Render audio data from a push source using event driven model

["Alfredo Rueda Unsain" <alfredo@vida-software.com>]

Hello!

I'm developing a Linux server that use poll as the event
notification schema.

The server must render audio that receives from a push 
source in real time.

When I receive a UDP Datagram (audio packet) I must render 
the packet using write in NON BLOCKING mode.

I have read all the ALSA documentation (including the 
examples) but I'm not sure how to proceed. The examples 
that use poll, use the snd_pcm_sw_params_set_avail_min 
function to specify when they want to be notified in order 
to write audio samples without blocking.

In this sense I could use the following schema:

When I receive an audio packet, store the packet in a FIFO 
queue.
Use snd_pcm_sw_params_set_avail_min to specify the number 
of samples of an audio packet.
When ALSA notifies the server, consume the next packet 
from the FIFO and write the packet.
  
I'm using another approach:

1. Open the PCM device (with SND_PCM_NONBLOCK flag).
2. Setup the PCM device (using 
snd_pcm_sw_params_set_start_threshold to prefetch 3 
packets).
3. Prepare the PCM device. (What is the purpose of 
snd_pcm_prepare function?
	I have read the states of a PCM device but, don't 
understand the changes between
	SETUP and PREPARED states).
4. When I receive an audio packet:
	a) If there is enough space in the audio buffer, render 
the packet directly using snd_pcm_writei.
	b) else, drop the packet.

    The problem is that before calling snd_pcm_writei, I 
call snd_pcm_avail_update and returns only 336 samples to 
deliver (only 1 channel).

    The examples show that event driven programs must use 
snd_pcm_sw_params_set_avail_min,
    but I don't understant why snd_pcm_avail_update 
returns only 336 samples if I call snd_pcm_prepare a lot 
of seconds before.

    What is the right approach to proceed in this 
scenario?

    Thank you very much for your help.

     Alfredo Rueda.


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