[alsa-devel] Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Takashi Iwai]

At Tue, 23 Jun 2009 02:40:35 -0300,
Guilherme wrote:
> 
> Hi all.
> 
> I am a bit confused on what I get with this function.
> I finished my first project but I did not understand what exactly the
> 
> _snd_pcm_channel_area at
> 
>  http://www.alsa-project.org/alsa-doc/alsa-lib/struct__snd__pcm__channel__area.html
> 
> does.
> 
> There are 3 fields in the documentation.
> 
> addr, first and step.
> 
> It seems that addr is the memory address of the channel samples... so 
> far so good. The "step" is the distance between 2 sample (but I am not 
> sure) and the least, "first", I could not figure out what does it means.

This information is needed to understand how the multi-channel samples
are assigned in a stream.  The first is the offset of the channel
position to the given addr.  The step is the bytes to the next sample
of that channel.

For example, suppose you have a 2-channel stereo interleaved stream
with 16bit samples.  Then you'll have an array of snd_pcm_channel_area
with two elements, for left and right channels, containing like

	area[0].addr = base_address;
	addr[0].first = 0;
	addr[0].step = 4;
	addr[1].addr = base_address;
	addr[1].first = 2;
	addr[1].step = 4;

Both channels share the same base address but have the different
"first" offset bytes.  The step size is 4 = #chanel * sample-size.

For a non-interleaved stereo stream, it'll look like

	addr[0].addr = base_addr_0;
	addr[0].first = 0;
	addr[0].step = 2;
	addr[1].addr = base_addr_1;
	addr[1].first = 0;
	addr[1].step = 2;

Thus it looks like two mono streams with 16bit samples.


HTH,

Takashi