[longo] Solução: tocando sons simultâneos no ALSA via software (mixagem)
Pessoal,
Pesquisando, acabei compreendendo melhor e descobri uma solução para tocar
sons ao mesmo tempo usando o Advanced Linux Sound Architecture (ALSA).
O que acontece é que algumas placas de som são capazes de processar
múltiplos sons por vez, outras não (geralmente onboard ou as placas mais
simples). Para estas últimas, existe um plugin nativo do ALSA, chamado dmix
(direct mixer) que é capaz de fazer a mixagem simultânea em tempo real, de
forma transparente, seja quantos forem os sons. Naturalmente há um certo
custo de processamento, mas segundo experiências próprias, é praticamente
desprezível.
E para habilitar o 'dmix', basta editar o arquivo de configuração
~/.asoundrc (ou /etc/asound.conf para escopo global).
Testei e funcionou muito bem para aplicativos que usam a saída ALSA
(diretamente), como por exemplo 'alsaplayer', 'aplay', 'mplayer' (-ao
alsa9), etc. No caso eu testei usando o 'alsaplayer' e 'mplayer', sendo
que abri cinco instâncias do 'alsaplayer' e uma do 'mplayer'. Funcionou
satisfatoriamente.
Ainda não consegui que algumas aplicações sem saída direta ALSA tire
proveito deste recurso, mas segundo o texto que replico abaixo, é possível
também.
O texto foi tirado da lista 'debian-user'[1]. Apesar de tê-lo achado depois
de ter resolvido o problema, ele está com uma abordagem bastante completa.
#############
"Inspired by a very good article about ALSA in the German magazine c't
26/2003, I finally got the ALSA dmix plugin working fine and thus rid of
problems with apps blocking each other on sound playback, without needing
any of the sound servers which all are unsatisfactory for me (not to mention
that I couldn't get many of them working correctly at all on my TiBook). I
thought I'd share my experience, hoping it will be useful for others.
First of all, you need to get ALSA working to the point where you basically
hear correct sound. This shouldn't be hard with the Debian ALSA packages, in
particular with a 2.6 kernel, so I won't go into details here.
The next step is setting up ALSA to use the dmix plugin (official
documentation at
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html#pcm_plugins_dmix).
Put something like this in /etc/asound.conf or ~/.asoundrc:
pcm.!default {
type plug
slave.pcm dmixer
}
pcm.dmixer {
type dmix
ipc_key 1024 # This must be a unique IPC key (see ipcs output)
ipc_key_add_uid true # UID will be added to IPC key
slave {
pcm "hw:0"
period_time 0
period_size 1024 # Must be power of 2 for alsa-oss
buffer_size 8192 # ditto
rate 44100
}
}
This sets up a PCM device called dmixer using the dmix plugin and overrides
the default PCM device to use it via the conversion plugin, which will
convert the sound format, rate and number of channels on the fly if
necessary (the dmix plugin only supports a fixed constellation of these
parameters due to the way it works).
Now you should be able to use any number of ALSA apps concurrently. The
problem is that still only a minority of apps use ALSA directly, at least by
default. Luckily, most (can) use a standard library for sound playback,
e.g. SDL; for those, you may have to set these environment variables:
SDL_AUDIODRIVER=alsa
AUDIODEV=default # This isn't necessary with the default device fix from
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=225252
For OpenAL, I use
(define devices '(alsa sdl native))
in ~/.openalrc, which tells it to try ALSA first (the current OpenAL
packages in Debian don't support ALSA directly though), then SDL and only
then direct access to /dev/dsp.
So far, so good, but there's still the odd app which can only
access /dev/dsp directly for sound. That's what alsa-oss is for, which
unfortunately isn't available in Debian yet (though I'm told that will
change after the next DeMuDi release, planned for mid January), but almost
as conveniently from
deb http://apt.agnula.org unstable main contrib non-free
(or the corresponding deb-src entry for non-i386 users). It works by
preloading a library (libaoss) which translates direct access to OSS devices
into ALSA library calls. It provides a script called aoss, but you probably
want to add /usr/lib/libaoss.so to the LD_PRELOAD environment variable. It
also requires the following additions to the ALSA configuration:
pcm.dsp0 pcm.default
ctl.mixer0 "hw:0"
The major remaining problem for me is that xine doesn't work well with dmix,
see http://sourceforge.net/mailarchive/message.php?msg_id=6793623.
My main usage scenario has been listening to music with alsaplayer while
playing armagetron. :)
Happy dmixing, and Happy New Year!
PS: I'm not subscribed to the debian-user list.
--
Earthling Michel Dänzer | Debian (powerpc), X and DRI developer
Software libre enthusiast | http://svcs.affero.net/rm.php?r=daenzer";
1. http://lists.debian.org/debian-user/2003/debian-user-200312/msg07973.html
#############
Referências adicionais:
http://www.alsa-project.org/alsa-doc/doc-php/asoundrc.php3#softmix
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html#pcm_plugins_dmix
http://alsa.opensrc.org/index.php?page=DmixPlugin
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm__dmix_8c.html
--
Douglas Augusto
[Netiqueta]
§ Cortar redundâncias nos textos, evitando poluição e uso desnecessário de
banda.
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