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Copyright (C) 4Front Technologies, 2002-2004. Released under GPLv2/CDDL.
This minimalistic program shows how to play udio with OSS. It outputs 1000 Hz sinewave signal (based on a 48 step lookup table).
This is pretty much the simpliest possible audio playback program one can imagine. It could be possible to make it even simplier by removing all error checking but that is in no way recommended.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <soundcard.h>
int fd_out;
int sample_rate = 48000;
static void
write_sinewave (void)
{
This routine is a typical example of application routine that produces audio signal using synthesis. This is actually a very basic "wave table" algorithm (btw). It uses precomputed sine function values for a complete cycle of a sine function. This is much faster than calling the sin() function once for each sample.
In other applications this routine can simply be replaced by whatever the application needs to do.
static unsigned int phase = 0; /* Phase of the sine wave */
unsigned int p;
int i;
short buf[1024]; /* 1024 samples/write is a safe choice */
int outsz = sizeof (buf) / 2;
static int sinebuf[48] = {
0, 4276, 8480, 12539, 16383, 19947, 23169, 25995,
28377, 30272, 31650, 32486, 32767, 32486, 31650, 30272,
28377, 25995, 23169, 19947, 16383, 12539, 8480, 4276,
0, -4276, -8480, -12539, -16383, -19947, -23169, -25995,
-28377, -30272, -31650, -32486, -32767, -32486, -31650, -30272,
-28377, -25995, -23169, -19947, -16383, -12539, -8480, -4276
};
for (i = 0; i < outsz; i++)
{
The sinebuf[] table was computed for 48000 Hz. We will use simple sample rate compensation.
We must prevent the phase variable from groving too large because that would cause cause arihmetic overflows after certain time. This kind of error posibilities must be identified when writing audio programs that could be running for hours or even months or years without interruption. When computing (say) 192000 samples each second the 32 bit integer range may get overflown very quickly. The number of samples played at 192 kHz will cause an overflow after about 6 hours.
p = (phase * sample_rate) / 48000;
phase = (phase + 1) % 4800;
buf[i] = sinebuf[p % 48];
}
Proper error checking must be done when using write. It's also important to reporte the error code returned by the system.
if (write (fd_out, buf, sizeof (buf)) != sizeof (buf))
{
perror ("Audio write");
exit (-1);
}
}
The open_audio_device opens the audio device and initializes it for the required mode.
static int
open_audio_device (char *name, int mode)
{
int tmp, fd;
if ((fd = open (name, mode, 0)) == -1)
{
perror (name);
exit (-1);
}
Setup the device. Note that it's important to set the sample format, number of channels and sample rate exactly in this order. Some devices depend on the order.
Set the sample format
tmp = AFMT_S16_NE; /* Native 16 bits */
if (ioctl (fd, SNDCTL_DSP_SETFMT, &tmp) == -1)
{
perror ("SNDCTL_DSP_SETFMT");
exit (-1);
}
if (tmp != AFMT_S16_NE)
{
fprintf (stderr,
"The device doesn't support the 16 bit sample format.\n");
exit (-1);
}
Set the number of channels
tmp = 1;
if (ioctl (fd, SNDCTL_DSP_CHANNELS, &tmp) == -1)
{
perror ("SNDCTL_DSP_CHANNELS");
exit (-1);
}
if (tmp != 1)
{
fprintf (stderr, "The device doesn't support mono mode.\n");
exit (-1);
}
Set the sample rate
sample_rate = 48000;
if (ioctl (fd, SNDCTL_DSP_SPEED, &sample_rate) == -1)
{
perror ("SNDCTL_DSP_SPEED");
exit (-1);
}
No need for error checking because we will automatically adjust the signal based on the actual sample rate. However most application must check the value of sample_rate and compare it to the requested rate.
Small differences between the rates (10% or less) are normal and the applications should usually tolerate them. However larger differences may cause annoying pitch problems (Mickey Mouse).
return fd;
}
int
main (int argc, char *argv[])
{
Use /dev/dsp as the default device because the system administrator may select the device using the Bad xlink 'ossctl' program or some other methods
char *name_out = "/dev/dsp";
It's recommended to provide some method for selecting some other device than the default. We use command line argument but in some cases an environment variable or some configuration file setting may be better.
if (argc > 1)
name_out = argv[1];
It's mandatory to use O_WRONLY in programs that do only playback. Other modes may cause increased resource (memory) usage in the driver. It may also prevent other applications from using the same device for recording at the same time.
fd_out = open_audio_device (name_out, O_WRONLY);
while (1)
write_sinewave ();
exit (0);
}