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9−1

9 Automatic Loudspeaker Equalizer

To simplify the process of designing digital filters for implementation in the TAS3001, Texas Instruments provides a

filter design tool called the Automatic Loudspeaker Equalizer or ALE. Details of this tool are provided in this section.

9.1

Automatic Generation of Equalization Filters Given a Measurement

ALE has the capability of reading in a speaker or environment measurement. The user can then provide the desired

speaker response. From these two inputs, ALE can automatically produce an optimized set of filters to equalize the

speaker. The patent-pending optimization algorithm was developed at Texas Instruments. An example of its operation

is shown in Figure 8−3 through Figure 8−6.

9.2

Automatic Approximation of an Equalization Curve

When the desired equalization transfer function is known, the problem becomes one of finding a set of digital

second-order IIR filters to implement it. In the case where H(s) is known, it can be converted to a digital filter using

a bilinear or impulse invariant method. The resulting transfer function (H(z) can then be factored and implemented

as a combination of second- and first-order sections using the TAS3001. (This factoring capability is not included in

ALE, but can be accomplished simply in Matlab.) However, in many cases the desired transfer function can be

specified as a curve. In this case, the curve can be read by ALE. Then an approximation to it can be generated

manually (see the Manual Filter Generation section below) or automatically. In the automatic mode, ALE again uses

the patent-pending optimization technique developed at Texas Instruments.

9.3

Manual Filter Design

In many cases, the user already has filter specifications and just needs a way of designing such filters. In such cases,

ordinary filter parameters, such as gain, bandwidth, and center frequency can be entered into ALE, which then

provides coefficients for the desired filters. As filters are generated, ALE displays the responses of individual filters

and the composite filter response. In addition, the manual filter generation capability can be used in conjunction with

the automatic capability (see Section 9.2, Automatic Approximation of an Equalization Curve). The user can specify

filters manually and allow ALE to optimize them and generate additional filters automatically as needed. The user

can also specify manual filters and let ALE automatically generate additional filters as needed but without disturbing

the manually-generated filters. In addition, the user can manually modify the filters created by ALE. A graphical user

interface allows easy manipulation of filters and movement between automatic and manual filter generation modes.

9.4

Conversion of Decimal Filter Coefficients to the TAS3001 Format

The filters generated in ALE can be saved in the TAS3001 hexadecimal format (4 integer bits, 20 fraction bits, 2’s

complement). ALE can read in filters in decimal and convert them to the hexadecimal format needed for

implementation in the TAS3001.

9.5

Editing of TAS3001 File Format

ALE can read and write filter files in the TAS3001 format. This allows the filters generated to be stored in the format

used by the TAS3001 control software. In addition, ALE can read TAS3001 filter files and display the filters.

9.6

Examples of the Filter Types Available From ALE 2.1

At the current time the following filter types can be designed by ALE:

Equalization (bell-shaped) filters with a variety of center frequencies (Qs) and gains, as shown in Figure 9−1.

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