FIR Filtering

FIR filtering is a powerful and advanced feature. It allows construction of complex arbitrary equalization and crossover filters with independent control of amplitude and phase.

FIR filtering overview

FIR ("finite impulse response") filtering differs from the IIR ("infinite impulse response") filters used in the PEQ and crossover blocks. While an IIR filter requires only a small number of filter coefficients, such as the five coefficients needed for a biquad, an FIR filter is specified by a large array of filter coefficients.

With the aid of suitable design software, FIR filtering allows very complex filters to be constructed. These filters can correct for amplitude only ("linear phase filters"), phase only, or a combination or both.

The coefficients of an FIR filter are often referred to as "taps." The miniDSP Flex Eight can compute a total of 4096 taps.

The FIR filter pane

Each input channel of the miniDSP Flex Eight has an FIR filter bank with a variable number of taps. Click on the FIR button to open the FIR filter settings window:

Enabling and disabling the filter

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

When the FIR filter is enabled, the graph is a solid colored line. When disabled, the graph is a dashed grey line.

Allocating taps

Taps are allocated to each channel according to the number of loaded taps. The miniDSP Flex Eight can compute a total of 4096 taps. These can be distributed as you wish across the two input channels, with the limitation that each channel must have 6 or more taps and can have no more than 2048 taps.

The current number of taps allocated to each channel, along with the number of taps still available for allocation, is shown at the lower right of the FIR filter window:

Loading filters

Filter filters are loaded in the area to the lower left of the FIR filter window.

File mode

In File Mode, a filter coefficient file generated by an FIR filter design program is loaded. The coefficient file must use IEEE 754 single-precision binary floating-point format.

To load coefficients, click Browse, navigate to the file containing the filter coefficients, and open it. The first few coefficients will be displayed and the buttons will change:

The displayed graph will also change to reflect the response of the loaded file.

• To accept the loaded filter, click APPLY.

• To cancel the operation, click UNLOAD FIR.

Manual mode

In Manual Mode, the FIR filter coefficients are pasted directly into the entry box in plain text, as shown below.

The pasted coefficients must be plain text in this format:

b0 = 1
b1 = -1
b2 = 0.5
b3 = -0.5
b4 = 0.2
b5 = 1

To set coefficients:

1. Cut and paste the coefficients from the text output of the design program.

2. Press the PROCESS INPUTS button.

3. Confirm that the frequency response graph is as you expect.

4. Press APPLY. This will write the coefficients into the processor's memory.

5. To clear the filter coefficients, click CLEAR TAPS and then APPLY.

Jumping between channels

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

FIR filter design software

The filter coefficients must be created with the aid of filter design software. miniDSP does not provide any such software, instead referring you to the many software packages available for this purpose (both freeware and commercial). The design program must be set for a 96 kHz sample rate.

See the following pages on our website for more information:

• FIR filter tools gives a list of FIR filter tools.

• The rePhase FIR tool gives worked examples using the rephase design tool.

Tips

Tip

If, after selecting a filter file or setting coefficients, the frequency response graph does not change as expected, make sure that the Enable FIR switch is turned on.

Tip

If you zero all filter taps, make sure that you also disable the filter, otherwise there will be no audio through that channel.