1 /*****************************************************************************
2 * mono.c : stereo2mono downmixsimple channel mixer plug-in
3 *****************************************************************************
4 * Copyright (C) 2006 M2X
7 * Authors: Jean-Paul Saman <jpsaman at m2x dot nl>
9 * This program is free software; you can redistribute it and/or modify
10 * it under the terms of the GNU General Public License as published by
11 * the Free Software Foundation; either version 2 of the License, or
12 * (at your option) any later version.
14 * This program is distributed in the hope that it will be useful,
15 * but WITHOUT ANY WARRANTY; without even the implied warranty of
16 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
17 * GNU General Public License for more details.
19 * You should have received a copy of the GNU General Public License
20 * along with this program; if not, write to the Free Software
21 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
22 *****************************************************************************/
24 /*****************************************************************************
26 *****************************************************************************/
27 #include <math.h> /* sqrt */
30 # include <stdint.h> /* int16_t .. */
31 #elif defined(HAVE_INTTYPES_H)
32 # include <inttypes.h> /* int16_t .. */
45 #include <vlc_block.h>
46 #include <vlc_filter.h>
49 /*****************************************************************************
51 *****************************************************************************/
52 static int OpenFilter ( vlc_object_t * );
53 static void CloseFilter ( vlc_object_t * );
55 static block_t *Convert( filter_t *p_filter, block_t *p_block );
57 static unsigned int stereo_to_mono( aout_filter_t *, aout_buffer_t *,
59 static unsigned int mono( aout_filter_t *, aout_buffer_t *, aout_buffer_t * );
60 static void stereo2mono_downmix( aout_filter_t *, aout_buffer_t *,
63 /*****************************************************************************
65 *****************************************************************************/
66 struct atomic_operation_t
68 int i_source_channel_offset;
69 int i_dest_channel_offset;
70 unsigned int i_delay;/* in sample unit */
71 double d_amplitude_factor;
78 unsigned int i_nb_channels; /* number of int16_t per sample */
79 int i_channel_selected;
82 size_t i_overflow_buffer_size;/* in bytes */
83 byte_t * p_overflow_buffer;
84 unsigned int i_nb_atomic_operations;
85 struct atomic_operation_t * p_atomic_operations;
88 #define MONO_DOWNMIX_TEXT N_("Use downmix algorithm")
89 #define MONO_DOWNMIX_LONGTEXT N_("This option selects a stereo to mono " \
90 "downmix algorithm that is used in the headphone channel mixer. It" \
91 "gives the effect of standing in a room full of speakers." )
93 #define MONO_CHANNEL_TEXT N_("Select channel to keep")
94 #define MONO_CHANNEL_LONGTEXT N_("This option silences all other channels " \
95 "except the selected channel. Choose one from (0=left, 1=right, " \
96 "2=rear left, 3=rear right, 4=center, 5=left front)")
98 static const int pi_pos_values[] = { 0, 1, 2, 4, 8, 5 };
99 static const char *ppsz_pos_descriptions[] =
100 { N_("Left"), N_("Right"), N_("Left rear"), N_("Right rear"), N_("Center"),
103 /* our internal channel order (WG-4 order) */
104 static const uint32_t pi_channels_out[] =
105 { AOUT_CHAN_LEFT, AOUT_CHAN_RIGHT, AOUT_CHAN_REARLEFT, AOUT_CHAN_REARRIGHT,
