[alsa-devel] Full duplex ALSA-plugin

Christian Gruber christian.gruber at voiceinterconnect.de
Wed Jul 11 09:04:36 CEST 2012

Am 10.07.2012 16:39, schrieb Jaroslav Kysela:
> Date 10.7.2012 16:28, Christian Gruber wrote:
>> I was looking for information on writing an echo cancellation ALSA-plugin. So I studied
>> the ALSA-plugin docs and also found several interesting threads in this mailing list. But
>> what I found out, was quiet disillusioning.
>> If I understood the ALSA-plugin concept right, there is no possibility to write a plugin
>> working on two streams, a playback and a record stream, simultaneously. One can only write
>> a filter plugin, which works either on a playback stream or a record stream but not both.
>> And there is no possibility to open a full duplex stream (consisting of playback and
>> record) with snd_pcm_open().
>> Is there a reason, why full duplex streams are not implemented yet? Or is it planned in
>> the future to implement this feature?
> If you want to do some synchronization between two streams, you have to
> use own "data sharing" mechanism such as IPC or so..
> In other words - implement filters on both playback and capture streams
> and implement the required communication between them.
> 					Jaroslav
Thanks, that's an important hint. I've seen, that such mechanisms are implemented in the 
plugins dmix, dshare and dsnoop for example. Are these plugins useful examples to see how 
it could work? And are the functions in pcm_direct.c necessary to implement my own IPC for 
playback and capture synchronization or are they specific to the three plugins dmix, 
dshare and dsnoop?


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