dc_elimination.h

void dcoe_state_init(dcoe_chan_state_t state[], const unsigned chan_count)

Initialize DCOE states.

The DC offset elimination state needs to be intialized before the filter can be applied. This function initializes it.

For correct behavior, the state vector state must persist between audio samples and is supplied with each call to dcoe_filter().

Parameters

  • state – [in] Array of dcoe_chan_state_t to be initialized.

  • chan_count – [in] Number of elements in state.

void dcoe_filter(int32_t new_output[], dcoe_chan_state_t state[], int32_t new_input[], const unsigned chan_count)

Apply DCOE filter.

Applies the DC offset elimination filter to get a new output sample and updates the filter state.

For correct behavior, this function should be called once per sample (here “sample” refers to a vector-valued quantity containing one element for each audio channel) of that stream.

The index of each array (state, new_input and new_output) corresponds to the audio channel. The update associated with each audio channel is independent of each other audio channel.

The equation used for each channel is:

y[t] = R * y[t-1] + x[t] - x[t-1]

where t is the current sample time index, y[] is the output signal, x[] is the input signal, and R is (255.0/256).

To filter a sample in-place use the same array for both the new_input and new_output arguments.

Parameters

  • new_output – [out] Array into which the output sample will be placed.

  • state – [in] DC offset elimination state vector.

  • new_input – [in] New input sample.

  • chan_count – [in] Number of channels to be processed.

struct dcoe_chan_state_t
#include <dc_elimination.h>

DC Offset Elimination (DCOE) State.

This is the required state information for a single channel to which the DC offset elimination filter is to be applied.

To apply the DC offset elimination filter to multiple channels simultaneously, an array of dcoe_chan_state_t should be used.

dcoe_state_init() is used once to initialize an array of state objects, and dcoe_filter() is used on each consecutive sample to apply the filter and get the resulting output sample.

DC offset elimination is an IIR filter. The state must persist between time steps.

Typical users of lib_mic_array will not need to directly use this type or any functions which take it as a parameter.

Use in lib_mic_array

The C++ class template mic_array::DcoeSampleFilter, if used in an application’s mic array unit, will allocate, initialize and apply the DCOE filter automatically.

The MicArray prefab mic_array::prefab::BasicMicArray has a bool template parameter USE_DCOE which indicates whether the mic_array::DcoeSampleFilter should be used. If true, DCOE will be enabled.

With MicArray Prefabs

For more information about MicArray prefabs, see [../../getting_started.html].

When using the ‘vanilla’ API, DCOE is enabled by default. To disable DCOE when using this API, add a preprocessor definition to the compiler flags, setting MIC_ARRAY_CONFIG_USE_DC_ELIMINATION to 0.

With Vanilla API

For more information about the vanilla API, see [../../vanilla_api.html].

Public Members

int64_t prev_y

Previous output sample value.