106 AOUT_CHAN_CENTER, AOUT_CHAN_LFE, 0 };
108 #define MONO_CFG "sout-mono-"
109 /*****************************************************************************
111 *****************************************************************************/
113 set_description( _("Audio filter for stereo to mono conversion") );
114 set_capability( "audio filter2", 0 );
116 add_bool( MONO_CFG "downmix", VLC_FALSE, NULL, MONO_DOWNMIX_TEXT,
117 MONO_DOWNMIX_LONGTEXT, VLC_FALSE );
118 add_integer( MONO_CFG "channel", -1, NULL, MONO_CHANNEL_TEXT,
119 MONO_CHANNEL_LONGTEXT, VLC_FALSE );
120 change_integer_list( pi_pos_values, ppsz_pos_descriptions, 0 );
122 set_category( CAT_AUDIO );
123 set_subcategory( SUBCAT_AUDIO_MISC );
124 set_callbacks( OpenFilter, CloseFilter );
125 set_shortname( "Mono" );
128 /* Init() and ComputeChannelOperations() -
129 * Code taken from modules/audio_filter/channel_mixer/headphone.c
130 * converted from float into int16_t based downmix
131 * Written by Boris Dorès <babal@via.ecp.fr>
134 /*****************************************************************************
135 * Init: initialize internal data structures
136 * and computes the needed atomic operations
137 *****************************************************************************/
138 /* x and z represent the coordinates of the virtual speaker
139 * relatively to the center of the listener's head, measured in meters :
148 * rear left rear right
152 static void ComputeChannelOperations( struct filter_sys_t * p_data,
153 unsigned int i_rate, unsigned int i_next_atomic_operation,
154 int i_source_channel_offset, double d_x, double d_z,
155 double d_compensation_length, double d_channel_amplitude_factor )
157 double d_c = 340; /*sound celerity (unit: m/s)*/
158 double d_compensation_delay = (d_compensation_length-0.1) / d_c * i_rate;
161 p_data->p_atomic_operations[i_next_atomic_operation]
162 .i_source_channel_offset = i_source_channel_offset;
163 p_data->p_atomic_operations[i_next_atomic_operation]
164 .i_dest_channel_offset = 0;/* left */
165 p_data->p_atomic_operations[i_next_atomic_operation]
166 .i_delay = (int)( sqrt( (-0.1-d_x)*(-0.1-d_x) + (0-d_z)*(0-d_z) )
167 / d_c * i_rate - d_compensation_delay );
170 p_data->p_atomic_operations[i_next_atomic_operation]
171 .d_amplitude_factor = d_channel_amplitude_factor * 1.1 / 2;
175 p_data->p_atomic_operations[i_next_atomic_operation]
176 .d_amplitude_factor = d_channel_amplitude_factor * 0.9 / 2;
180 p_data->p_atomic_operations[i_next_atomic_operation]
181 .d_amplitude_factor = d_channel_amplitude_factor / 2;
185 p_data->p_atomic_operations[i_next_atomic_operation + 1]
186 .i_source_channel_offset = i_source_channel_offset;
187 p_data->p_atomic_operations[i_next_atomic_operation + 1]
188 .i_dest_channel_offset = 1;/* right */
189 p_data->p_atomic_operations[i_next_atomic_operation + 1]
190 .i_delay = (int)( sqrt( (0.1-d_x)*(0.1-d_x) + (0-d_z)*(0-d_z) )
191 / d_c * i_rate - d_compensation_delay );
194 p_data->p_atomic_operations[i_next_atomic_operation + 1]
195 .d_amplitude_factor = d_channel_amplitude_factor * 0.9 / 2;
199 p_data->p_atomic_operations[i_next_atomic_operation + 1]
200 .d_amplitude_factor = d_channel_amplitude_factor * 1.1 / 2;
204 p_data->p_atomic_operations[i_next_atomic_operation + 1]
205 .d_amplitude_factor = d_channel_amplitude_factor / 2;
209 static int Init( vlc_object_t *p_this, struct filter_sys_t * p_data,
210 unsigned int i_nb_channels, uint32_t i_physical_channels,
211 unsigned int i_rate )
213 double d_x = config_GetInt( p_this, "headphone-dim" );
