CODEWITHSUNDEEP
clinuxaudioalsa
captcha
captcha

Reputation: 348

how to play short tones with ALSA

I'm trying to generate short tones of variable lengths through ALSA in a small C program. A few examples I've tried work fine when playing one second worth of sound but anything shorter than that just doesn't produce any sound at all.

I'm filling a buffer with a sine wave like so:

#define BUFFER_LEN 44100
int freq = 700;             //audio frequency
int fs = 44100;             //sampling frequency
float buffer [BUFFER_LEN];
for (k=0; k<BUFFER_LEN; k++) {
   buffer[k] = (sin(2*M_PI*freq/fs*k));                                 
}

Setting the pcm device parameters:

if ((err = snd_pcm_set_params(handle,
                              SND_PCM_FORMAT_FLOAT,
                              SND_PCM_ACCESS_RW_INTERLEAVED,
                              1,
                              44100,
                              1,
                              500000)) < 0) {
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}

Playback:

frames = snd_pcm_writei(handle, buffer, BUFFER_LEN);

But if I want to change it to play the tone, say a quarter of a second (i.e. changing BUFFER_LEN to 11025) nothing comes out of the speaker anymore.

I've tried changing types from floats to shorts, setting the PCM_FORMAT to other values and trying different ways of filling the buffer with sine waves.

If anything, I hear a little 'bip' sound in the speakers but just not what I expect. The program doesn't segfaults or crash but I'm just puzzled how to make ALSA play shorter samples.

I don't know if I need to work with some exact frame or buffer size multiple but I'm very open to suggestions.

Upvotes: 4

Views: 2521

Answers (1)

CL.
CL.

Reputation: 180210

Your application writing samples to the buffer, and the hardware reading samples from the buffer and playing them are two different processes that run asynchronously.

If there is not enough free space in the buffer for the amount of samples you're trying to write, then snd_pcm_writei() will wait until enough space is available. But when snd_pcm_writei() returns, up to a full buffer of samples might not yet have been played.

To wait until the buffer is empty, use snd_pcm_drain().

Upvotes: 3

Related Questions