[gst-devel] Linux audio is a mess? [was: JACK and GStreamer, from the horse's mouth]

[Wim Taymans]

On Thu, 2006-11-30 at 13:10 -0300, Edgard Lima wrote:
> Hi Paul,
> 
> ok, Im a bit late on this discussion (on vacation), but...
> 
> if you want to implement a pull based audiosink easily just look at 
> sdlaudiosink (in gst-plugins-bad)
> 
> we don't "write" to the device, instead a callback named 
> "fill_this_buffer" is called
> 
> the only thing you have to do is use a LOCK to protect producer and 
> consumer, like this:
> 
> filesrc -> decoder -> sdlaudiosink
> 
> in closer look
> 
> "decoder" writes to a ringbuffer and will be blocked when it is full. 
> (we are in push mode here and not problem at all)
> 
> from the sdlaudiosink what we do is to write data from this ringbuffer 
> to a temporary buffer and them send to device when the callback is 
> called, like this:
> 
> func_write_from_ring_buffer_to_temp_buffer() {
>   lock(A)
>   memcpy(temp_buffer, ring_buffer)
>   unlock(B)
> }
> 
> func_sdl_callback_fill_my_buffer(*buff) {
>   lock(B)
>   memcpy(buff, temp_buff);
>   unlock(A);
> }
> 
> if the return of "func_sdl_callback_fill_my_buffer" is too late, them 
> silent will be played.
> 
> I dont know about JACK, so please comment this approach, does it work 
> for you?

Just had a look at SDL audiosink. It's entirely not needed to use locks
around the ringbuffer. Also, the SDL audioosink does not seem to use a
ringbuffer at all, instead the data is passed between GStreamer and SDL
using a buffer of exactly one segment. This means a context switch
(forced because of the lock) for each segment exchanged between
gstreamer and SDL. You can improve SDL audiosink enormously by taking a
look at how jackaudiosink is implemented (hint, extend
GstBaseAudioSink).

Wim
> 
> BR,
> Edgard
> 
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-- 
Wim Taymans <wim at fluendo.com>