Parametric EQ

Parametric equalization (PEQ) is a flexible type of equalization filter. It can be used to correct for errors in loudspeaker output, to compensate for acoustic room effects, and to tailor the overall system response for best sound.

Overview

Click on the PEQ button to open the parametric equalizer settings window:

The miniDSP DDRC-24 has 10 parametric EQ filters on each output channel. The graph displays the response of each filter in a light color and the combined response of all enabled filters as a heavy line. For example, the curve shown above has a low-shelf boost at 100 Hz, a dip at 500 Hz, and a high-shelf cut at 5000 Hz.

Editing filters

Change the frequency and gain of each filter by clicking and dragging on the drag-points (colored circles) on the graph. The full set of parameters can be edited as described below. Hovering the mouse over any curve brings up an overlay showing the frequency and the gain at that frequency.

Disabling individual filters

All 10 filters are enabled by default. To disable a filter, select it with the row of buttons. Click on the Enable button to turn it off.

Note that all other filters are still operational unless individually bypassed. A filter will also have no effect if its gain is set to 0.0.

Filter mode

The parametric EQ filters can be set in either basic mode or advanced mode. Basic mode allows each filter to be specified simply by providing four parameters, while advanced mode requires the use of a separate design program.

Basic mode

The shape of the parametric EQ filters is set with a small set of intuitive parameters.

Frequency

For PEAK filters, this is the center frequency of the peak or dip. For HIGH_SHELF and LOW_SHELF filters, this is the frequency at which the gain is half of the set value. For ALL_PASS filters, this is the center frequency of the phase shift.

Gain

For PEAK filters, this is the gain in dB at the center frequency. For HIGH_SHELF and LOW_SHELF filters, this is the gain in dB reached at high or low frequencies respectively. A filter has no effect if its gain is set to 0 dB. Gain can be adjusted in increments of 0.1 dB up to +/- 16 dB. This item is not present for the ALL_PASS filter type.

Q

Q controls the "sharpness" of the filter. For PEAK filters, lower Q gives a broader peak or dip, while higher Q gives a narrower peak or dip. For HIGH_SHELF and LOW_SHELF filters, Q controls how quickly the filter transitions from no gain to maximum gain. For ALL_PASS filters, higher Q gives a steeper phase transition.

Filter type

Selects the type of filter:

PEAK

Create a dip or a peak in the frequency response.

LOW_SHELF

Reduce or increase part of the frequency spectrum below a given frequency.

HIGH_SHELF

Reduce or increase part of the frequency spectrum above a given frequency.

ALL_PASS

Create a phase shift across the frequency band. This can be useful for correcting phase issues and for simulating analog crossovers.

Keyboard and mouse operations

Except for filter type, parameters can be edited by:

1. Clicking and dragging on a slider. Once the focus is on a slider, you can also change the value with the left and right arrow keys on the keyboard.
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.
3. 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 parametric EQ 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 PEQ settings of that channel are mirrored to the other. Typically, corresponding channels on the left and right are linked: for example, left and right tweeter and left and right woofer.

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.

Reset to default

To reset all parametric EQ bands of the current channel to the default settings, enable the Menu switch at the top right and click the RESET TO DEFAULT button:

Warning

This operation is not reversible, so be sure you mean to reset all bands! Save your settings to a file first if you need to keep any.

Advanced mode

Advanced mode enables custom biquad programming for almost infinite flexibility in filter implementation. In this mode, each individual filter to be specified by its biquad coefficients. The coefficients must be generated by a design program such as Room EQ Wizard (REW). The design program must be set for a 48 kHz sample rate.

Programming a single biquad

Each parametric EQ filter can be individually switched to advanced mode. Paste in the coefficients in the format shown and click on the PROCESS AND APPLY button for them to take effect. Click the RESET button to set the coefficients back to their default values.

Info

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

Loading a biquad file

Multiple biquads can be set at once by loading a file containing biquad coefficients. This example illustrates the correct file 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,
..
biquad10,
b0=1.0010192374642126,
b1=-1.9950555192569264,
b2=0.9940580112181501,
a1=1.995060938714333,
a2=-0.9950718292249559

To load the file, select the Menu switch at the top right and click the LOAD BIQUADS FILE button. The filters are set to advanced mode starting at EQ1.

If the loaded file contains fewer than 10 biquads, then only that number of EQ filters will be set to Advanced mode. For example, if loading a file with six biquads, EQ1 to EQ6 will be set to Advanced mode, and EQ7 to EQ10 will not be changed. This can be used to good effect in cases where manually-set parametric EQ (in Basic mode) is required in addition to parametric EQ developed with a design program.

If the loaded file contains more than 10 biquads, then only the first 10 will be loaded.

Biquad design software

Following are programs that can be used to design your biquad coefficients. The design program must be set for a 48 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.