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# Sample Filters

Following the two-stage decimation procedure is an optional post-processing stage called the sample filter. This stage operates on each sample emitted by the second stage decimator, one at a time, before the samples are handed off for framing or transfer to the rest of the application’s audio pipeline.

Note

This is represented by the `SampleFilter` sub-component of the `MicArray` class template.

An application may implement its own sample filter in the form of a C++ class which implements the `Filter()` function as required by `MicArray`. See the implementation of `DcoeSampleFilter` for a simple example.

## DC Offset Elimination

The current version of this library provides a simple IIR filter called DC Offset Elimination (DCOE) that can be used as the sample filter. This is a high-pass filter meant to ensure that each audio channel will tend towards a mean sample value of zero.

### Enabling/Disabling DCOE

Whether the DCOE filter is enabled by default and how to enable or disable it depends on which approach your project uses to include the mic array component in the application.

#### Vanilla Model

If your project uses the vanilla model (see Vanilla API) to include the mic array unit in your application, then DCOE is enabled by default. To disable DCOE your build script must add a compiler option to your application target that sets the `MIC_ARRAY_CONFIG_USE_DC_ELIMINATION` preprocessor macro to the value `0`.

For example, in a typical application’s `CMakeLists.txt`, that may look like the following.

    # Gather sources and create application target
    # ...
    # Add vanilla source to application build
    mic_array_vanilla_add(my_app  ${MCLK_FREQ} ${PDM_FREQ}
                                ${MIC_COUNT} ${FRAME_SIZE} )
    # ...
    # Disable DCOE
    target_compile_definitions(my_app
        PRIVATE MIC_ARRAY_CONFIG_USE_DC_ELIMINATION=0 )

#### Prefab Model

If your project instantiates the `BasicMicArray` class template to include the mic array unit, DC offset elimination is enabled or disabled with the `USE_DCOE` boolean template parameter (there is no default).

    template <unsigned MIC_COUNT, unsigned FRAME_SIZE, bool USE_DCOE>
        class BasicMicArray : public ...

The sample filter chosen is based on the `USE_DCOE` template parameter when the class template gets instantiated. If `true`, `DcoeSampleFilter` will be selected as the `MicArray` `SampleFilter` sub-component. Otherwise `NopSampleFilter` will be used.

Note

`NopSampleFilter` is a no-op filter – it does not modify the samples given to it and ultimately will be completely optimized out at compile time.

For example, in your application source:

    #include "mic_array/mic_array.h"
    ...
    // Controls whether DCOE is enabled
    static constexpr bool enable_dcoe = true;
    auto mics = mic_array::prefab::BasicMicArray<MICS, FRAME_SIZE, enable_dcoe>();
    ...

#### General Model

If your project does not use either the vanilla or prefab models to include the mic array unit in your application, then precisely how the DCOE filter is included may depend on the specifics of your application. In general, however, the DCOE filter will be enabled by using `DcoeSampleFilter` as the `TSampleFilter` template parameter for the `MicArray` class template.

For example, sub-classing `mic_array::MicArray` as follows will enable DCOE for any `MicArray` implementation deriving from that sub-class.

    #include "mic_array/cpp/MicArray.hpp"
    using namespace mic_array;
    ...
    template <unsigned MIC_COUNT, class TDecimator,
              class TPdmRx, class TOutputHandler>
    class DcoeEnabledMicArray : public MicArray<MIC_COUNT, TDecimator, TPdmRx,
                                        DcoeSampleFilter, TOutputHandler>
    {
      ...
    };

### DCOE Filter Equation

As mentioned above, the DCOE filter is a simple IIR filter given by the following equation, where `x[t]` and `x[t-1]` are the current and previous input sample values respectively, and `y[t]` and `y[t-1]` are the current and previous output sample values respectively.

    R = 252.0 / 256.0
    y[t] = R * y[t-1] + x[t] - x[t-1]

### DCOE Filter Frequency Response

The plot below indicates the frequency response of DCOE filter DCOE filter frequency response.

Fig. 7 DCOE filter frequency response