maximum PEQ center frequency to twice the highest anticipated crossover frequency or 200 Hz, whichever is smaller.

For main speakers, also set the maximum PEQ center frequency to twice the highest anticipated crossover frequency or 200 Hz, whichever is smaller. Set the minimum PEQ center frequency to one-half the lowest anticipated crossover frequency or 20 Hz, whichever is larger.

Rearranging and Normalizing Gains and Delays

These commands help you achieve realizable gain and delay parameter values after performing an optimization.

Rearranging Gains and Delays

When using a conventional AVR or pre-pro with an add-on DSP device at its subwoofer output, the most practical and useful arrangement of sub gains and delays usually involves having a wide adjustment range for the shared delay and gain using the AVR's sub distance and gain trims respectively, and a narrower adjustment range for the individual sub gains and delays using the add-on DSP device. However, MSO has a restriction that when using N individual sub channels, you can have at most N gain blocks and N delay blocks. This could mean having e.g. N per-sub delay blocks as per Figure 3 of the section "Adding Filters, Delays and Gains" or 1 shared delay block and N-1 per-sub delay blocks as per Figure 2 of that same section. The latter case with a shared block is usually the desired one, but it can be challenging to figure out which individual sub channel should be the one without a gain or delay block. This problem can be solved by starting out with an arrangement like that of Figure 2 with no shared delay or gain block, then doing a Rearrange Delays or Rearrange Gains operation after the optimization is run. When using this method with delay blocks, allowing negative delays in each delay block's parameter value limits can be very useful to get the best possible results. A negative delay just means "increase the sub distance relative to the value that was used when the measurement was performed." Of course, this interpretation only makes sense if the delay is a shared one, acting as a stand-in for the AVR's sub distance setting. When doing the Rearrange Delays command in connection with negative delays, any negative delay is lumped into the shared delay block, which is the only place it makes sense. This allows for the appropriate change to the AVR's sub distance to be made, without needing a physically unrealizable negative delay in the add-on DSP device. When performed in succession, these operations will toggle the delay or gain block arrangement between that of Figures 2 and 3 linked above.

To perform these operations, go to the Config View and locate the Subwoofer Channels node of the configuration you wish to modify, then right-click and choose Rearrange Delays or Rearrange Gains from the context menu. These commands may not always be enabled in the menu. The following list describes the conditions required for them to be enabled.

Normalizing Gains and Delays

The Normalize Delays and Normalize Gains commands are similar to the Rearrange Delays and Rearrange Gains commands. These commands ensure that all per-sub gain blocks have an attenuation in dB that is positive or 0, and that all per-sub delay blocks have a delay that is positive or zero. They do this by lumping all positive gains into the shared gain block and all negative delays into the shared delay block. For sub-only configurations, no shared delay block is allowed, but Normalize Delays still works by adding the appropriate and identical delay to each sub channel until all delays are positive or zero. This can be done because only the subs' relative delays matter in a sub-only configuration. These commands can result in one of the per-sub gain or delay blocks shifting from one channel to another, so in connection with allowing negative per-sub delays or positive per-sub gains, they can be used to determine which individual sub channel should end up with no delay block, and which should have no gain block. This is done by running an optimization, then performing the Normalize Delays and Normalize Gains commands afterwards.

Similar to the Rearrange Delays and Rearrange Gains commands, the Normalize Delays and Normalize Gains commands are found on the context menu for the Subwoofer Channels node of the desired configuration you wish to modify via the Config View. The following list describes the conditions required for these commands to be enabled.

Double-Checking Parameter Value Limits

When rearranging or normalizing gains or delays, having equivalent arrangements of gain or delay blocks sometimes requires that the new computed value for one or more gain or delay parameters exceeds the limits you originally set for the parameter. In this case, MSO will widen the minimum or maximum limit for the affected parameter just enough to allow the relevant limit not to be exceeded. For this reason, it's important to check the delay and gain parameter limits of the potentially affected gain and delay blocks in the Properties Window after performing one of these commands to ensure the limits are still compatible with your hardware. This might sometimes require some iteration. This is especially true for the Rearrange Delays and Rearrange Gains commands, for which a gain or delay block can move around from a per-sub position to a shared position or vice versa. Usually, the allowable delay or gain range should be made wider for shared delay and gain blocks than for per-sub ones.

Saving Graph Traces as FRD Files

You can export any trace to FRD format for import into other programs. To do so, perform the following steps:

Saving Traces as FRD Files
Saving Traces as FRD Files

How Many Subs are Needed?

Earlier versions of the MSO documentation suggested that three or more subwoofers might be needed to get significant improvements with MSO. This appears to have been an overly cautious suggestion. The AVSForum user genesplitter has posted his MSO results with a two-sub system at five listening positions. I've summarized them for clarity below.

First, the data before using MSO:

Two Subwoofers - Before MSO
Two Subwoofers - Before MSO

Next, the data after using MSO:

Two Subwoofers - After MSO
Two Subwoofers - After MSO

Substantial improvements have been made at all listening positions. Based on this new information, usage of MSO can be recommended for systems having two or more subwoofers.