From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: Play multiple sounds concurrently
Date: Fri, 18 Dec 2009 15:02:51 +0100
Message-ID: <4B2B8B8B.505@ladisch.de>
References: <4B2B873E.6010906@avibit.com>
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To: Markus Luttenberger <m.luttenberger@avibit.com>
Cc: alsa-devel@alsa-project.org
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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