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92 lines
4.7 KiB
HTML
92 lines
4.7 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Mozilla/4.77 [en]C-gatewaynet (Win98; U) [Netscape]">
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<meta name="Author" content="Phil Burk">
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<meta name="Description" content="Tutorial for PortAudio, a cross platform, open-source, audio I/O library.It provides a very simple API for recording and/or playing sound using a simple callback function.">
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<meta name="KeyWords" content="audio, tutorial, library, portable, open-source, DirectSound,sound, music, JSyn, synthesis,">
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<title>PortAudio Tutorial</title>
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</head>
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<body>
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<center><table COLS=1 WIDTH="100%" BGCOLOR="#FADA7A" >
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<tr>
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<td>
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<center>
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<h1>
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PortAudio Tutorial</h1></center>
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</td>
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</tr>
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</table></center>
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<h2>
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Writing a Callback Function</h2>
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<blockquote>To write a program using PortAudio, you must include the "portaudio.h"
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include file. You may wish to read "<a href="portaudio_h.txt">portaudio.h</a>"
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because it contains a complete description of the PortAudio functions and
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constants.
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<blockquote>
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<pre>#include "portaudio.h"</pre>
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</blockquote>
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The next task is to write your custom callback function. It is a function
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that is called by the PortAudio engine whenever it has captured audio data,
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or when it needs more audio data for output.
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<p>Your callback function is often called by an interrupt, or low level
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process so you should not do any complex system activities like allocating
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memory, or reading or writing files, or printf(). Just crunch numbers and
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generate audio signals. What is safe or not safe will vary from platform
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to platform. On the Macintosh, for example, you can only call "interrupt
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safe" routines. Also do not call any PortAudio functions in the callback
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except for Pa_StreamTime() and Pa_GetCPULoad().
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<p>Your callback function must return an int and accept the exact parameters
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specified in this typedef:
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<blockquote>
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<pre>typedef int (PortAudioCallback)(
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void *inputBuffer, void *outputBuffer,
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unsigned long framesPerBuffer,
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PaTimestamp outTime, void *userData );</pre>
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</blockquote>
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Here is an example callback function from the test file "patests/patest_saw.c".
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It calculates a simple left and right sawtooth signal and writes it to
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the output buffer. Notice that in this example, the signals are of <tt>float</tt>
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data type. The signals must be between -1.0 and +1.0. You can also use
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16 bit integers or other formats which are specified during setup. You
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can pass a pointer to your data structure through PortAudio which will
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appear as <tt>userData</tt>.
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<blockquote>
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<pre>int patestCallback( void *inputBuffer, void *outputBuffer,
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unsigned long framesPerBuffer,
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PaTimestamp outTime, void *userData )
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{
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unsigned int i;
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/* Cast data passed through stream to our structure type. */
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paTestData *data = (paTestData*)userData;
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float *out = (float*)outputBuffer;
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for( i=0; i<framesPerBuffer; i++ )
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{
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/* Stereo channels are interleaved. */
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*out++ = data->left_phase; /* left */
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*out++ = data->right_phase; /* right */
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/* Generate simple sawtooth phaser that ranges between -1.0 and 1.0. */
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data->left_phase += 0.01f;
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/* When signal reaches top, drop back down. */
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if( data->left_phase >= 1.0f ) data->left_phase -= 2.0f;
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/* higher pitch so we can distinguish left and right. */
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data->right_phase += 0.03f;
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if( data->right_phase >= 1.0f ) data->right_phase -= 2.0f;
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}
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return 0;
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}</pre>
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</blockquote>
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</blockquote>
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<font size=+2><a href="http://www.portaudio.com/">home</a> |
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<a href="pa_tutorial.html">contents</a>
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| <a href="pa_tut_over.html">previous</a> | <a href="pa_tut_init.html">next</a></font>
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</body>
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</html>
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