.. include:: ../../README.rst

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Overview
--------

Introduction
............

This demo application shows the minimum code to interface to the microphone array.

Block diagram
.............

.. figure:: block_diagram.pdf
   :width: 80%

   Application block diagram

How to use lib_mic_array 
------------------------

The Makefile
............

To start using the  microphone array library you need to 
add ``lib_mic_array`` to you Makefile::

  USED_MODULES = .. lib_mic_array ...

Includes
........

This application requires the system headers that defines XMOS xCORE specific
defines for declaring and initialising hardware:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: include <platform.h>
   :end-on: include <string.h>

The microphone array library functions are defined in ``lib_mic_array.h``. This header must
be included in your code to use the library. The support functions for the board are defined in
``mic_array_board_support.h`` and the logging functions are provided by ``debug_print.h``.

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: include "mic_array.h"
   :end-before:  in port p_pdm_clk

Allocating hardware resources
.............................

A PDM microphone requires a clock and a data pin. For eight PDM microphones a single 
clock can be shared between all microphones and the data can be sampled on a single 
8 bit port. On an xCORE the pins are controlled by ``ports``. The application therefore 
declares one 1-bit port and one 8-bit port:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: in port p_pdm_clk
   :end-on: in buffered port:32 p_pdm_mics

To generate the PDM clock a 24.576MHz master clock is divided by 8 using a clock block. These two hardware
resources are declared with:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: in port p_mclk 
   :end-on:  clock pdmclk      

and are configured with:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: configure_clock_src_divide(pdmclk
   :end-on:  start_clock(pdmclk);     

The result begin a 3.072MHz PDM clock is used for clocking the microphone data into the xCORE.


Task setup
----------

The PDM microphones interface task and the decimators have to be connected 
together and to the application (``example()``). There needs to be one 
``mic_array_decimate_to_pcm_4ch()`` task per four channels that need processing. 
The PDM interface task, ``mic_array_pdm_rx()`` can process eight channels so only 
one is needed for this application. The PDM interface needs to be connected to the two 
decimators via streaming channels. Finally, the decimators have to be connected to 
the application.

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: streaming chan c_pdm_to_dec
   :end-before: return 0;

Note that the decimators have to be on the same tile as the application due to shared frame memory.	

Frame memory
------------

For each decimator an block of memory must be allocated for storing FIR data. The size of the data 
block must be::

  Number of channels for that decimator * THIRD_STAGE_COEFS_PER_STAGE * Decimation factor * sizeof(int)

bytes. The data must also be double word aligned. For example:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: int data[
   :end-before: void example
  
Note that on the xCORE-200 all global arrays are guaranteed to be double-word aligned.

Configuration
-------------

Configuration of the microphone array for the example is achieved through:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: mic_array_decimator_conf_common_t dcc
   :end-before:  mic_array_init_time_domain_frame

All configuration options are described in the Microphone array library guide. Once configured 
then the decimators require initialization via:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: mic_array_init_time_domain_frame
   :end-before: while(1)

The decimators will start presenting samples in the form of frames that can be accessed with:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: mic_array_frame_time_domain *  current =
   :end-on: mic_array_get_next_time_domain_frame

The return value of ``mic_array_get_next_time_domain_frame()`` is a pointer to the frame that 
the application is allowed to access. The ``current`` frame contains the frame data in the 
``data`` member. ``data`` is a 2D array with the first index denoting the channel number and 
the second index denoting the frame index. The frame index used 0 for the oldest samples and 
increasing indices for newer samples.

Changing the sample rate
........................

The sample rate chane be change easily with the example code by modifying:

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: #define DECIMATION_FACTOR
   :end-before: #define DECIMATOR_COUNT 

The supported ``DECIMATION_FACTORS`` that come as standard from ``lib_mic_array`` are 2, 4, 6, 8 and 12. These
correspond to 48kHz, 24kHz, 16kHz, 12kHz, 8kHz and 6kHz. In order to change the define successfully you must
also ensure that the coefficients to the decimators are correct for the selected decimation factor.
The coefficients are declared in the header ``fir_coefs.h`` which is included in ``mic_array.h``. The ``coefs``
member of ``mic_array_decimator_config_common`` must match the ``output_decimation_factor`` member. Also,
the FIR compensation must be made to match. For example, to change to 24kHz output, the config should look like::

       mic_array_decimator_config_common dcc = {
		   0, // Frame size log 2 is set to 0, i.e. one sample per channel will be present in each frame
		   1, // DC offset elimination is turned on
		   0, // Index bit reversal is off
		   0, // No windowing function is being applied
		   4,// The decimation factor is set to 6
		   g_third_stage_div_4_fir, // This corresponds to a 24kHz output hence this coef array is used
		   0, // Gain compensation is turned off
		   FIR_COMPENSATOR_DIV_4, // FIR compensation is set to the corresponding coefficients
		   DECIMATOR_NO_FRAME_OVERLAP, // Frame overlapping is turned off
		   FRAME_BUFFER_COUNT  // The number of buffers in the audio array
       };

Changing the frame buffer count
...............................

The number of frame buffer in use can be modified through the define 

.. literalinclude:: app_phase_aligned_example.xc
   :start-on: #define FRAME_BUFFER_COUNT  2
   :end-before: #define DECIMATOR_CH_COUNT 4 
   
within this application. Increasing this would allow sample to be known for a longer period of time 
before the memory is reused for the present samples at the cost of increased memory usage. 


Changing the frame size
.......................

The length of a frame is an arbitrary value < 2**15. The maximum allowed at run time is given by two to the
power of ``MIC_ARRAY_MAX_FRAME_SIZE_LOG2``. At run time the length can be dynamically configured by
setting the ``len`` member of ``mic_array_decimator_config_common``. If ``len``
is less than 16, the frame size will be ``2^len``. If ``len`` is 16 or greater, the
frame size will be ``len``.

Changing the buffering type
...........................

The buffering type can be either ``DECIMATOR_NO_FRAME_OVERLAP`` or ``DECIMATOR_HALF_FRAME_OVERLAP``.
This is configured through the ``buffering_type`` member of ``mic_array_decimator_config_common``. 
In half frame overlap mode the rate that the ``mic_array_get_next_time_domain_frame()`` function must
be serviced is doubled.

Disabling the DC offset elimination
...................................

The DC offset elimination can be disabled by configuring the ``mic_array_decimator_config_common`` with
0 in the ``apply_dc_offset_removal`` member. Setting ``apply_dc_offset_removal`` to non-zero will enable
it.

Enabling microphone gain compensation
.....................................

To enable the microphone gain compensation first the ``apply_mic_gain_compensation`` member of 
``mic_array_decimator_config_common`` must be set to non-zero. Then for each ``mic_array_decimator_config`` 
structure used to configure a 4 channel decimator the array member ``mic_gain_compensation`` must be set.
If the gain a microphone i is g\ :sub:`i`\, then the compensation factor should be 
``INT_MAX`` / min(g\ :sub:`i`\) for each microphone i.
	   
References
----------

.. nopoints::

  * XMOS Tools User Guide

    http://www.xmos.com/published/xtimecomposer-user-guide

  * XMOS xCORE Programming Guide

    http://www.xmos.com/published/xmos-programming-guide

  * XMOS Microphone Array Library

    http://www.xmos.com/support/libraries/lib_mic_array

	
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Full source code listing
------------------------

Source code for app_phase_aligned_example.xc
....................................................

.. literalinclude:: app_phase_aligned_example.xc
  :largelisting:

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