Re: Interrupt-driven (or callback/handler?), Full-duplex (simultaneous capture/playback) without using JACK

[Paul Davis <paul@linuxaudiosystems.com>]

>I am trying to write an application that uses ALSA interrupt-driven
>(which I guess is obtained by adding PCM handler(s) in ALSA?),
>full-duplex (simultaneous capture/playback) using the ALSA PCM API
>directly (i.e. not using JACK).
>
>My question: do I only need to register one callback/handler function to
>take care of both capture and playback?  My hope is that when I link the
>capture and playback streams, and register a single handler with either
>capture or playback, this handler will be called whenever there is BOTH
>capture data to be read and a free playback buffer to be written (?).
>Is this the case?

the ALSA PCM API doesn't have "handlers". it doesn't provide any kind
of callback mechanism. what it does do is to provide a poll(2)
interface, allowing callback-like mechanisms to be built around it
(like JACK). 

the snd_pcm_wait() function isn't enough for what you want to do, and
using it to what you want to do is equivalent to reimplenting the JACK
ALSA driver. a couple of people have asked that the core of JACK's
ALSA driver's inner loop be merged into alsa-lib, since it would do
what you want (take N PCM handles, block and return when all of them
have at least X frames of audio ready to read or write). at this time,
this has not happened, and there is no sign that it will be.

if you really don't want to use JACK, then i would suggest that,
assuming your program is be either private or distributed under the
GPL, you just take the JACK ALSA driver and use that as a template for
your efforts. its not simple, because full-duplex, interrupt-driven
audio i/o is not simple unless you're a JACK client :) if you want to
write the "server", you have to work a lot harder.

either that, or take a look at ecasound's ALSA i/o code, since i think
that kai implemented that "correctly" also.

--p



-------------------------------------------------------
This SF.Net email sponsored by: ApacheCon 2003,
16-19 November in Las Vegas. Learn firsthand the latest
developments in Apache, PHP, Perl, XML, Java, MySQL,
WebDAV, and more! http://www.apachecon.com/