Play multiple sounds concurrently

Play multiple sounds concurrently

[Markus Luttenberger]

Hey,


I want to play multiple sounds concurrently with ALSA. The audio data is 
mono and G.711 a-law encoded. I can successfully play a single source of 
audio data but the sound is distorted when I try to play two sources.

Basically, this is what I'm doing:

-----8<-----------------
char buf1[1024];    // filled with audio data from source 1
char buf2[1024];    // filled with audio data from source 2
char wr_buf[1024];

for (int count = 0; count < 1024; count++)
   wr_buf[count] = (char) (0.5 * buf1[count]) +
                   (char) (0.5 * buf2[count]);

snd_pcm_writei(dev_handle, wr_buf, 1024);
-----8<-----------------


I think I have to use a scale value to correct the values or else 
clipping occures. However, this is the problem. How do I have to modify 
the audio bytes in order to add them together and write them into one 
buffer?




greetings..
..Markus

Re: Play multiple sounds concurrently

[Clemens Ladisch]

Markus Luttenberger wrote:
> I want to play multiple sounds concurrently with ALSA. The audio data is 
> mono and G.711 a-law encoded. I can successfully play a single source of 
> audio data but the sound is distorted when I try to play two sources.
> 
>    wr_buf[count] = (char) (0.5 * buf1[count]) +
>                    (char) (0.5 * buf2[count]);

A-law is a somewhat logarithmic encoding; simply adding two values with
linear scale factors will not work.

If both your source data and the output device use A-law, you have to
decode the samples to linear PCM, add them, and encode them back.  See
<http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_alaw.c;hb=HEAD>
for the algorithm that alsa-lib uses; you would use something like
buf[] = s16_to_alaw((alaw_to_s16(buf1[]) + alaw_to_s16(buf2[])) / 2);

If it is possible for your output device to use a linear PCM format,
you might want to use that to avoid the additional encoding step.


HTH
Clemens