Hallo,
currently I am working on sound localization an for this reason I would like
to extract
- an amount of 1024 samples (contiguous ones)
- from a never stopping ring buffer (stop threshold = boundary)
- simultanously from two different sound cards
with ALSA when I need a set of samples for further processing.
I have already tried the snd_pcm_drain() function but this did not work with
my capture stream. Now I am trying a direct MMAP access - call hesync,
receive pointer, begin, read sample data, commit. However, the official ALSA
example code does not help me a lot because it is only for playback.
Additionally I do not know how the capture works an how to properly read
from it.
I spent very much time on reading the documentations espacially the
reference. Nevertheless I stil haven't got the information whether it is
possible to demand a certain amount of samples with the simple
snd_pcm_readi() function (for just one sound card) if the ring buffer is
looping without any stop. Can I be sure that I really receive the latest -
lets say 1024 - (stereo) samples and that they have not been overwritten,
for example. Does snd_pcm_readi() require a continuous stream?
Thank you very much for your answer!
Sincerely,
TobiU
PS: This is my code I would like to use for a simpe data acquisition:
#include <alsa/asoundlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <alsa/asoundlib.h>
#include <sched.h>
#include <errno.h>
#include <math.h>
static unsigned int buffer_time = 110000; /* ring buffer
length in us */
static unsigned int period_time = 22000;
static unsigned int channels = 2;
static unsigned int rate = 48000;
static unsigned int format = SND_PCM_FORMAT_S16_LE;
static snd_pcm_sframes_t period_size, buffer_size;
static snd_pcm_uframes_t wantedframes = 1240;
//function to set hardware parameters
static int set_hwparams(snd_pcm_t *handle,
snd_pcm_hw_params_t *params)
{
unsigned int rrate, value;
snd_pcm_uframes_t size;
int err, dir;
/* choose all parameters */
err = snd_pcm_hw_params_any(handle, params);
if (err < 0) {
printf("Broken configuration for playback: no configurations
available: %s\n", snd_strerror(err));
return err;
}
/* set hardware resampling, Restrict a configuration space to
contain only real hardware rates (1 = enabled). */
err = snd_pcm_hw_params_set_rate_resample(handle, params, 1);
if (err < 0) {
printf("Resampling setup failed for playback: %s\n",
snd_strerror(err));
return err;
}
/* set the interleaved read/write format */
err = snd_pcm_hw_params_set_access(handle, params,
SND_PCM_ACCESS_RW_INTERLEAVED);
if (err < 0) {
printf("Access type not available for playback: %s\n",
snd_strerror(err));
return err;
}
/* set the sample format */
err = snd_pcm_hw_params_set_format(handle, params, format);
if (err < 0) {
printf("Sample format not available for playback: %s\n",
snd_strerror(err));
return err;
}
/* set the count of channels */
err = snd_pcm_hw_params_set_channels(handle, params, channels);
if (err < 0) {
printf("Channels count 2 not available for capture: %s\n",
snd_strerror(err));
return err;
}
/* set the stream rate */
rrate = rate;
err = snd_pcm_hw_params_set_rate_near(handle, params, &rrate, 0);
if (err < 0) {
printf("Rate %iHz not available for capture: %s\n", rate,
snd_strerror(err));
return err;
}
if (rrate != rate) {
printf("Rate doesn't match (requested %iHz, get %iHz)\n",
rate, err);
return -EINVAL;
}
/* set the buffer size */
value = buffer_time;
dir = 1;
err = snd_pcm_hw_params_set_buffer_time_near(handle, params, &value,
&dir);
if (err < 0) {
printf("Unable to set buffer time %i for capture: %s\n",
dir, snd_strerror(err));
return err;
}
err = snd_pcm_hw_params_get_buffer_size(params, &size);
if (err < 0) {
printf("Unable to get buffer size for capture: %s\n",
snd_strerror(err));
return err;
}
buffer_size = size;
printf("buffer size is: %i\n", buffer_size);
err = snd_pcm_hw_params_get_buffer_time(params, &value, &dir);
if (err < 0) {
printf("Unable to get buffer time for capture: %s\n",
snd_strerror(err));
return err;
}
printf("buffer time %i with direction %i\n", value, dir);
/* set the period size */
value = period_time;
dir = 1;
err = snd_pcm_hw_params_set_period_time_near(handle, params, &value,
&dir);
if (err < 0) {
printf("Unable to set period time %i for capture: %s\n",
size, snd_strerror(err));
return err;
}
err = snd_pcm_hw_params_get_period_size(params, &size, &dir);
if (err < 0) {
printf("Unable to get period size for capture: %s\n",
snd_strerror(err));
return err;
}
period_size = size;
printf("period size: %i\n", period_size);
err = snd_pcm_hw_params_get_period_time(params, &value, &dir);
if (err < 0) {
printf("Unable to get period time for capture: %s\n",
snd_strerror(err));
return err;
}
printf("period time %i with direction %i\n", value, dir);
/* write the parameters to device */
err = snd_pcm_hw_params(handle, params);
if (err < 0) {
printf("Unable to set hw params for capture: %s\n",
snd_strerror(err));
return err;
}
return 0;
