1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
|
<HTML
><HEAD
><TITLE
>SDL_OpenAudio</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
"><LINK
REL="HOME"
TITLE="SDL Library Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Audio"
HREF="audio.html"><LINK
REL="PREVIOUS"
TITLE="SDL_AudioSpec"
HREF="sdlaudiospec.html"><LINK
REL="NEXT"
TITLE="SDL_PauseAudio"
HREF="sdlpauseaudio.html"></HEAD
><BODY
CLASS="REFENTRY"
BGCOLOR="#FFF8DC"
TEXT="#000000"
LINK="#0000ee"
VLINK="#551a8b"
ALINK="#ff0000"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>SDL Library Documentation</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="sdlaudiospec.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="sdlpauseaudio.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><H1
><A
NAME="SDLOPENAUDIO"
></A
>SDL_OpenAudio</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN6650"
></A
><H2
>Name</H2
>SDL_OpenAudio -- Opens the audio device with the desired parameters.</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN6653"
></A
><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN6654"
></A
><P
></P
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include "SDL.h"</PRE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <B
CLASS="FSFUNC"
>SDL_OpenAudio</B
></CODE
>(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN6660"
></A
><H2
>Description</H2
><P
>This function opens the audio device with the <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> parameters, and
returns 0 if successful, placing the actual hardware parameters in the
structure pointed to by <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
>. If <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
> is NULL, the audio
data passed to the callback function will be guaranteed to be in the
requested format, and will be automatically converted to the hardware
audio format if necessary. This function returns -1 if it failed
to open the audio device, or couldn't set up the audio thread.</P
><P
>To open the audio device a <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> <A
HREF="sdlaudiospec.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
></A
> must be created.
<PRE
CLASS="PROGRAMLISTING"
>SDL_AudioSpec *desired;
.
.
desired = malloc(sizeof(SDL_AudioSpec));</PRE
>
You must then fill this structure with your desired audio specifications.</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><SPAN
CLASS="STRUCTNAME"
>desired</SPAN
>-><TT
CLASS="STRUCTFIELD"
><I
>freq</I
></TT
></DT
><DD
><P
>The desired audio frequency in samples-per-second.</P
></DD
><DT
><SPAN
CLASS="STRUCTNAME"
>desired</SPAN
>-><TT
CLASS="STRUCTFIELD"
><I
>format</I
></TT
></DT
><DD
><P
>The desired audio format (see <A
HREF="sdlaudiospec.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
></A
>)</P
></DD
><DT
><SPAN
CLASS="STRUCTNAME"
>desired</SPAN
>-><TT
CLASS="STRUCTFIELD"
><I
>samples</I
></TT
></DT
><DD
><P
>The desired size of the audio buffer in samples. This number should be a power of two, and may be adjusted by the audio driver to a value more suitable for the hardware. Good values seem to range between 512 and 8192 inclusive, depending on the application and CPU speed. Smaller values yield faster response time, but can lead to underflow if the application is doing heavy processing and cannot fill the audio buffer in time. A stereo sample consists of both right and left channels in LR ordering. Note that the number of samples is directly related to time by the following formula: ms = (samples*1000)/freq</P
></DD
><DT
><SPAN
CLASS="STRUCTNAME"
>desired</SPAN
>-><TT
CLASS="STRUCTFIELD"
><I
>callback</I
></TT
></DT
><DD
><P
>This should be set to a function that will be called when the audio device is ready for more data. It is passed a pointer to the audio buffer, and the length in bytes of the audio buffer. This function usually runs in a separate thread, and so you should protect data structures that it accesses by calling <A
HREF="sdllockaudio.html"
><TT
CLASS="FUNCTION"
>SDL_LockAudio</TT
></A
> and <A
HREF="sdlunlockaudio.html"
><TT
CLASS="FUNCTION"
>SDL_UnlockAudio</TT
></A
> in your code. The callback prototype is:
<PRE
CLASS="PROGRAMLISTING"
>void callback(void *userdata, Uint8 *stream, int len);</PRE
>
<TT
CLASS="PARAMETER"
><I
>userdata</I
></TT
> is the pointer stored in <TT
CLASS="STRUCTFIELD"
><I
>userdata</I
></TT
> field of the <SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
>. <TT
CLASS="PARAMETER"
><I
>stream</I
></TT
> is a pointer to the audio buffer you want to fill with information and <TT
CLASS="PARAMETER"
