Takashi...
The base_address and the step sounds pretty clear to me.... just the offset I could not understand.
What the difference in being .first = 0 or .first = 2??? Could you provide please a more in depth explanation. I tried really hard find this but there is nothing related in the documentation.
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;
P.S. being a stereo or a mono pipeline I understand... just the way the offset works that is not so clear to me.
Thanks in advanced!
Tks!
-------------------
Guilherme Longo Dept. Eng. da Computação Unaerp
Linux User - #484927
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Takashi Iwai wrote:
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__are...
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