}
//function to set software parameters
static int set_swparams(snd_pcm_t *handle, snd_pcm_sw_params_t *swparams)
{
int err;
unsigned int value;
snd_pcm_uframes_t size;
/* get the current swparams */
err = snd_pcm_sw_params_current(handle, swparams);
if (err < 0) {
printf("Unable to determine current swparams for capture:
%s\n", snd_strerror(err));
return err;
}
/* start the transfer when the buffer contains period_size */
/* (buffer_size / avail_min) * avail_min */
err = snd_pcm_sw_params_set_start_threshold(handle, swparams,
period_size);
if (err < 0) {
printf("Unable to set start threshold mode for capture:
%s\n", snd_strerror(err));
return err;
}
/* allow the transfer when at least period_size samples can be
processed */
err = snd_pcm_sw_params_set_avail_min(handle, swparams,
period_size);
if (err < 0) {
printf("Unable to set avail min for capture: %s\n",
snd_strerror(err));
return err;
}
/* align all transfers period_size samples */
err = snd_pcm_sw_params_set_xfer_align(handle, swparams,
period_size);
if (err < 0) {
printf("Unable to set transfer align for capture: %s\n",
snd_strerror(err));
return err;
}
err = snd_pcm_sw_params_set_tstamp_mode(handle, swparams,
SND_PCM_TSTAMP_MMAP);
if (err < 0){
printf("Unable to set timestamp mode for capture: %s\n",
snd_strerror(err));
return err;
}
/*never stopping ring buffer (automatic stop threshold = boundary)*/
err = snd_pcm_sw_params_get_boundary(swparams, &size);
if (err < 0){
printf("Unable to get boundary for capture: %s\n",
snd_strerror(err));
return err;
}
printf("boundary: %i frames\n", size);
err = snd_pcm_sw_params_set_stop_threshold(handle, swparams, size);
//changed on 17 Apr
if (err < 0){
printf("Unable to set stop threshold for capture: %s\n",
snd_strerror(err));
return err;
}
err = snd_pcm_sw_params_get_stop_threshold(swparams, &size); //changed
on
17 Apr
if (err < 0){
printf("Unable to set stop threshold for capture: %s\n",
snd_strerror(err));
return err;
}
printf("stop threshold: %i\n", size);
/* write the parameters to the capture device */
err = snd_pcm_sw_params(handle, swparams);
if (err < 0) {
printf("Unable to set sw params for capture: %s\n",
snd_strerror(err));
return err;
}
return 0;
}
int main() {
//Deklaration von Variablen
int z;
long loops;
int rc;
snd_pcm_sframes_t rcsframes;
int size;
//unsigned char buffer
int space;
unsigned char *buffer;
signed short *shortsamples;
snd_pcm_t *handle;
snd_pcm_hw_params_t *hwparams;
snd_pcm_sw_params_t *swparams;
unsigned int val;
int dir;
snd_pcm_access_t access = SND_PCM_ACCESS_RW_INTERLEAVED;
snd_pcm_state_t state;
/* Open PCM device for capture */
rc = snd_pcm_open(&handle, "hw:0,0", SND_PCM_STREAM_CAPTURE, 0);
if (rc < 0) {
fprintf(stderr, "unable to open first pcm device: %s\n",
snd_strerror(rc));
exit(1);
}
/* set the hardware parameters and write them to driver */
snd_pcm_hw_params_alloca( &hwparams );
rc = set_hwparams( handle, hwparams );
if (rc < 0) {
fprintf(stderr, "unable to set hw parameters for first capture: %s\n",
snd_strerror(rc));
exit(1);
}
/* set the software parameters and write them to driver */
snd_pcm_sw_params_alloca( &swparams );
rc = set_swparams( handle, swparams );
if (rc < 0) {
fprintf(stderr, "unable to set sw parameters for first capture: %s\n",
snd_strerror(rc));
exit(1);
}
/*prepare handles for use*/
rc = snd_pcm_prepare(handle);
if (rc < 0) {
printf("Unable to prepare first capture: %s\n", snd_strerror(rc));
exit(1);
}
/*wait for handles to be ready for use*/
rc = snd_pcm_wait(handle, 1000);
if (rc < 0) {
printf("Waiting failed for capture: %s\n", snd_strerror(rc));
exit(1);
}
space = wantedframes * 4; /* 2 bytes/sample, 2 channels */
buffer = (unsigned char *) malloc(space);
rc = snd_pcm_start(handle);
if (rc < 0) {
fprintf(stderr, "Unable to stsrt capture %s\n", snd_strerror(rc));
exit(1);
}
/* We want to loop for 5 seconds */
snd_pcm_hw_params_get_period_time(hwparams, &val, &dir);
loops = 5000000 / val;
while (loops > 0) {
loops--;
printf("state of capture: %i\n", snd_pcm_state(handle));
rcsframes = snd_pcm_readi(handle, buffer, wantedframes);
/*if (rcsframes == -EPIPE) {
/* EPIPE means overrun /
fprintf(stderr, "overrun occurred\n");
snd_pcm_prepare(handle);
} else if (rcsframes < 0) {
fprintf(stderr,"error from read: %s\n", snd_strerror(rcsframes));
} else if (rcsframes != (int)wantedframes) {
fprintf(stderr, "short read, read %d frames\n", rcsframes);
}*/
if ((int)rcsframes >= wantedframes)
printf("number of read frames: %i\n", (int)rcsframes);
else {
printf("Failure at read\n");
exit(1);
}
//write the sample values to stdout
shortsamples = (signed short *) buffer;
for ( z=0; z<(int)wantedframes; z++) {
printf("short1: %d\n", (*(shortsamples+z)));
printf("short2: %d\n", (*(shortsamples+z+1)));
}
printf("Ende eines Satzes\n");
printf("state of capture: %i\n",snd_pcm_state(handle) );
sleep(0.6);
}
snd_pcm_drain(handle);
snd_pcm_close(handle);
free(buffer);
return 0;
}
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