215 double d_z_rear = -d_x/3;
217 unsigned int i_next_atomic_operation;
218 int i_source_channel_offset;
223 msg_Dbg( p_this, "passing a null pointer as argument" );
227 if( config_GetInt( p_this, "headphone-compensate" ) )
229 /* minimal distance to any speaker */
230 if( i_physical_channels & AOUT_CHAN_REARCENTER )
240 /* Number of elementary operations */
241 p_data->i_nb_atomic_operations = i_nb_channels * 2;
242 if( i_physical_channels & AOUT_CHAN_CENTER )
244 p_data->i_nb_atomic_operations += 2;
246 p_data->p_atomic_operations = malloc( sizeof(struct atomic_operation_t)
247 * p_data->i_nb_atomic_operations );
248 if( p_data->p_atomic_operations == NULL )
250 msg_Err( p_this, "out of memory" );
254 /* For each virtual speaker, computes elementary wave propagation time
256 i_next_atomic_operation = 0;
257 i_source_channel_offset = 0;
258 if( i_physical_channels & AOUT_CHAN_LEFT )
260 ComputeChannelOperations( p_data , i_rate
261 , i_next_atomic_operation , i_source_channel_offset
262 , -d_x , d_z , d_min , 2.0 / i_nb_channels );
263 i_next_atomic_operation += 2;
264 i_source_channel_offset++;
266 if( i_physical_channels & AOUT_CHAN_RIGHT )
268 ComputeChannelOperations( p_data , i_rate
269 , i_next_atomic_operation , i_source_channel_offset
270 , d_x , d_z , d_min , 2.0 / i_nb_channels );
271 i_next_atomic_operation += 2;
272 i_source_channel_offset++;
274 if( i_physical_channels & AOUT_CHAN_MIDDLELEFT )
276 ComputeChannelOperations( p_data , i_rate
277 , i_next_atomic_operation , i_source_channel_offset
278 , -d_x , 0 , d_min , 1.5 / i_nb_channels );
279 i_next_atomic_operation += 2;
280 i_source_channel_offset++;
282 if( i_physical_channels & AOUT_CHAN_MIDDLERIGHT )
284 ComputeChannelOperations( p_data , i_rate
285 , i_next_atomic_operation , i_source_channel_offset
286 , d_x , 0 , d_min , 1.5 / i_nb_channels );
287 i_next_atomic_operation += 2;
288 i_source_channel_offset++;
290 if( i_physical_channels & AOUT_CHAN_REARLEFT )
292 ComputeChannelOperations( p_data , i_rate
293 , i_next_atomic_operation , i_source_channel_offset
294 , -d_x , d_z_rear , d_min , 1.5 / i_nb_channels );
295 i_next_atomic_operation += 2;
296 i_source_channel_offset++;
298 if( i_physical_channels & AOUT_CHAN_REARRIGHT )
300 ComputeChannelOperations( p_data , i_rate
301 , i_next_atomic_operation , i_source_channel_offset
302 , d_x , d_z_rear , d_min , 1.5 / i_nb_channels );
303 i_next_atomic_operation += 2;
304 i_source_channel_offset++;
306 if( i_physical_channels & AOUT_CHAN_REARCENTER )
308 ComputeChannelOperations( p_data , i_rate
309 , i_next_atomic_operation , i_source_channel_offset
310 , 0 , -d_z , d_min , 1.5 / i_nb_channels );
311 i_next_atomic_operation += 2;
312 i_source_channel_offset++;
314 if( i_physical_channels & AOUT_CHAN_CENTER )
316 /* having two center channels increases the spatialization effect */
317 ComputeChannelOperations( p_data , i_rate
318 , i_next_atomic_operation , i_source_channel_offset
319 , d_x / 5.0 , d_z , d_min , 0.75 / i_nb_channels );
320 i_next_atomic_operation += 2;
321 ComputeChannelOperations( p_data , i_rate
322 , i_next_atomic_operation , i_source_channel_offset
323 , -d_x / 5.0 , d_z , d_min , 0.75 / i_nb_channels );
324 i_next_atomic_operation += 2;
325 i_source_channel_offset++;
327 if( i_physical_channels & AOUT_CHAN_LFE )
329 ComputeChannelOperations( p_data , i_rate
330 , i_next_atomic_operation , i_source_channel_offset
331 , 0 , d_z_rear , d_min , 5.0 / i_nb_channels );
332 i_next_atomic_operation += 2;
