In this app note we will show you how to equalize a surround sound or home theater system using the miniDSP 10x10 Hd. This unit has eight channels of analog input, eight channels of analog output, one stereo digital input, and one stereo digital output. Whether you have a traditional 4.0 quadraphonic surround sound setup, a 5.1 setup, or a 7.1 setup, the 10x10 Hd will fit into your system.

• What you will need
• 1. Getting set up for measurement
• 2. Set up your routing
• 3. Establish your baseline
• 4. Equalize the subwoofer and speakers
• 5. Apply bass management
• Advanced routing

What you will need [Top]

• A miniDSP 10x10 Hd. Alternatively, if you are into DIY, you can use the 8x8 kit.

• The miniDSP 10x10 plugin software.

• Room EQ Wizard (REW). Be sure to download the latest version from the Downloads Area for UMIK-1 support.

• A miniDSP UMIK-1. After receiving the UMIK-1, go to the UMIK-1 page to get the calibration file for your unique serial number, and save it as a .TXT file.

• A microphone stand with boom arm, available in music supply stores. (While the UMIK-1 is supplied with a small "table top" stand, best results will be obtained if the microphone is placed in free space, away from sofas or tables.)

Note: if you already have a measurement microphone and/or measurement software, those can be used instead of the UMIK-1 and REW. The auto-EQ function will only work on REW though.

1. Getting set up for measurement [Top]

The diagram below illustrates the setup used for acoustic measurement. Typically, running the computer measurement signal through the 10x10 Hd will require disconnecting the analog connections from the source and connecting the computer output to the appropriate 10x10 Hd input.

Setting up the computer to use REW and the UMIK-1 is covered in the following app note:

• UMIK-1 setup with REW

2. Set up your routing [Top]

The miniDSP 10x10 HD has three screens that are used to configure all of its settings, called Inputs, Routing, and Outputs. To begin with, let's just give all of our channels useful names. On the Inputs screen, click on the name of each input channel and type in a new name. Here is a typical setup for a 7.1 system:

Do the same for the Outputs screen:

On the Routing screen, use the On/Off buttons to set up which input channels are routed through to which output channels. To start with, pass each input straight through to the corresponding output, like this:

In the above example, we also set the digital inputs to pass through to the left and right output channels, which is appropriate if you wish to play stereo content through the digital input.

3. Establish your baseline [Top]

Use REW to measure the in-room frequency response of each of your speaker and subwoofer channels, one at a time. Position the microphone at the center of the listening area, and for best results and consistency, point the UMIK-1 towards the speaker being measured.

You will end up with a set of frequency response measurements. Use the REW Overlays window and 1/3rd, 1/6th, or 1/12th octave smoothing to view them. Here is an example showing the left channel in red, the center channel in blue, and the subwoofer in green. (The other speakers are omitted from the example to avoid cluttering the graph.)

Performing this baseline measurement ensures that your setup is ready to proceed with equalization. In some cases, the baseline measurements may reveal issues that will need to be corrected before proceeding. Remember also that the subwoofer response will vary a lot with the location of the subwoofer. Experiment with different subwoofer locations in order to get the best response prior to equalization.

You may also wish to try making additional measurements with the microphone in different locations around your seating area. This is a "sanity check" so that you can understand how the frequency response varies around the lsitening area. It can help you to ensure that you are not equalizing for issues that vary a lot over the seating area and are not "over-equalizing."

4. Equalize the subwoofer and speakers[Top]

Start by equalizing the subwoofer. This is most easily and quickly done using the REW auto-EQ function. The method for doing this is described in the app note:

• Auto-EQ tuning with REW

For the speaker channels, you may be interested in trying manual equalization, where you set the EQ settings directly in the user interface. It is more time-consuming but gives you more control over the final result.

The sequence when doing manual equalization is 1. measure, 2. perform an EQ adjustment, 3. measure again. Of the various types of EQ available in the 10x10 plugin, there are two that we suggest as the ones to start with. The first is the PEAK type filter with a negative gain, used to "flatten" peaks in the response. Here is an example setting:

The second is shelving filters. These are very useful for "shaping" the response of your speakers, as they elevate or lower the whole spectrum above or below the specified frequency. For example, you can use a HIGH_SHELF filter to create a gentle high-frequency slope that adjusts the "tilt" of the in-room frequency response curve:

You may find that the left and right speakers measure a bit differently. In that case, you will need to pick a compromise. Overall, you are looking to get to an in-room curve that is reasonably smooth, and is consistent between all speakers. The in-room curve should typically have a gentle slope down in the treble (sometimes called a "house curve").

