ALSA tutorial required

Question

I am New to audio programming.I want to create small application which is capable of playing and gives volume control . I am using alsa-lib.

I want to know what is the purpose of switch (ex.Master Playback switch), enum in mixer elements and what value should i set to those switchs .

Please suggest me some tutorial for mixer settings as well as alsa programming .

Gaucho

Answer 1

Just collecting some here, that have example code:

• ALSA Programming HOWTO v.1.0.0 [alsamodular.sourceforge.net]
• A tutorial on using the ALSA Audio API [equalarea.com] 2002
• A close look at ALSA [volkerschatz.com]
• ALSA API - Sample Programs With Source Code By Aquiles Yanez 2005
• Introduction to Sound Programming with ALSA | Linux Journal (pg3 with example code) 2004

Note that some of these are old, and API may have changed in the meantime... you can also look up aplay.c (the source for the command line arecord and aplay), but that one is not the easiest to read for starters...

Transformism

Answer 2

You'll have a tough time finding anything concrete on ALSA, as I have have found from just starting learning it too. The best place to begin is the ALSA project homepage where they link to a number of tutorials, the best one being Dr Nagorni's one IMO.

From what it sounds like you're trying to do, JACK would most likely be a quicker and easier solution, though.

Burnejones

Answer 3

Check out the docs. There are some good examples.

http://www.alsa-project.org/alsa-doc/alsa-lib/examples.html

Be aware of the safe alsa subset.

https://www.winehq.org/pipermail/wine-bugs/2009-June/179698.html

Here's something small I put together using the various sources I could find. It miiiiiiiiiight be a good starting point.

/* Compile with gcc -lasound -pthread threadaudio.c */

#include <alsa/asoundlib.h>
#include <pthread.h>
#include <stdio.h>

unsigned char audiobuffer[0x400];
pthread_mutex_t audiomutex = PTHREAD_MUTEX_INITIALIZER;

void changeaudio (int volume) {
  int i;
  pthread_mutex_lock(&audiomutex);
  for (i = 0; i < sizeof(audiobuffer); i++)
    audiobuffer[i] = (random() & 0xff) * volume / 10;
  pthread_mutex_unlock(&audiomutex);
}

void *startaudio (void *param)
{
  static char *device = "default";
  snd_output_t *output = NULL;
  int *audiostop = (int*)param;
  int err;
  snd_pcm_t *handle;
  snd_pcm_sframes_t frames;

  changeaudio(5);

  if ((err = snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
    printf("Playback open error: %s\n", snd_strerror(err));
    exit(EXIT_FAILURE);
  }

  if ((err = snd_pcm_set_params(handle,
                SND_PCM_FORMAT_U8,
                SND_PCM_ACCESS_RW_INTERLEAVED,
                1,
                48000,
                1,
                100000)) < 0) {   /* 0.1sec */
    printf("Playback open error: %s\n", snd_strerror(err));
    exit(EXIT_FAILURE);
  }

  while (!*audiostop) {
    err = snd_pcm_wait(handle, 1000);
    if (err < 0) {
      fprintf (stderr, "poll failed (%d)\n", err);
      break;
    }

    pthread_mutex_lock(&audiomutex);
    frames = snd_pcm_writei(handle, audiobuffer, sizeof(audiobuffer));
    pthread_mutex_unlock(&audiomutex);

    if (frames < 0)
      err = snd_pcm_recover(handle, frames, 0);
    if (err < 0) {
      printf("snd_pcm_writei failed: %s\n", snd_strerror(err));
      break;
    }

    if (frames > 0 && frames < (long)sizeof(audiobuffer))
      printf("Short write (expected %li, wrote %li)\n", (long)sizeof(audiobuffer), frames);
  }
  snd_pcm_close(handle);  
}

int main(void)
{
  pthread_t audiothread;
  int audiostop = 0;
  int volume;

  pthread_create(&audiothread, NULL, startaudio, &audiostop);

  while (1) {
    printf("Enter volume 1 through 10. [0 to quit.]: ");
    scanf("%d", &volume);
    if (volume == 0) break;
    changeaudio(volume);
  }

  audiostop = 1;
  pthread_join(audiothread, NULL);

  return 0;
}

And after reading the code above you'll probably want to read this article regarding (among other things) not using locks.

http://www.rossbencina.com/code/real-time-audio-programming-101-time-waits-for-nothing

Unearthly