A Minimal ALSA Loopback Program

alsa_loopback_min.c

#include <stdio.h>
#include <stdlib.h>
#include <alsa/asoundlib.h>
      
#define BUF_BYTES 128

int main (int argc, char *argv[]) {
    int err;
    unsigned char buf[BUF_BYTES];
    
    snd_pcm_t *playback_handle;
    snd_pcm_t *capture_handle;
    
    char* device = "default";
    if (argc > 1) device = argv[1];
    
    unsigned int rate = 8000;
    unsigned int nchannels = 2;
    snd_pcm_format_t format = SND_PCM_FORMAT_S16_LE;
    
    unsigned int buf_frames = BUF_BYTES / nchannels / 2;

    if ((err = snd_pcm_open (&playback_handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
        fprintf (stderr, "cannot open audio device %s (%s)\n", device, snd_strerror (err)); exit (1);
    }
    
    if ((err = snd_pcm_set_params(playback_handle, format,  SND_PCM_ACCESS_RW_INTERLEAVED, nchannels, rate, 1, 500000)) < 0) {   /* 0.5sec */
        fprintf(stderr, "Playback open error: %s\n", snd_strerror(err)); exit(1);
    }
    
    if ((err = snd_pcm_open (&capture_handle, device, SND_PCM_STREAM_CAPTURE, 0)) < 0) {
        fprintf (stderr, "cannot open audio device %s (%s)\n", device, snd_strerror (err)); exit (1);
    }
    
    if ((err = snd_pcm_set_params(capture_handle, format,  SND_PCM_ACCESS_RW_INTERLEAVED, nchannels, rate, 1, 500000)) < 0) {   /* 0.5sec */
        fprintf(stderr, "capture open error: %s\n", snd_strerror(err)); exit(1);
    }
    
    while(1) {
        if ((err = snd_pcm_readi (capture_handle, buf, buf_frames)) != buf_frames) {
            fprintf (stderr, "read from audio interface failed (%s)\n", snd_strerror (err)); exit (1);
        }
        if ((err = snd_pcm_writei (playback_handle, buf, buf_frames)) != buf_frames) {
            fprintf (stderr, "write to audio interface failed (%s)\n", snd_strerror (err)); exit (1);
        }
    }

    fprintf (stderr, "close handles\n");
    snd_pcm_close (playback_handle);
    snd_pcm_close (capture_handle);
    
    return 0;
}

Its about tweaking the sample rate converter and sync, there are two parameters -A 5 and -S 5 that if set to automatic will improve the performance of alsaloop. This a raspberrypi3 A with 512 MB and QUAD CORE ARM get correct stream line with ~10-30ms