333 i_source_channel_offset++;
336 /* Initialize the overflow buffer
337 * we need it because the process induce a delay in the samples */
338 p_data->i_overflow_buffer_size = 0;
339 for( i = 0 ; i < p_data->i_nb_atomic_operations ; i++ )
341 if( p_data->i_overflow_buffer_size
342 < p_data->p_atomic_operations[i].i_delay * 2 * sizeof (int16_t) )
344 p_data->i_overflow_buffer_size
345 = p_data->p_atomic_operations[i].i_delay * 2 * sizeof (int16_t);
348 p_data->p_overflow_buffer = malloc( p_data->i_overflow_buffer_size );
349 if( p_data->p_atomic_operations == NULL )
351 msg_Err( p_this, "out of memory" );
354 memset( p_data->p_overflow_buffer, 0, p_data->i_overflow_buffer_size );
360 /*****************************************************************************
362 *****************************************************************************/
363 static int OpenFilter( vlc_object_t *p_this )
365 filter_t * p_filter = (filter_t *)p_this;
366 filter_sys_t *p_sys = NULL;
368 if( aout_FormatNbChannels( &(p_filter->fmt_in.audio) ) == 1 )
370 msg_Dbg( p_filter, "filter discarded (incompatible format)" );
374 if( (p_filter->fmt_in.i_codec != AOUT_FMT_S16_NE) ||
375 (p_filter->fmt_out.i_codec != AOUT_FMT_S16_NE) )
377 msg_Err( p_this, "filter discarded (invalid format)" );
381 if( (p_filter->fmt_in.audio.i_format != p_filter->fmt_out.audio.i_format) &&
382 (p_filter->fmt_in.audio.i_rate != p_filter->fmt_out.audio.i_rate) &&
383 (p_filter->fmt_in.audio.i_format != AOUT_FMT_S16_NE) &&
384 (p_filter->fmt_out.audio.i_format != AOUT_FMT_S16_NE) &&
385 (p_filter->fmt_in.audio.i_bitspersample !=
386 p_filter->fmt_out.audio.i_bitspersample))
388 msg_Err( p_this, "couldn't load mono filter" );
392 /* Allocate the memory needed to store the module's structure */
393 p_sys = p_filter->p_sys = malloc( sizeof(filter_sys_t) );
396 msg_Err( p_filter, "out of memory" );
400 var_Create( p_this, MONO_CFG "downmix",
401 VLC_VAR_BOOL | VLC_VAR_DOINHERIT );
402 p_sys->b_downmix = var_GetBool( p_this, MONO_CFG "downmix" );
404 var_Create( p_this, MONO_CFG "channel",
405 VLC_VAR_INTEGER | VLC_VAR_DOINHERIT );
406 p_sys->i_channel_selected =
407 (unsigned int) var_GetInteger( p_this, MONO_CFG "channel" );
409 if( p_sys->b_downmix )
411 msg_Dbg( p_this, "using stereo to mono downmix" );
412 p_filter->fmt_out.audio.i_physical_channels = AOUT_CHAN_CENTER;
413 p_filter->fmt_out.audio.i_channels = 1;
417 msg_Dbg( p_this, "using pseudo mono" );
418 p_filter->fmt_out.audio.i_physical_channels =
419 (AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT);
420 p_filter->fmt_out.audio.i_channels = 2;
423 p_filter->fmt_out.audio.i_rate = p_filter->fmt_in.audio.i_rate;
424 p_filter->fmt_out.audio.i_format = p_filter->fmt_out.i_codec;
426 p_sys->i_nb_channels = aout_FormatNbChannels( &(p_filter->fmt_in.audio) );
427 p_sys->i_bitspersample = p_filter->fmt_out.audio.i_bitspersample;
429 p_sys->i_overflow_buffer_size = 0;
430 p_sys->p_overflow_buffer = NULL;
431 p_sys->i_nb_atomic_operations = 0;
432 p_sys->p_atomic_operations = NULL;
434 if( Init( VLC_OBJECT(p_filter), p_filter->p_sys,
435 aout_FormatNbChannels( &p_filter->fmt_in.audio ),
436 p_filter->fmt_in.audio.i_physical_channels,
437 p_filter->fmt_in.audio.i_rate ) < 0 )
442 p_filter->pf_audio_filter = Convert;
444 msg_Dbg( p_this, "%4.4s->%4.4s, channels %d->%d, bits per sample: %i->%i",
445 (char *)&p_filter->fmt_in.i_codec,
446 (char *)&p_filter->fmt_out.i_codec,
447 p_filter->fmt_in.audio.i_physical_channels,
