buffer underrun issue

Ferenc Engárd <ferenc.engard@xxxxxxxxx> · Thu, 9 Mar 2017 00:03:59 +0100

Hi all,
Welcome everybody, as this is my first mail to the list.

I am trying to write my first ALSA audio program, and I have intermittent issues I can't explain.

I am playing ~68ms of sound from memory, in 1-channel U8 setup, and quite frequently I get the following errors:

ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred

The test project can be found here: https://github.com/circum1/alsa-underrun-demo
Compile simply with g++ -o alsa-test alsa-test.cpp -lasound

The program writes 3*1000 samples, then drains, sleeps 1 second, and loops. My example output running this program:

fery@fery-E6540:~/tmp/alsa-demo/alsa-underrun-demo$ g++ -o alsa-test alsa-test.cpp -lasound && ./alsa-test
SOUND: setupWithFreq sampling rate: 44100
SOUND: number of periods: 3
SOUND: period size: 1000 frames
0 {0}
1 {1}
ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred
1 {2}
1 before drain
48 after drain

1049 {0}
1050 {1}
1050 {2}
1050 before drain
1119 after drain

2120 {0}
2121 {1}
ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred
2121 {2}
ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred
2122 before drain
2122 after drain

3123 {0}
3124 {1}
3124 {2}
3124 before drain
3194 after drain

4195 {0}
4195 {1}
4195 {2}
4195 before drain
4265 after drain

5266 {0}
5266 {1}
ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred
5267 {2}
ALSA error: Broken pipe
ALSA lib pcm.c:7963:(snd_pcm_recover) underrun occurred
5268 before drain
5268 after drain

6269 {0}
6270 {1}
6270 {2}
6270 before drain
6339 after drain

^C

The numbers at the beginning of the lines are elapsed milliseconds. It is visible that the program writes 2000 samples in 1-2ms, but nevertheless the underrun occurs.

I don't know if the error is in my code, or is it some bug or maybe hw issue?

Possibly relevant env info:

Ubuntu 16.04, Linux version 4.4.0-59-generic
libasound2: Version: 1.1.0-0ubuntu1
pulseaudio: Version: 1:8.0-0ubuntu3.2

00:03.0 Audio device: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller (rev 06)
(Dell E6540 notebook)

Independent of this issue, I also have another question: is it safe to write fewer samples than period size, even if it is not the last pcm_writei in the stream?

Thanks in advance,
Ferenc

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