><I
>len</I
></TT
> is the length of the audio buffer in bytes.</P
></DD
><DT
><SPAN
CLASS="STRUCTNAME"
>desired</SPAN
>-><TT
CLASS="STRUCTFIELD"
><I
>userdata</I
></TT
></DT
><DD
><P
>This pointer is passed as the first parameter to the <TT
CLASS="FUNCTION"
>callback</TT
> function.</P
></DD
></DL
></DIV
><P
><TT
CLASS="FUNCTION"
>SDL_OpenAudio</TT
> reads these fields from the <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> <SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
> structure pass to the function and attempts to find an audio configuration matching your <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
>. As mentioned above, if the <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
> parameter is <TT
CLASS="LITERAL"
>NULL</TT
> then SDL with convert from your <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> audio settings to the hardware settings as it plays.</P
><P
>If <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
> is <TT
CLASS="LITERAL"
>NULL</TT
> then the <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> <SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
> is your working specification, otherwise the <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
> <SPAN
CLASS="STRUCTNAME"
>SDL_AudioSpec</SPAN
> becomes the working specification and the <TT
CLASS="PARAMETER"
><I
>desirec</I
></TT
> specification can be deleted. The data in the working specification is used when building <SPAN
CLASS="STRUCTNAME"
>SDL_AudioCVT</SPAN
>'s for converting loaded data to the hardware format.</P
><P
><TT
CLASS="FUNCTION"
>SDL_OpenAudio</TT
> calculates the <TT
CLASS="STRUCTFIELD"
><I
>size</I
></TT
> and <TT
CLASS="STRUCTFIELD"
><I
>silence</I
></TT
> fields for both the <TT
CLASS="PARAMETER"
><I
>desired</I
></TT
> and <TT
CLASS="PARAMETER"
><I
>obtained</I
></TT
> specifications. The <TT
CLASS="STRUCTFIELD"
><I
>size</I
></TT
> field stores the total size of the audio buffer in bytes, while the <TT
CLASS="STRUCTFIELD"
><I
>silence</I
></TT
> stores the value used to represent silence in the audio buffer</P
><P
>The audio device starts out playing <TT
CLASS="STRUCTFIELD"
><I
>silence</I
></TT
> when it's opened, and should be enabled for playing by calling <A
HREF="sdlpauseaudio.html"
><TT
CLASS="FUNCTION"
>SDL_PauseAudio</TT
>(<TT
CLASS="PARAMETER"
><I
>0</I
></TT
>)</A
> when you are ready for your audio <TT
CLASS="STRUCTFIELD"
><I
>callback</I
></TT
> function to be called. Since the audio driver may modify the requested <TT
CLASS="STRUCTFIELD"
><I
>size</I
></TT
> of the audio buffer, you should allocate any local mixing buffers after you open the audio device.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN6747"
></A
><H2
>Examples</H2
><PRE
CLASS="PROGRAMLISTING"
>/* Prototype of our callback function */
void my_audio_callback(void *userdata, Uint8 *stream, int len);
/* Open the audio device */
SDL_AudioSpec *desired, *obtained;
SDL_AudioSpec *hardware_spec;
/* Allocate a desired SDL_AudioSpec */
desired = malloc(sizeof(SDL_AudioSpec));
/* Allocate space for the obtained SDL_AudioSpec */
obtained = malloc(sizeof(SDL_AudioSpec));
/* 22050Hz - FM Radio quality */
desired->freq=22050;
/* 16-bit signed audio */
desired->format=AUDIO_S16LSB;
/* Mono */
desired->channels=0;
/* Large audio buffer reduces risk of dropouts but increases response time */
desired->samples=8192;
/* Our callback function */
desired->callback=my_audio_callback;
desired->userdata=NULL;
/* Open the audio device */
if ( SDL_OpenAudio(desired, obtained) < 0 ){
fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
exit(-1);
}
/* desired spec is no longer needed */
free(desired);
hardware_spec=obtained;
.
.
/* Prepare callback for playing */
.
.
.
/* Start playing */
SDL_PauseAudio(0);</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN6750"
></A
><H2
>See Also</H2
><P
><A
HREF="sdlaudiospec.html"
><TT
CLASS="FUNCTION"
>SDL_AudioSpec</TT
></A
>,
<A
HREF="sdllockaudio.html"
><TT
CLASS="FUNCTION"
>SDL_LockAudio</TT
></A
>,
<A
HREF="sdlunlockaudio.html"
><TT
CLASS="FUNCTION"
>SDL_UnlockAudio</TT
></A
>,
<A
HREF="sdlpauseaudio.html"
><TT
CLASS="FUNCTION"
>SDL_PauseAudio</TT
></A
></P
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="sdlaudiospec.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="sdlpauseaudio.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>SDL_AudioSpec</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="audio.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>SDL_PauseAudio</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|
