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.

Default model¶

DCOE is enabled by default in mic_array_conf_default.h. To disable, override MIC_ARRAY_CONFIG_USE_DC_ELIMINATION through mic_array_conf.h file in the application or in the application’s CMakeLists.txt.

Advanced usage model¶

If the project does not use the default models to include the mic array unit in the application, then precisely how the DCOE filter is included may depend on the specifics of the 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.

../../../_images/dcoe_freq_response.png — Fig. 8 DCOE filter frequency response¶