View source for Hacker Guide/Audio Filters

=Overview=

=Writing an audio filter=

==Module descriptor==
 static int  Open     ( [http://www.videolan.org/developers/vlc/doc/doxygen/html/structvlc__object__t.html vlc_object_t] *p_this );
 static void Close    ( [http://www.videolan.org/developers/vlc/doc/doxygen/html/structvlc__object__t.html vlc_object_t] *p_this );
 static void DoWork   ( [http://www.videolan.org/developers/vlc/doc/doxygen/html/structaout__instance__t.html aout_instance_t] *p_aout, [http://www.videolan.org/developers/vlc/doc/doxygen/html/structaout__filter__t.html aout_filter_t] *p_filter, [http://www.videolan.org/developers/vlc/doc/doxygen/html/structaout__buffer__t.html aout_buffer_t] *p_in_buf, [http://www.videolan.org/developers/vlc/doc/doxygen/html/structaout__buffer__t.html aout_buffer_t] *p_out_buf );
 
 vlc_module_begin()
     set_description( N_("audio filter x") )
     set_shortname( N_("audio filter x") )
     set_category( CAT_AUDIO )
     set_subcategory( SUBCAT_AUDIO_AFILTER )
     add_shortcut( "afx" )
     set_capability( "audio filter", 0 )
     set_callbacks( Open, Close )
 vlc_module_end()

==Mandatory structures==
*aout_filter_sys_t

==I/O Buffers==
Data comes in and out the plugin in an [http://www.videolan.org/developers/vlc/doc/doxygen/html/structaout__buffer__t.html aout_buffer_t], one sample at a time, and in each sample, one block by channel.

{|border="1" cellpadding="5" cellspacing="0" align="center"
|-
|data stream
| ch 1
| ch 2
| ch 1
| ch 2
| ch 1
| ch 2
| ...
|-
|
| colspan="2" | Sample 1
| colspan="2" | Sample 2
| colspan="2" | Sample 3
| ...
|}



An audio filter module is constituted of a constructor, a destructor, and a p_filter->pf_do_work function. The constructor is passed a p_filter structure, and it returns 0 if it is able to do the whole transformation between p_filter->input and p_filter->output. If you can do only part of the transformation, say you can't do it (if the aout core doesn't find a fitting filter, it will split the transformation and ask you again).

==Note==

Audio filters can be of three types :

*Converters : change i_format (for instance from float32 to s16).
*Resamplers : change i_rate (for instance from 48 kHz to 44.1 kHz).
*Channel mixers : change i_physical_channels/i_original_channels (for instance from 5.1 to stereo). 

Audio filters can also combine any of these types. For instance you can have an audio filter which transforms A/52 5.1 to float32 stereo.

The constructor must also set p_filter->b_in_place. If it's 0, the aout core will allocate a new buffer for the output. If it's 1, when you write a byte in the output buffer, it destroys the same byte in the input buffer (they share the same memory area). Some filters can work in place because they just do a linear transformation (like float32->s16), but most filters will want b_in_place = 0. The filter can allocate private data in p_filter->p_sys. Do not forget to deallocate it in the destructor.

The p_filter->pf_do_work gets an input and an output buffer as arguments, and process them. At the end of the processing, do not forget to set p_out_buf->i_nb_samples and p_out_buf->i_nb_bytes, since they aren't inited by the aout core (their values can change between input and output and it's not quite predictible).