At this time, you will also want to roughly match the levels of the subwoofer and the other speakers. This is easily done using the output gain controls (it's usually better to use negative gain than positive). Here are the left, center, and subwoofer channels of our example system after completing this process:

Here are some additional guidelines when equalizing the speaker channels:

• Avoid using boost filters (filters with positive gain). As a general rule, negative gain is preferred as it ensures that clipping will not be introduced.

• If boost filters are used, they should have a fairly low Q (1 or less) and a moderate amount of gain (4-5 dB max). Don't try and use a narrow boost to fill in a deep notch in the response.

• View measurements at 1/3rd or 1/6th octave smoothing as a general rule. Switch to 1/12th octave smoothing to check that you are not attempting to correct a narrow peak with a broad filter.

• Even with negative gain, use high-Q filters (Q>4) carefully if at all.

• Link the PEQ filters for the various left and right channels (generally, it's best to avoid using different EQ settings for the left and right speakers):

5. Apply bass management [Top]

Bass management is used to direct low-frequency content to the subwoofer or subwoofers. For example, if the center and surround speakers are not able to produce low frequency content, they can be routed to the subwoofer with the Routing matrix, like this:

The subwoofer output channel must have a steep low pass filter applied (typically at 80 Hz) to remove all high-frequency content. Usually, high pass filters will also be used on the bass-managed channels to remove low-frequency content from them. The signal flow path being implemented with this routing and is illustrated in this diagram (EQ is left off the diagram to simplify it):

Here are the left and center channels of our example system after applying bass management, annotated to show our "house curve":

For more detailed information on bass management and how to apply it, see the related app note Bass management with the nanoAVR. Be aware that summing multiple channels can potentially cause clipping of the signal. If you detect clipping, reduce the input gains and if necessary, increase output gains on selected channels.

Advanced routing [Top]

More advanced routing configurations than those described so far can be set up in the Routing matrix. There are dozens if not hundreds of possible combinations, so we will illustrate just a few here.

Re-routing LFE content

This is like bass management "in reverse". In a system in which there is no subwoofer, but the left and right front speakers are able to produce low frequencies, the LFE channel input signal can be routed to those speakers like this:

(Adjust the level of the LFE content using the input gain control on that channel. Note that it is not possible to use this method to send low-frequency content from the center and surrounds to the front left and right speakers.)

Multiple subwoofers

If you have unused output channels (e.g. you are running 4.x or 5.x), then you can use those spare channels to independently equalize and delay multiple subwoofers. Here is an example routing matrix with three independent subwoofer output channels:

To send low-frequency signals from other channels to these subwoofers (for bass management), turn on the routing to those subs as described above and add high pass filtering as appropriate.

Active crossovers

Above we assumed that each output channel was driving a full-range speaker. The 10x10 Hd can also be used to implement active crossovers, in any combination within the limits of the number of output channels (eight analog + two digital). For example, here is a routing matrix that implements two-way active crossovers on the front left and right speakers of a 5.1 system:

You can of course combine this with bass management by sending some of the input channels to the subwoofer output channel. Note also that in this example, the left and right rear inputs (if used) are routed to the left and right surround outputs.

With use of the digital outputs, a total of ten output channels are available, leading to even more advanced configurations. Here is a routing matrix for two-way active crossovers on the front left, front right, and center, with two separate subwoofer outputs on the digital outputs (just add an inexpensive DAC):

Here is a routing matrix for a 4.0 system with two-way active speakers on each of the four main channels, as well as two derived subwoofer output channels:

If you need more output channels, you can use two (or more!) 10x10 Hd units. For example, a pair of 10x10 Hd units would allow for a three-way active crossover on the front left and right, two-way crossovers on the center and four surround channels, and a number of LFE/sub channels. (Note: only one miniDSP can be connected to the computer for programming/configuration at a time.)

For more information on setting up active crossovers, see these App notes:

• Building a 2way crossover
• Stereo 3/4 way Crossover