Crossover

Crossovers "split" the frequency band to send to different drivers. In a two-way loudspeaker, for example, a low pass filter is used to remove high frequencies from the signal sent to the woofer, and a high pass filter is used to remove low frequencies from the signal sent to the tweeter.

When integrating a subwoofer, high pass filters are used on the speakers and a low pass filter on the subwoofer.

Crossover filters can also be used to limit low frequency content delivered to a speaker or subwoofer, to help protect it from over-excursion.

Unlike conventional analog crossovers, the flexibility of DSP allows a completely arbitrary mix of different filter slopes and types. Filters can be set at any frequency or disabled completely. This allows maximum flexibility in matching your crossover to the acoustic characteristics of the loudspeaker drivers.

Overview

Each output channel has independent high pass and low pass crossover filters. Click on the CROSSOVER button to open the crossover settings window:

The current channel is displayed in orange, with the others displayed in grey. Hovering the mouse over the curve brings up an overlay showing the frequency and the attenuation at that frequency.

Info

The channel names on the right hand side may be out of order. This is an unavoidable artifact of the library used to draw the graphs: the graph that displays on top also comes first in the list.

Crossover mode

The crossovers on each output channel can be set in either basic mode or advanced mode. Basic mode allows each crossover filter to be specified simply by providing two parameters, while advanced mode requires the use of a separate design program.

Basic mode

In basic mode, there are two crossover filters on each output channel: high pass (removes low frequencies) and low pass (removes high frequencies). In basic mode, these two filters are completely independent and each has its own settings.

Enable

The crossover filters are disabled by default. To enable a crossover filter, click on the Enable switch.

Cut-off frequency

Sets the nominal cutoff frequency of the crossover. In actual fact, the crossover has a more or less gradual transition from "full on" to "full off," as determined by the filter slope.

Filter type

Selects the type and slope of the filter. The steeper the slope, the more quickly frequencies above or below the cutoff frequency are attenuated. There are three types of filter:

Butterworth (BW)

Available in 6, 12, 18, 24, 30, 36, 42, and 48 dB/octave, Butterworth crossover filters are 3 dB down at the cutoff frequency.

Linkwitz-Riley (LR)

Available in 12, 24, and 48 dB/octave, Linkwitz-Riley crossover filters are 6 dB down at the cutoff frequency.

Bessel

Available in 12 dB/octave only, a Bessel filter gives a more gradual roll-off through the crossover region.

Keyboard and mouse operations

Except for filter type, parameters can be edited by:

1. Clicking and dragging on a slider. Once the focus is in a value field, you can also change the value with the left and right arrow keys on the keyboard, or by using the mouse scroll wheel or equivalent gesture on a trackpad.
2. Clicking in the value field and typing a new value. Once the focus is in a value field, you can also change the value with the up and down arrow keys on the keyboard, or by using the mouse scroll wheel or equivalent gesture on a trackpad.
3. If the focus is on the Enable button, it can be toggled with the Space key.
4. Move the mouse cursor over the parameter and use the mouse scroll wheel or equivalent gesture on a trackpad.

The Tab key will move focus between fields; Shift-Tab will move in the opposite direction.

Jumping between channels

To jump quickly between the crossover window of different channels, enable the Menu switch at the top right. Jump to a different channel by clicking on the row of buttons:

Linking channels

Each channel can be linked to one other channel. When a channel is linked to another, the crossover settings of that channel are mirrored to the other. Typically, the corresponding drivers on the left and right channels are linked: left and right tweeter, left and right woofer, and so on.

To link a channel, enable the Menu switch at the top right. Click on the channel that you want to link the current channel to. After confirming, an indicator appears to show the new link:

Info

A channel can be linked to only one other channel. That is, you cannot link three or more channels.

Hiding channels

By default, the crossover graph shows the crossovers of all four output channels. The current channel is displayed in orange and the others in black.

If you wish to view only a subset of channels, use the switches to the right of the graph to select channels to hide. In this example, channel 2 is the current channel. Channels 1 to 3 are visible, while channels 4 to 8 are hidden:

The crossover settings of channels 1, 2 and 3 can be changed by using the channel selection buttons at the top of the window to jump between them. As long as you stay on these channels, the set of hidden channels stays the same. However, if you switch to a hidden channel, that channel then becomes visible. (For example, if you switch to channel 6, then channels 1, 2, 3 and 6 will be visible.)

Advanced mode

The crossover block on each output channel has eight biquads. In Basic mode, four are allocated to the high pass filter and four to the low pass filter. In Advanced mode, however, the eight biquads are all specified by their coefficients, in the following format:

biquad1,
b0=0.998191200483864,
b1=-1.9950521500467384,
b2=0.996920046761057,
a1=1.9950521500467384,
a2=-0.9951112472449212,
biquad2,
b0=0.999640139948623,
b1=-1.9981670485581222,
b2=0.9985489719847982,
a1=1.9981670485581222,
a2=-0.9981891119334211,
biquad3,
..
biquad4,
..
biquad8,
b0=1.0010192374642126,
b1=-1.9950555192569264,
b2=0.9940580112181501,
a1=1.995060938714333,
a2=-0.9950718292249559

These biquads can be used for almost infinite flexibility (within the limits of eight biquads). For example, you could use two biquads for a fourth-order high pass filter, and the remaining six for parametric EQ.

Setting biquads for the crossover block

Paste the coefficients into the text box, then click on the PROCESS AND APPLY button for them to take effect.

Info

Switching between basic and advanced mode does not convert between Frequency/Filter type and biquad parameters. The basic parameters and advanced biquad coefficients for each filter are stored independently and the selected set is used for processing.

Biquad design software

Following are programs that can be used to design your biquad coefficients. The design program must be set for a 96 kHz sample rate.

Biquad calculation spreadsheet

The community-developed biquad calculation spreadsheet allows many filter types to be calculated, including notch filters, Linkwitz transforms, and filters with arbitrary Q-factor. Access this spreadsheet here (requires Microsoft Excel):

• Download zipped spreadsheet from miniDSP.com

Room EQ Wizard (REW)

Room EQ Wizard is a free acoustic measurement and analysis tool, available for Windows, Mac and Linux platforms. It includes the ability to automatically generate a bank of parametric EQ biquads based on a measurement. These coefficients can be saved to a file from REW and loaded using the Device Console. Room EQ Wizard can be downloaded from:

• Room EQ Wizard downloads

For guidance on using this feature, refer to the app note Auto EQ with Room EQ Wizard.