Filter Adjustment

Having obtained a set of measurement data for a channel it is time to adjust the EQ filters to counter the effects of resonances. Before starting this, you may wish to establish communications with the equaliser or TAG McLaren AV processor as explained in the linked sections below (you can skip this step if entering filter settings manually).

Communicating with a TAG McLaren AV Processor

REW can communicate with AV32R or AV192R processors to retrieve and send settings (see Communicating with AV32R DP or AV192R).

Communicating with a BFD Pro

REW can communicate with a BFD Pro via a Midi interface to send filter settings (see Communicating with the BFD Pro). Select the Midi port in the Comms settings and connect to the BFD Pro using the Midi interface.

Setting the Target Level

The Target Level for the measurement must be set using the button in the Target Settings pane in the Measurements Panel. This will provide a reference level against which to compare the results of measurements, before running Set Target Level... make sure the Speaker type has been set correctly as this determines the type of calibration signal which is used.

The Target Level is important for getting the right correction filter gains and widths. When measuring the subwoofer, the target level will usually measure higher than it should be due to the effect of the room's resonances, after making a measurement of the subwoofer's frequency response you can manually reduce the target level setting to something that looks more sensible, or go through the filter setting process, apply the filters to correct the peaks to your initial target, then re-measure the target level to get a more accurate figure - use the new level to repeat the filter setting process.

Adjusting Filter Settings

The sample data below will be used to provide an example. The blue curve is the Target Response which in this case is for a Bass Limited speaker with a 12dB/octave slope.

Sample Measurement Data

Find the Peaks

First we need to locate the peaks in the measurement. For this we use the Find Peaks action in the Filter Tasks pane, which searches within the frequency range specified and generates a table of the peaks found, sorted by the amplitude of the peaks relative to the target level.
Find Peaks

If the Target Level has been set during the measurement process then the Target Response will be at the correct level to act as a reference for identifying the peaks. If not, the level should be set before proceeding, either via the Set Target Level... action in the Target Settings pane or by manually adjusting the level until the target response appears to be at the level this channel would have without the influence of the peaks and dips contributed by the room. The default target shape is a flat line at the Target Level, alternative shapes may be specified by loading a House Curve data file.

Find Peaks comes up with the following results:
Peaks Found

Assign Correction Filters

The next step is to use Assign Filters to configure peaking filters to counter the resonances that produce the peaks. Filters are assigned in order until either all peaks have been assigned a filter or all available filters have been used. Only filters with "Control" set to "Automatic" are used.

The results of that inital assignment are shown below, the corrected trace (dotted red) shows the predicted effect of the filters. The filter numbers are shown along the top of the graph. The Invert Filters option has been used to draw the filter responses inverted to more easily see their match to the peaks they are assigned to correct.

Initial Assignment of Filters

Initial Assignment of Filters

Before starting the filter optimisation the centre frequencies should be compared to the measurements and adjusted if necessary - REW rounds the filter frequencies to the nearest frequency the equaliser supports and makes allowance for characteristics of the response that may indicate that the resonance is actually offset slightly compared to the position of the peak on the graph, but this may result in a centre frequency a little higher or lower than ideal.

Optimise the Filter Settings

Next Optimise PK Gain and Q is used to adjust the filter settings to bring the Corrected trace closer to the Target Response. REW iteratively adjusts the gain and Q settings of the filters to obtain the best match to the shape of the peaks and hence the flattest corrected response. The optimiser tries to match the shape over a region either side of each peak's center whose boundaries are determined by the characteristics of the peak and the nature of the frequency response nearby. Here are the results:

Result of Optimising Filters

Filter Settings after Optimisation

Note the following:

Manually Tweak Settings as Required

The filter settings can be manually adjusted to improve the correction, typically adjusting the Q values and using Adjust PK Gains to readjust the gains to suit. The graph traces update as the adjustments are made making the process fast and easy. Adjust PK Gains alters the gains of all Automatic filters to bring the response at their centre frequencies to the target level. Q is not altered.

Transfer the Settings to the Equaliser

Having arrived at a set of filters to use for this measurement, they can be transferred to the AV processor or equaliser (e.g. BFD Pro) using the Send Filter Settings to Equaliser (Ctrl+Shift+F) entry in the Equaliser menu. Note: This will overwrite any existing filter settings. The measurement can then be repeated with the filters active to check the effect they have had, or you can move on to measuring and correcting the another speaker.

Notes

  1. After applying correction filters it may be necessary to adjust the trim for the speaker that has been corrected as removing resonant peaks will reduce the overall SPL. This is almost always necessary for the subwoofer, but less so for the other speakers if the corrections are all below 200Hz. Carry out the normal speaker trim calibration using your AV processor's internal test tones.
  2. Repeating Set Target Level after applying correction filters can give a more accurate figure, especially for the subwoofer. That new Target Level can be used with the original measurement to repeat the process of finding peaks and adjusting filters, which will typically result in slightly different gains and bandwidths for the corrections.
  3. Filters which are disabled will download to the BFD Pro DSP1124P with their frequency, gain and bandwidth settings as configured, but the filter mode will be set to off (OF) rather than parametric (PA). On FBQ2496 the gain will be set to zero.
  4. Downloading a set of filters to DSP1124P takes about 5 seconds, downloading to FBQ2496 takes about 1 second per filter, 20 seconds for the full set.

Save the Settings

Once you have arrived at filter settings for a measurement it is advisable to use Save Filters to store them as a .req file.

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