448 p_filter->fmt_out.audio.i_physical_channels,
449 p_filter->fmt_in.audio.i_bitspersample,
450 p_filter->fmt_out.audio.i_bitspersample );
455 /*****************************************************************************
457 *****************************************************************************/
458 static void CloseFilter( vlc_object_t *p_this)
460 filter_t *p_filter = (filter_t *) p_this;
461 filter_sys_t *p_sys = p_filter->p_sys;
463 var_Destroy( p_this, MONO_CFG "channel" );
464 var_Destroy( p_this, MONO_CFG "downmix" );
468 /*****************************************************************************
470 *****************************************************************************/
471 static block_t *Convert( filter_t *p_filter, block_t *p_block )
473 aout_filter_t aout_filter;
474 aout_buffer_t in_buf, out_buf;
475 block_t *p_out = NULL;
476 unsigned int i_samples;
479 if( !p_block || !p_block->i_samples )
482 p_block->pf_release( p_block );
486 i_out_size = p_block->i_samples * p_filter->p_sys->i_bitspersample/8 *
487 aout_FormatNbChannels( &(p_filter->fmt_out.audio) );
489 p_out = p_filter->pf_audio_buffer_new( p_filter, i_out_size );
492 msg_Warn( p_filter, "can't get output buffer" );
493 p_block->pf_release( p_block );
496 p_out->i_samples = (p_block->i_samples / p_filter->p_sys->i_nb_channels) *
497 aout_FormatNbChannels( &(p_filter->fmt_out.audio) );
498 p_out->i_dts = p_block->i_dts;
499 p_out->i_pts = p_block->i_pts;
500 p_out->i_length = p_block->i_length;
502 aout_filter.p_sys = (struct aout_filter_sys_t *)p_filter->p_sys;
503 aout_filter.input = p_filter->fmt_in.audio;
504 aout_filter.input.i_format = p_filter->fmt_in.i_codec;
505 aout_filter.output = p_filter->fmt_out.audio;
506 aout_filter.output.i_format = p_filter->fmt_out.i_codec;
508 in_buf.p_buffer = p_block->p_buffer;
509 in_buf.i_nb_bytes = p_block->i_buffer;
510 in_buf.i_nb_samples = p_block->i_samples;
513 unsigned int i_in_size = in_buf.i_nb_samples * (p_filter->p_sys->i_bitspersample/8) *
514 aout_FormatNbChannels( &(p_filter->fmt_in.audio) );
515 if( (in_buf.i_nb_bytes != i_in_size) && ((i_in_size % 32) != 0) ) /* is it word aligned?? */
517 msg_Err( p_filter, "input buffer is not word aligned" );
518 /* Fix output buffer to be word aligned */
522 out_buf.p_buffer = p_out->p_buffer;
523 out_buf.i_nb_bytes = p_out->i_buffer;
524 out_buf.i_nb_samples = p_out->i_samples;
526 memset( p_out->p_buffer, 0, i_out_size );
527 if( p_filter->p_sys->b_downmix )
529 stereo2mono_downmix( &aout_filter, &in_buf, &out_buf );
530 i_samples = mono( &aout_filter, &out_buf, &in_buf );
534 i_samples = stereo_to_mono( &aout_filter, &out_buf, &in_buf );
537 p_out->i_buffer = out_buf.i_nb_bytes;
538 p_out->i_samples = out_buf.i_nb_samples;
540 p_block->pf_release( p_block );
544 /* stereo2mono_downmix - stereo channels into one mono channel.
545 * Code taken from modules/audio_filter/channel_mixer/headphone.c
546 * converted from float into int16_t based downmix
547 * Written by Boris Dorès <babal@via.ecp.fr>
549 static void stereo2mono_downmix( aout_filter_t * p_filter,
550 aout_buffer_t * p_in_buf, aout_buffer_t * p_out_buf )
552 filter_sys_t *p_sys = (filter_sys_t *)p_filter->p_sys;
554 int i_input_nb = aout_FormatNbChannels( &p_filter->input );
555 int i_output_nb = aout_FormatNbChannels( &p_filter->output );
557 int16_t * p_in = (int16_t*) p_in_buf->p_buffer;
562 size_t i_overflow_size; /* in bytes */
563 size_t i_out_size; /* in bytes */
567 int i_source_channel_offset;
568 int i_dest_channel_offset;
569 unsigned int i_delay;
570 double d_amplitude_factor;
572 /* out buffer characterisitcs */
573 p_out_buf->i_nb_samples = p_in_buf->i_nb_samples;
574 p_out_buf->i_nb_bytes = p_in_buf->i_nb_bytes * i_output_nb / i_input_nb;
575 p_out = p_out_buf->p_buffer;
576 i_out_size = p_out_buf->i_nb_bytes;
580 /* Slide the overflow buffer */
581 p_overflow = p_sys->p_overflow_buffer;
582 i_overflow_size = p_sys->i_overflow_buffer_size;
584 if ( i_out_size > i_overflow_size )
585 memcpy( p_out, p_overflow, i_overflow_size );
587 memcpy( p_out, p_overflow, i_out_size );
589 p_slide = p_sys->p_overflow_buffer;
590 while( p_slide < p_overflow + i_overflow_size )
592 if( p_slide + i_out_size < p_overflow + i_overflow_size )
594 memset( p_slide, 0, i_out_size );
595 if( p_slide + 2 * i_out_size < p_overflow + i_overflow_size )
596 memcpy( p_slide, p_slide + i_out_size, i_out_size );
598 memcpy( p_slide, p_slide + i_out_size,
599 p_overflow + i_overflow_size - ( p_slide + i_out_size ) );
603 memset( p_slide, 0, p_overflow + i_overflow_size - p_slide );
605 p_slide += i_out_size;
608 /* apply the atomic operations */
609 for( i = 0; i < p_sys->i_nb_atomic_operations; i++ )
611 /* shorter variable names */
612 i_source_channel_offset
613 = p_sys->p_atomic_operations[i].i_source_channel_offset;
614 i_dest_channel_offset
615 = p_sys->p_atomic_operations[i].i_dest_channel_offset;
616 i_delay = p_sys->p_atomic_operations[i].i_delay;
618 = p_sys->p_atomic_operations[i].d_amplitude_factor;
620 if( p_out_buf->i_nb_samples > i_delay )
622 /* current buffer coefficients */
623 for( j = 0; j < p_out_buf->i_nb_samples - i_delay; j++ )
625 ((int16_t*)p_out)[ (i_delay+j)*i_output_nb + i_dest_channel_offset ]
626 += p_in[ j * i_input_nb + i_source_channel_offset ]
627 * d_amplitude_factor;
630 /* overflow buffer coefficients */
631 for( j = 0; j < i_delay; j++ )
633 ((int16_t*)p_overflow)[ j*i_output_nb + i_dest_channel_offset ]
634 += p_in[ (p_out_buf->i_nb_samples - i_delay + j)
635 * i_input_nb + i_source_channel_offset ]
636 * d_amplitude_factor;
641 /* overflow buffer coefficients only */
642 for( j = 0; j < p_out_buf->i_nb_samples; j++ )
644 ((int16_t*)p_overflow)[ (i_delay - p_out_buf->i_nb_samples + j)
645 * i_output_nb + i_dest_channel_offset ]
646 += p_in[ j * i_input_nb + i_source_channel_offset ]
647 * d_amplitude_factor;
654 memset( p_out, 0, i_out_size );
658 /* Simple stereo to mono mixing. */
659 static unsigned int mono( aout_filter_t *p_filter,
660 aout_buffer_t *p_output, aout_buffer_t *p_input )
662 filter_sys_t *p_sys = (filter_sys_t *)p_filter->p_sys;
663 int16_t *p_in, *p_out;
664 unsigned int n = 0, r = 0;
666 p_in = (int16_t *) p_input->p_buffer;
667 p_out = (int16_t *) p_output->p_buffer;
669 while( n < (p_input->i_nb_samples * p_sys->i_nb_channels) )
671 p_out[r] = (p_in[n] + p_in[n+1]) >> 1;
678 /* Simple stereo to mono mixing. */
679 static unsigned int stereo_to_mono( aout_filter_t *p_filter,
680 aout_buffer_t *p_output, aout_buffer_t *p_input )
682 filter_sys_t *p_sys = (filter_sys_t *)p_filter->p_sys;
683 int16_t *p_in, *p_out;
686 p_in = (int16_t *) p_input->p_buffer;
687 p_out = (int16_t *) p_output->p_buffer;
689 for( n = 0; n < (p_input->i_nb_samples * p_sys->i_nb_channels); n++ )
691 /* Fake real mono. */
692 if( p_sys->i_channel_selected == -1)
694 p_out[n] = p_out[n+1] = (p_in[n] + p_in[n+1]) >> 1;
697 else if( (n % p_sys->i_nb_channels) == (unsigned int) p_sys->i_channel_selected )
699 p_out[n] = p_out[n+1] = p_in[n];